Increasing role of Anopheles funestus and Anopheles arabiensis in malaria transmission in the Kilombero Valley, Tanzania

Entomological survey of sibling species in the Anopheles funestus group in Tanzania confirms the role of Anopheles parensis as a secondary malaria vector

BACKGROUND

Malaria transmission in Tanzania is driven by mosquitoes of the Anopheles gambiae complex and Anopheles funestus group. The latter includes An. funestus s.s., an anthropophilic vector, which is now strongly resistant to public health insecticides, and several sibling species, which remain largely understudied despite their potential as secondary vectors. This paper provides the initial results of a cross-country study of the species composition, distribution and malaria transmission potential of members of the Anopheles funestus group in Tanzania.

METHODS

Mosquitoes were collected inside homes in 12 regions across Tanzania between 2018 and 2022 using Centres for Disease Control and Prevention (CDC) light traps and Prokopack aspirators. Polymerase chain reaction (PCR) assays targeting the noncoding internal transcribed spacer 2 (ITS2) and 18S ribosomal DNA (18S rDNA) were used to identify sibling species in the An. funestus group and presence of Plasmodium infections, respectively. Where DNA fragments failed to amplify during PCR, we sequenced the ITS2 region to identify any polymorphisms.

RESULTS

The following sibling species of the An. funestus group were found across Tanzania: An. funestus s.s. (50.3%), An. parensis (11.4%), An. rivulorum (1.1%), An. leesoni (0.3%). Sequencing of the ITS2 region in the nonamplified samples showed that polymorphisms at the priming sites of standard species-specific primers obstructed PCR amplification, although the ITS2 sequences closely matched those of An. funestus s.s., barring these polymorphisms. Of the 914 samples tested for Plasmodium infections, 11 An. funestus s.s. (1.2%), and 2 An. parensis (0.2%) individuals were confirmed positive for P. falciparum. The highest malaria transmission intensities [entomological inoculation rate (EIR)] contributed by the Funestus group were in the north-western region [108.3 infectious bites/person/year (ib/p/y)] and the south-eastern region (72.2 ib/p/y).

CONCLUSIONS

Whereas An. funestus s.s. is the dominant malaria vector in the Funestus group in Tanzania, this survey confirms the occurrence of Plasmodium-infected An. parensis, an observation previously made in at least two other occasions in the country. The findings indicate the need to better understand the ecology and vectorial capacity of this and other secondary malaria vectors in the region to improve malaria control.

Distribution and insecticide resistance profile of the major malaria vector Anopheles funestus group across the African continent

There has been significant progress in malaria control in the last 2 decades, with a decline in mortality and morbidity. However, these gains are jeopardised by insecticide resistance, which negatively impacts the core interventions, such as insecticide-treated nets (ITN) and indoor residual spraying (IRS). While most malaria control and research efforts are still focused on Anopheles gambiae complex mosquitoes, Anopheles funestus remains an important vector in many countries and, in some cases, contributes to most of the local transmission. As countries move towards malaria elimination, it is important to ensure that all dominant vector species, including An. funestus, an important vector in some countries, are targeted. The objective of this review is to compile and discuss information related to A. funestus populations' resistance to insecticides and the mechanisms involved across Africa, emphasising the sibling species and their resistance profiles in relation to malaria elimination goals. Data on insecticide resistance in An. funestus malaria vectors in Africa were extracted from published studies. Online bibliographic databases, including Google Scholar and PubMed, were used to search for relevant studies. Articles published between 2000 and May 2023 reporting resistance of An. funestus to insecticides and associated mechanisms were included. Those reporting only bionomics were excluded. Spatial variation in species distribution and resistance to insecticides was recorded from 174 articles that met the selection criteria. It was found that An. funestus was increasingly resistant to the four classes of insecticides recommended by the World Health Organisation for malaria vector control; however, this varied by country. Insecticide resistance appears to reduce the effectiveness of vector control methods, particularly IRS and ITN. Biochemical resistance due to detoxification enzymes (P450s and glutathione-S-transferases [GSTs]) in An. funestus was widely recorded. However, An. funestus in Africa remains susceptible to other insecticide classes, such as organophosphates and neonicotinoids. This review highlights the increasing insecticide resistance of An. funestus mosquitoes, which are important malaria vectors in Africa, posing a significant challenge to malaria control efforts. While An. funestus has shown resistance to the recommended insecticide classes, notably pyrethroids and, in some cases, organochlorides and carbamates, it remains susceptible to other classes of insecticides such as organophosphates and neonicotinoids, providing potential alternative options for vector control strategies. The study underscores the need for targeted interventions that consider the population structure and geographical distribution of An. funestus, including its sibling species and their insecticide resistance profiles, to effectively achieve malaria elimination goals.

Rapid classification of epidemiologically relevant age categories of the malaria vector, Anopheles funestus

BACKGROUND

Accurately determining the age and survival probabilities of adult mosquitoes is crucial for understanding parasite transmission, evaluating the effectiveness of control interventions and assessing disease risk in communities. This study was aimed at demonstrating the rapid identification of epidemiologically relevant age categories of Anopheles funestus, a major Afro-tropical malaria vector, through the innovative combination of infrared spectroscopy and machine learning, instead of the cumbersome practice of dissecting mosquito ovaries to estimate age based on parity status.

METHODS

Anopheles funestus larvae were collected in rural south-eastern Tanzania and reared in an insectary. Emerging adult females were sorted by age (1-16 days old) and preserved using silica gel. Polymerase chain reaction (PCR) confirmation was conducted using DNA extracted from mosquito legs to verify the presence of An. funestus and to eliminate undesired mosquitoes. Mid-infrared spectra were obtained by scanning the heads and thoraces of the mosquitoes using an attenuated total reflection-Fourier transform infrared (ATR-FT-IR) spectrometer. The spectra (N = 2084) were divided into two epidemiologically relevant age groups: 1-9 days (young, non-infectious) and 10-16 days (old, potentially infectious). The dimensionality of the spectra was reduced using principal component analysis, and then a set of machine learning and multi-layer perceptron (MLP) models were trained using the spectra to predict the mosquito age categories.

RESULTS

The best-performing model, XGBoost, achieved overall accuracy of 87%, with classification accuracy of 89% for young and 84% for old An. funestus. When the most important spectral features influencing the model performance were selected to train a new model, the overall accuracy increased slightly to 89%. The MLP model, utilizing the significant spectral features, achieved higher classification accuracy of 95% and 94% for the young and old An. funestus, respectively. After dimensionality reduction, the MLP achieved 93% accuracy for both age categories.

CONCLUSIONS

This study shows how machine learning can quickly classify epidemiologically relevant age groups of An. funestus based on their mid-infrared spectra. Having been previously applied to An. gambiae, An. arabiensis and An. coluzzii, this demonstration on An. funestus underscores the potential of this low-cost, reagent-free technique for widespread use on all the major Afro-tropical malaria vectors. Future research should demonstrate how such machine-derived age classifications in field-collected mosquitoes correlate with malaria in human populations.

Comparison of different trapping methods to collect malaria vectors indoors and outdoors in western Kenya

BACKGROUND

Vector surveillance is among the World Health Organization global vector control response (2017-2030) pillars. Human landing catches are a gold standard but difficult to implement and potentially expose collectors to malaria infection. Other methods like light traps, pyrethrum spray catches and aspiration are less expensive and less risky to collectors.

METHODS

Three mosquito sampling methods (UV light traps, CDC light traps and Prokopack aspiration) were evaluated against human landing catches (HLC) in two villages of Rarieda sub-county, Siaya County, Kenya. UV-LTs, CDC-LTs and HLCs were conducted hourly between 17:00 and 07:00. Aspiration was done indoors and outdoors between 07:00 and 11:00 a.m. Analyses of mosquito densities, species abundance and sporozoite infectivity were performed across all sampling methods. Species identification PCR and ELISAs were done for Anopheles gambiae and Anopheles funestus complexes and data analysis was done in R.

RESULTS

Anopheles mosquitoes sampled from 608 trapping efforts were 5,370 constituting 70.3% Anopheles funestus sensu lato (s.l.), 19.7% Anopheles coustani and 7.2% An. gambiae s.l. 93.8% of An. funestus s.l. were An. funestus sensu stricto (s.s.) and 97.8% of An. gambiae s.l. were Anopheles arabiensis. Only An. funestus were sporozoite positive with 3.1% infection prevalence. Indoors, aspiration captured higher An. funestus (mean = 6.74; RR = 8.83, P < 0.001) then UV-LT (mean = 3.70; RR = 3.97, P < 0.001) and CDC-LT (mean = 1.74; RR = 1.89, P = 0.03) compared to HLC. UV-LT and CDC-LT indoors captured averagely 0.18 An. arabiensis RR = 5.75, P = 0.028 and RR = 5.87, P = 0.028 respectively. Outdoors, UV-LT collected significantly higher Anopheles mosquitoes compared to HLC (An. funestus: RR = 5.18, P < 0.001; An. arabiensis: RR = 15.64, P = 0.009; An. coustani: RR = 11.65, P < 0.001). Anopheles funestus hourly biting indoors in UV-LT and CDC-LT indicated different peaks compared to HLC.

CONCLUSIONS

Anopheles funestus remains the predominant mosquito species. More mosquitoes were collected using aspiration, CDC-LTs and UV-LTs indoors and UV-LTs and CD-LTs outdoors compared to HLCs. UV-LTs collected more mosquitoes than CDC-LTs. The varied trends observed at different times of the night suggest that these methods collect mosquitoes with diverse activities and care must be taken when interpreting the results.

Effect of non-human hosts on the human biting rate of primary and secondary malaria vectors in Tanzania

BACKGROUND

Malaria vectors vary in feeding preference depending on their innate behaviour, host availability and abundance. Host preference and human biting rate in malaria vectors are key factors in establishing zooprophylaxis and zoopotentiation. This study aimed at assessing the impact of non-human hosts in close proximity to humans on the human biting rate of primary and secondary malaria vectors, with varying host preferences.

METHODS

The effect of the presence of non-human hosts in close proximity to the human host on the mean catches per person per night, as a proxy for mosquito biting rate, was measured using mosquito-electrocuting traps (METs), in Sagamaganga, Kilombero Valley, Tanzania. Two experiments were designed: (1) a human versus a calf, each enclosed in a MET, and (2) a human surrounded by three calves versus a human alone, with each human volunteer enclosed individually in a MET spaced 10 m apart. Each experiment was conducted on alternate days and lasted for 36 nights per experiment. During each experiment, the positions of hosts were exchanged daily (except the human in experiment 2). All anopheline mosquitoes caught were assayed for Plasmodium sporozoites using enzyme-linked immunosorbent assay.

RESULTS

A total of 20,574 mosquitoes were captured and identified during the study, of which 3608 were anophelines (84.4% primary and 15.6% secondary malaria vectors) and 17,146 were culicines. In experiment 1, the primary malaria vector, Anopheles arabiensis, along with Culex spp. demonstrated a preference for cattle, while the primary vectors, Anopheles funestus, preferred humans. In experiment 2, both primary vectors, An. arabiensis and An. funestus, as well as the secondary vector Anopheles rivolurum, demonstrated behaviours amenable to zooprophylaxis, whereas Culex spp. increased their attraction to humans in the presence of nearby cattle. All anopheline mosquitoes tested negative for sporozoites.

CONCLUSIONS

The findings of this study provide support for the zooprophylaxis model for malaria vectors present in the Kilombero Valley, and for the zoopotentiation model, as it pertains to the Culex spp. in the region. However, the factors regulating zooprophylaxis and zoopotentiation are complex, with different species-dependent mechanisms regulating these behaviours, that need to be considered when designing integrated vector management programmes.

Changes in contributions of different Anopheles vector species to malaria transmission in east and southern Africa from 2000 to 2022

BACKGROUND

Malaria transmission in Africa is facilitated by multiple species of Anopheles mosquitoes. These vectors have different behaviors and vectorial capacities and are affected differently by vector control interventions, such as insecticide-treated nets and indoor residual spraying. This review aimed to assess changes in the contribution of different vector species to malaria transmission in east and southern Africa over 20 years of widespread insecticide-based vector control.

METHODS

We searched PubMed, Global Health, and Web of Science online databases for articles published between January 2000 and April 2023 that provided species-specific sporozoite rates for different malaria vectors in east and southern Africa. We extracted data on study characteristics, biting rates, sporozoite infection proportions, and entomological inoculation rates (EIR). Using EIR data, the proportional contribution of each species to malaria transmission was estimated.

RESULTS

Studies conducted between 2000 and 2010 identified the Anopheles gambiae complex as the primary malaria vector, while studies conducted from 2011 to 2021 indicated the dominance of Anopheles funestus. From 2000 to 2010, in 57% of sites, An. gambiae demonstrated higher parasite infection prevalence than other Anopheles species. Anopheles gambiae also accounted for over 50% of EIR in 76% of the study sites. Conversely, from 2011 to 2021, An. funestus dominated with higher infection rates than other Anopheles in 58% of sites and a majority EIR contribution in 63% of sites. This trend coincided with a decline in overall EIR and the proportion of sporozoite-infected An. gambiae. The main vectors in the An. gambiae complex in the region were Anopheles arabiensis and An. gambiae sensu stricto (s.s.), while the important member of the An. funestus group was An. funestus s.s.

CONCLUSION

The contribution of different vector species in malaria transmission has changed over the past 20 years. As the role of An. gambiae has declined, An. funestus now appears to be dominant in most settings in east and southern Africa. Other secondary vector species may play minor roles in specific localities. To improve malaria control in the region, vector control should be optimized to match these entomological trends, considering the different ecologies and behaviors of the dominant vector species.

Geospatial based model for malaria risk prediction in Kilombero valley, South-eastern, Tanzania

BACKGROUND

Malaria continues to pose a major public health challenge in tropical regions. Despite significant efforts to control malaria in Tanzania, there are still residual transmission cases. Unfortunately, little is known about where these residual malaria transmission cases occur and how they spread. In Tanzania for example, the transmission is heterogeneously distributed. In order to effectively control and prevent the spread of malaria, it is essential to understand the spatial distribution and transmission patterns of the disease. This study seeks to predict areas that are at high risk of malaria transmission so that intervention measures can be developed to accelerate malaria elimination efforts.

METHODS

This study employs a geospatial based model to predict and map out malaria risk area in Kilombero Valley. Environmental factors related to malaria transmission were considered and assigned valuable weights in the Analytic Hierarchy Process (AHP), an online system using a pairwise comparison technique. The malaria hazard map was generated by a weighted overlay of the altitude, slope, curvature, aspect, rainfall distribution, and distance to streams in Geographic Information Systems (GIS). Finally, the risk map was created by overlaying components of malaria risk including hazards, elements at risk, and vulnerability.

RESULTS

The study demonstrates that the majority of the study area falls under moderate risk level (61%), followed by the low risk level (31%), while the high malaria risk area covers a small area, which occupies only 8% of the total area.

CONCLUSION

The findings of this study are crucial for developing spatially targeted interventions against malaria transmission in residual transmission settings. Predicted areas prone to malaria risk provide information that will inform decision-makers and policymakers for proper planning, monitoring, and deployment of interventions.

Life expectancy of Anopheles funestus is double that of Anopheles arabiensis in southeast Tanzania based on mark-release-recapture method

Anopheles arabiensis and Anopheles funestus sensu stricto mosquitoes are major East African malaria vectors. Understanding their dispersal and population structure is critical for developing effective malaria control tools. Three mark-release-recapture (MRR) experiments were conducted for 51 nights to assess daily survival and flight range of An. arabiensis and An. funestus mosquitoes in south-eastern, Tanzania. Mosquitoes were marked with a fluorescent dye as they emerged from breeding sites via a self-marking device. Mosquitoes were collected indoors and outdoors using human landing catches (HLC) and Centers for Disease Control and Prevention light traps (CDC-LT). In total, 4210 An. arabiensis and An. funestus were collected with 316 (7.5%) marked and recaptured (MR). Daily mean MR was 6.8, standard deviation (SD ± 7.6) for An. arabiensis and 8.9 (SD ± 8.3) for An. funestus. Probability of daily survival was 0.76 for An. arabiensis and 0.86 for An. funestus translating into average life expectancy of 3.6 days for An. arabiensis and 6.5 days for An. funestus. Dispersal distance was 654 m for An. arabiensis and 510 m for An. funestus. An. funestus life expectancy was substantially longer than that of An. arabiensis. The MRR method described here could be routinely utilized when evaluating the impact of new vector control tools on mosquito survival.

Sterilized Anopheles funestus can autodisseminate sufficient pyriproxyfen to the breeding habitat under semi-field settings

BACKGROUND

Anopheles funestus, the main malaria vector, prefer to oviposit in permanent and/or semi-permanent breeding habitats located far from human dwellings. Difficulties in identifying and accessing these habitats jeopardize the feasibility of conventional larviciding. In this way, a semi-field study was conducted to assess the potential of autodissemination of pyriproxyfen (PPF) by An. funestus for its control.

METHODS

The study was conducted inside a semi-field system (SFS). Therein, two identical separate chambers, the treatment chamber with a PPF-treated clay pot (0.25 g AI), and the control chamber with an untreated clay pot. In both chambers, one artificial breeding habitat made of a plastic basin with one litre of water was provided. Three hundred blood-fed female An. funestus aged 5-9 days were held inside untreated and treated clay pots for 30 min and 48 h before being released for oviposition. The impact of PPF on adult emergence, fecundity, and fertility through autodissemination and sterilization effects were assessed by comparing the treatment with its appropriate control group.

RESULTS

Mean (95% CI) percentage of adult emergence was 15.5% (14.9-16.1%) and 70.3% (69-71%) in the PPF and control chamber for females exposed for 30 min (p < 0.001); and 19% (12-28%) and 95% (88-98%) in the PPF and control chamber for females exposed for 48 h (p < 0.001) respectively. Eggs laid by exposed mosquitoes and their hatch rate were significantly reduced compared to unexposed mosquitoes (p < 0.001). Approximately, 90% of females exposed for 48 h retained abnormal ovarian follicles and only 42% in females exposed for 30 min.

CONCLUSION

The study demonstrated sterilization and adult emergence inhibition via autodissemination of PPF by An. funestus. Also, it offers proof that sterilized An. funestus can transfer PPF to prevent adult emergence at breeding habitats. These findings warrant further assessment of the autodissemination of PPF in controlling wild population of An. funestus, and highlights its potential for complementing long-lasting insecticidal nets.

Efficacy of the spatial repellent product Mosquito Shield™ against wild pyrethroid-resistant Anopheles arabiensis in south-eastern Tanzania

BACKGROUND

Spatial repellents that create airborne concentrations of an active ingredient (AI) within a space offer a scalable solution to further reduce transmission of malaria, by disrupting mosquito behaviours in ways that ultimately lead to reduced human-vector contact. Passive emanator spatial repellents can protect multiple people within the treated space and can last for multiple weeks without the need for daily user touchpoints, making them less intrusive interventions. They may be particularly advantageous in certain use cases where implementation of core tools may be constrained, such as in humanitarian emergencies and among mobile at-risk populations. The purpose of this study was to assess the efficacy of Mosquito Shield™ deployed in experimental huts against wild, free-flying, pyrethroid-resistant Anopheles arabiensis mosquitoes in Tanzania over 1 month.

METHODS

The efficacy of Mosquito Shield™ transfluthrin spatial repellent in reducing mosquito lands and blood-feeding was evaluated using 24 huts: sixteen huts were allocated to Human Landing Catch (HLC) collections and eight huts to estimating blood-feeding. In both experiments, half of the huts received no intervention (control) while the remaining received the intervention randomly allocated to huts and remained fixed for the study duration. Outcomes measured were mosquito landings, blood-fed, resting and dead mosquitoes. Data were analysed by multilevel mixed effects regression with appropriate dispersion and link function accounting for volunteer, hut and day.

RESULTS

Landing inhibition was estimated to be 70% (57-78%) [IRR 0.30 (95% CI 0.22-0.43); p < 0.0001] and blood-feeding inhibition was estimated to be 69% (56-79%) [IRR 0.31 (95% CI 0.21-0.44; p < 0.0001] There was no difference in the protective efficacy estimates of landing and blood-feeding inhibition [IRR 0.98 (95% CI 0.53-1.82; p = 0.958].

CONCLUSIONS

This study demonstrated that Mosquito Shield™ was efficacious against a wild pyrethroid-resistant strain of An. arabiensis mosquitoes in Tanzania for up to 1 month and could be used as a complementary or stand-alone tool where gaps in protection offered by core malaria vector control tools exist. HLC is a suitable technique for estimating bite reductions conferred by spatial repellents especially where direct blood-feeding measurements are not practical or are ethically limited.

Common predators and factors influencing their abundance in Anopheles funestus aquatic habitats in rural south-eastern Tanzania

BACKGROUND

The role of larval predators in regulating the Anopheles funestus population in various malaria-endemic countries remains relatively unknown. This study aimed to investigate the common predators that co-exist with Anopheles funestus group larvae and evaluate factors that influence their abundance in rural south-eastern Tanzania.

METHODS

Mosquito larvae and predators were sampled concurrently using standard dipper (350 ml) or 10 L bucket in previously identified aquatic habitats in selected villages in southern Tanzania. Predators and mosquito larvae were identified using standard identification keys. All positive habitats were geo-located and their physical features characterized. Water physicochemical parameters such as dissolved oxygen (DO), pH, electrical conductivity (EC), total dissolved solids (TDS) and temperature were also recorded.

RESULTS

A total of 85 previously identified An. funestus aquatic habitats in nine villages were sampled for larvae and potential predators. A total of 8,295 predators were sampled. Of these Coenagrionidae 57.7% (n = 4785), Corixidae 12.8% (n = 1,060), Notonectidae 9.9% (n = 822), Aeshnidae 4.9% (n = 405), Amphibian 4.5% (n = 370), Dytiscidae 3.8% (n = 313) were common. A total of 5,260 mosquito larvae were sampled, whereby Anopheles funestus group were 60.3% (n = 3,170), Culex spp. 24.3% (n = 1,279), An. gambie s.l. 8.3% (n = 438) and other anophelines 7.1% (n = 373). Permanent and aquatic habitats larger than 100m2 were positively associated with An. funestus group larvae (P<0.05) and predator abundance (P<0.05). Habitats with submerged vegetation were negatively associated with An. funestus group larvae (P<0.05). Only dissolved oxygen (DO) was positively and significantly affect the abundance of An. funestus group larvae (P<0.05). While predators' abundance was not impacted by all physicochemical parameters.

CONCLUSION

Six potential predator families were common in aquatic habitats of An. funestus group larvae. Additional studies are needed to demonstrate the efficacy of different predators on larval density and adult fitness traits. Interventions leveraging the interaction between mosquitoes and predators can be established to disrupt the transmission potential and survival of the An. funestus mosquitoes.

What incentives encourage local communities to collect and upload mosquito sound data by using smartphones? A mixed methods study in Tanzania

BACKGROUND

To detect and identify mosquitoes using their characteristic high-pitched sound, we have developed a smartphone application, known as the 'HumBug sensor', that records the acoustic signature of this sound, along with the time and location. This data is then sent remotely to a server where algorithms identify the species according to their distinctive acoustic signature. Whilst this system works well, a key question that remains is what mechanisms will lead to effective uptake and use of this mosquito survey tool? We addressed this question by working with local communities in rural Tanzania and providing three alternative incentives: money only, short message service (SMS) reminders and money, and SMS reminders only. We also had a control group with no incentive.

METHODS

A multi-site, quantitative empirical study was conducted in four villages in Tanzania from April to August 2021. Consenting participants (n = 148) were recruited and placed into one of the three intervention arms: monetary incentives only; SMS reminders with monetary incentives; and SMS reminders only. There was also a control group (no intervention). To test effectiveness of the mechanisms, the number of audio uploads to the server of the four trial groups on their specific dates were compared. Qualitative focus group discussions and feedback surveys were also conducted to explore participants' perspectives on their participation in the study and to capture their experiences of using the HumBug sensor.

RESULTS

Qualitative data analysis revealed that for many participants (37 out of 81), the main motivation expressed was to learn more about the types of mosquitoes present in their houses. Results from the quantitative empirical study indicate that the participants in the 'control' group switched on their HumBug sensors more over the 14-week period (8 out of 14 weeks) when compared to those belonging to the 'SMS reminders and monetary incentives' trial group. These findings are statistically significant (p < 0.05 or p > 0.95 under a two-sided z-test), revealing that the provision of monetary incentives and sending SMS reminders did not appear to encourage greater number of audio uploads when compared to the control.

CONCLUSIONS

Knowledge on the presence of harmful mosquitoes was the strongest motive for local communities to collect and upload mosquito sound data via the HumBug sensor in rural Tanzania. This finding suggests that most efforts should be made to improve flow of real-time information back to the communities on types and risks associated with mosquitoes present in their houses.

Human landing catches provide a useful measure of protective efficacy for the evaluation of volatile pyrethroid spatial repellents

BACKGROUND

The human landing catch (HLC) method, in which human volunteers collect mosquitoes that land on them before they can bite, is used to quantify human exposure to mosquito vectors of disease. Comparing HLCs in the presence and absence of interventions such as repellents is often used to measure protective efficacy (PE). Some repellents have multiple actions, including feeding inhibition, whereby mosquitoes may be unable to bite even if they land on a host. A comparison was made between the PE of the volatile pyrethroid spatial repellent (VPSR) transfluthrin determined using a landing method (HLC) and a biting method (allowing the mosquitoes that landed to blood-feed) to evaluate whether HLC is a suitable method for the estimation of the personal PE of a VPSR.

METHODS

A fully balanced, two-arm crossover design study was conducted using a 6 × 6 × 2-m netted cage within a semi-field system. Hessian strips (4 m × 0.1 m) treated with a 5-, 10-, 15-, or 20-g dose of transfluthrin were evaluated against a paired negative control for three strains of laboratory-reared Anopheles and Aedes aegypti mosquitoes. Six replicates were performed per dose using either the landing or the biting method. The number of recaptured mosquitoes was analysed by negative binomial regression, and the PEs calculated using the two methods were compared by Bland-Altman plots.

RESULTS

For Anopheles, fewer mosquitoes blood-fed in the biting arm than landed in the landing arm (incidence rate ratio = 0.87, 95% confidence interval 0.81-0.93, P < 0.001). For Ae. aegypti, biting was overestimated by around 37% with the landing method (incidence rate ratio = 0.63, 95% confidence interval 0.57-0.70, P = 0.001). However, the PEs calculated for each method were in close agreement when tested by the Bland Altman plot.

CONCLUSIONS

The HLC method led to underestimation of mosquito feeding inhibition as a mode of action of transfluthrin, and there were species- and dose-dependent differences in the relationship between landing and biting. However, the estimated PEs were similar between the two methods. The results of this study indicate that HLC can be used as a proxy for personal PE for the evaluation of a VPSR, especially when the difficulties associated with enumerating blood-fed mosquitoes in a field setting are taken into consideration.

Species Composition, Seasonal Abundance, and Biting Behavior of Malaria Vectors in Rural Conhane Village, Southern Mozambique

Malaria vector surveillance provides important data to inform the effective planning of vector control interventions at a local level. The aim of this study was to determine the species diversity and abundance, biting activity, and Plasmodium infectivity of Anopheles mosquitoes from a rural village in southern Mozambique. Human landing catches were performed monthly between December 2020 and August 2021. All collected Anopheles were identified to the species level and tested for the presence of malaria parasites. Eight Anopheles species were identified among the 1802 collected anophelines. Anopheles gambiae sensu lato (s.l.) were the most abundant (51.9%) and were represented by Anopheles quadriannulatus and Anopheles arabiensis. Anopheles funestus s.l. represented 4.5%. The biting activity of An. arabiensis was more pronounced early in the evening and outdoors, whereas that of An. funestus sensu stricto (s.s.) was more intense late in the night, with no significant differences in location. One An. funestus s.s. and one An. arabiensis, both collected outdoors, were infected with Plasmodium falciparum. The overall entomologic inoculation rate was estimated at 0.015 infective bites per person per night. The significant outdoor and early evening biting activity of An. arabiensis and An. funestus found in this village may negatively impact the effectiveness of current vector control interventions. Additional vector control tools that can target these mosquitoes are needed.

The ability of Anopheles funestus and A. arabiensis to penetrate LLINs and its effect on their mortality

Background: Variation in mosquito body size and the ability to penetrate long-lasting insecticide-treated nets (LLINs) remains unknown. This study evaluated the ability of Anopheles funestus and A. arabiensis to penetrate commercially available treated and untreated bednets and how this behaviour affects mosquito mortality. Methods: Three types of LLINs; DawaPlus 2.0, PermaNet 2.0, Olyset 2.0, and untreated (Safi Net) were tested inside a semi-field system. One hundred 3–5-day-old female A. funestus and A. arabiensis were released in a chamber with a sleeping adult volunteer under a treated or untreated bednet. Mosquitoes that penetrated inside the nets were collected every two hours using a mouth aspirator. Live mosquitoes were put in paper cups, fed on glucose ad libitum and their mortality rate was monitored for 48 h. Results: The ability of A. funestus to penetrate treated and untreated bednets was significantly higher than for A. arabiensis for all three LLIN net types (P<0.001). For both species the penetration rate was higher for untreated bednets than treated ones except for the Olyset net. Regardless of the assessed mosquito species, all the mosquitoes that penetrated the net, successfully blood-fed on the sleeping volunteer. Compared to A. arabiensis, significant mortality was recorded for A. funestus that were caught inside Olyset nets within 48 hrs of monitoring (P<0.001). Conclusions: These findings demonstrate the ability of A. funestus and A. arabiensis mosquitoes to penetrate the human-occupied treated and untreated bednets. Despite this ability, mosquitoes that penetrated the bednet succumbed to death within two days.

Small-scale field evaluation of transfluthrin-treated eave ribbons and sandals for the control of malaria vectors in rural Tanzania

BACKGROUND

Early-evening and outdoor-biting mosquitoes may compromise the effectiveness of frontline malaria interventions, notably insecticide-treated nets (ITNs). This study aimed to evaluate the efficacy of low-cost insecticide-treated eave ribbons and sandals as supplementary interventions against indoor-biting and outdoor-biting mosquitoes in south-eastern Tanzania, where ITNs are already widely used.

METHODS

This study was conducted in three villages, with 72 households participating (24 households per village). The households were divided into four study arms and assigned: transfluthrin-treated sandals (TS), transfluthrin-treated eave ribbons (TER), a combination of TER and TS, or experimental controls. Each arm had 18 households, and all households received new ITNs. Mosquitoes were collected using double net traps (to assess outdoor biting), CDC light traps (to assess indoor biting), and Prokopack aspirators (to assess indoor resting). Protection provided by the interventions was evaluated by comparing mosquito densities between the treatment and control arms. Additional tests were done in experimental huts to assess the mortality of wild mosquitoes exposed to the treatments or controls.

RESULTS

TERs reduced indoor-biting, indoor-resting and outdoor-biting Anopheles arabiensis by 60%, 73% and 41%, respectively, while TS reduced the densities by 18%, 40% and 42%, respectively. When used together, TER & TS reduced indoor-biting, indoor-resting and outdoor-biting An. arabiensis by 53%, 67% and 57%, respectively. Protection against Anopheles funestus ranged from 42 to 69% with TER and from 57 to 74% with TER & TS combined. Mortality of field-collected mosquitoes exposed to TER, TS or both interventions was 56-78% for An. arabiensis and 47-74% for An. funestus.

CONCLUSION

Transfluthrin-treated eave ribbons and sandals or their combination can offer significant household-level protection against malaria vectors. Their efficacy is magnified by the transfluthrin-induced mortality, which was observed despite the prevailing pyrethroid resistance in the study area. These results suggest that TER and TS could be useful supplementary tools against residual malaria transmission in areas where ITN coverage is high but additional protection is needed against early-evening and outdoor-biting mosquitoes. Further research is needed to validate the performance of these tools in different settings, and assess their long-term effectiveness and feasibility for malaria control.

Habitat Diversity, Stability, and Productivity of Malaria Vectors in Irrigated and Nonirrigated Ecosystems in Western Kenya

Several sub-Saharan African countries rely on irrigation for food production. This study examined the impact of environmental modifications resulting from irrigation on the ecology of aquatic stages of malaria vectors in a semi-arid region of western Kenya. Mosquito larvae were collected from irrigated and non-irrigated ecosystems during seasonal cross-sectional and monthly longitudinal studies to assess habitat availability, stability, and productivity of anophelines in temporary, semipermanent, and permanent habitats during the dry and wet seasons. The duration of habitat stability was also compared between selected habitats. Emergence traps were used to determine the daily production of female adult mosquitoes from different habitat types. Malaria vectors were morphologically identified and sibling species subjected to molecular analysis. Data was statistically compared between the two ecosystems. After aggregating the data, the overall malaria vector productivity for habitats in the two ecosystems was estimated. Immatures of the malaria vector (Anopheles arabiensis) Patton (Diptera: Culicidae) comprised 98.3% of the Anopheles in both the irrigated and non-irrigated habitats. The irrigated ecosystem had the most habitats, higher larval densities, and produced 85.8% of emerged adult females. These results showed that irrigation provided conditions that increased habitat availability, stability, and diversity, consequently increasing the An. arabiensis production and potential risk of malaria transmission throughout the year. The irrigated ecosystems increased the number of habitats suitable for Anopheles breeding by about 3-fold compared to non-irrigated ecosystems. These results suggest that water management in the irrigation systems of western Kenya would serve as an effective method for malaria vector control.

The Effect of Plasmodium falciparum (Welch) (Haemospororida: Plasmodiidae) Infection on the Susceptibility of Anopheles gambiae s.l. and Anopheles funestus (Diptera: Culicidae) to Pyrethroid Insecticides in the North-Western and South-Eastern, Tanzania

The rapid development of insecticide resistance in malaria vectors threatens insecticide-based interventions. It is hypothesized that infection of insecticide-resistant vectors with Plasmodium parasites increases their vulnerability to insecticides, thus assuring the effectiveness of insecticide-based strategies for malaria control. Nonetheless, there is limited field data to support this. We investigated the effect of the Plasmodium falciparum infection on the susceptibility of Anopheles gambiae s.l. and Anopheles funestus to pyrethroids in south-eastern (Kilombero) and north-western (Muleba), Tanzania. The wild-collected mosquitoes were tested against 0.05% deltamethrin and 0.75% permethrin, then assessed for sporozoite rate and resistant gene (kdr) mutations. All Anopheles gambiae s.l. from Kilombero were An. arabiensis (Patton, 1905) while those from Muleba were 87% An. gambiae s.s (Giles, 1902) and 13% An. Arabiensis. High levels of pyrethroid resistance were observed in both areas studied. The kdr mutation was only detected in An. gambiae s.s. at the frequency of 100% in survivors and 97% in dead mosquitoes. The P. falciparum sporozoite rates were slightly higher in susceptible than in resistant mosquitoes. In Muleba, sporozoite rates in An. gambiae s.l. were 8.1% and 6.4% in dead mosquitoes and survivors, respectively (SRR = 1.28, p = 0.19). The sporozoite rates in Kilombero were 1.3% and 0.7% in the dead and survived mosquitoes, respectively (sporozoite rate ratio (SRR) = 1.9, p = 0.33). In An. funestus group sporozoite rates were 6.2% and 4.4% in dead and survived mosquitoes, respectively (SRR = 1.4, p = 0.54). These findings indicate that insecticides might still be effective in malaria control despite the rapid development of insecticide resistance in malaria vectors.

Seasonal variation in abundance and blood meal sources of primary and secondary malaria vectors within Kilombero Valley, Southern Tanzania

BACKGROUND

Existing control tools have significantly reduced malaria over the past two decades. However, progress has been stalled due to increased resistance in primary vectors and the increasing role of secondary vectors. This study aimed to investigate the impact of seasonal change on primary and secondary vector abundance and host preference. Understanding the impact of seasonal dynamics of primary and secondary vectors on disease transmission will inform effective strategies for vector management and control.

METHODS

Vector abundance was measured through longitudinal collection of mosquitoes, conducted monthly during the wet and dry seasons, in Sagamaganga, a village in the Kilombero Valley, Tanzania. Mosquitoes were collected indoors using CDC light traps and backpack aspirators, and outdoors using resting buckets baited with cattle urine. In addition, a direct measure of host preference was taken monthly using human- and cattle-baited mosquito electrocuting traps. A host census was conducted to provide an indirect measure of host preference together with monthly blood meal source analysis. All collected mosquitoes were assayed for Plasmodium sporozoites.

RESULTS

A total of 2828 anophelines were collected, of which 78.5% and 21.4%, were primary and secondary vectors, respectively. The abundance of the primary vectors, Anopheles arabiensis and Anopheles funestus, and of the secondary vectors varied seasonally. Indirect measures of host preference indicated that all vectors varied blood meal choice seasonally, with the direct measure confirming this for An. arabiensis. All anopheline mosquitoes tested negative for sporozoites.

CONCLUSIONS

At the study location, the abundance of both primary and secondary vectors changed seasonally. Indirect and direct measures of host preference demonstrated that An. arabiensis varied from being zoophilic to being more opportunistic during the wet and dry seasons. A similar trend was observed for the other vectors.

Participatory development of practical, affordable, insecticide-treated mosquito proofing for a range of housing designs in rural southern Tanzania

Background

Insecticidal mosquito-proof netting screens could combine the best features of insecticide-treated nets (ITNs) and indoor residual spraying (IRS), the two most important front line vector control interventions in Africa today, and also overcome the most important limitations of these methods. This study engaged members of a rural Tanzanian community in developing and evaluating simple, affordable and scalable procedures for installing readily available screening materials on eave gaps and windows of their own houses, and then treating those screens with a widely used IRS formulation of the organophosphate insecticide pirimiphos-methyl (PM).

Methods

A cohort of 54 households recruited upon consent, following which the structural features and occupant demographics of their houses were surveyed. Indoor mosquito densities were surveyed longitudinally, for approximately 3 months before and over 5 months after participatory house modification and screening using locally available materials. Each house was randomly assigned to one of three study arms: (1) No screens installed until the end of the study (negative control), (2) untreated screens installed, and (3) screened installed and then treated with PM, the insecticidal activity of which was subsequently assessed using standard cone assays.

Results

Almost all (52) recruited households participated until the end, at which point all houses had been successfully screened. In most cases, screening was only installed after making enabling structural modifications that were accepted by the enrolled households. Compared to unscreened houses, houses with either treated or untreated screens both almost entirely excluded Anopheles arabiensis (Relative reduction (RR) ≥ 98%, P < < 0.0001), the most abundant local malaria vector. However, screens were far less effective against Culex quinquefasciatus (RR ≤ 46%, P < < 0.0001), a non-malaria vector causing considerable biting nuisance, regardless of their treatment status. While PM did not augment household level protection by screens against either mosquito species (P = 0.676 and 0.831, respectively), 8 months after treatment it still caused 73% and 89% mortality among susceptible insectary-reared Anopheles gambiae following exposures of 3 and 30 min, respectively.

Conclusions

Participatory approaches to mosquito proofing houses may be acceptable and effective, and installed screens may be suitable targets for residual insecticide treatments.

The ability of Anopheles funestus and A. arabiensis to penetrate LLINs and its effect on their mortality

Background: Variation in mosquito body size and the ability to penetrate long-lasting insecticide-treated nets (LLINs) remains unknown. This study evaluated the ability of Anopheles funestus and A. arabiensis to penetrate commercially available treated and untreated bednets and how this behaviour affects mosquito mortality. Methods: Three types of LLINs; DawaPlus 2.0, PermaNet 2.0, Olyset 2.0, and untreated (Safi Net) were tested inside a semi-field system. One hundred 3–5-day-old female A. funestus and A. arabiensis were released in a chamber with a sleeping adult volunteer under a treated or untreated bednet. Mosquitoes that penetrated inside the nets were collected every two hours using a mouth aspirator. Live mosquitoes were put in paper cups, fed on glucose ad libitum and their mortality rate was monitored for 48 h. Results: The ability of A. funestus to penetrate treated and untreated bednets was significantly higher than for A. arabiensis for all three LLIN net types (P<0.001). For both species the penetration rate was higher for untreated bednets than treated ones except for the Olyset net. Of all mosquitoes that penetrated the net, regardless of the species, >90% successfully blood-fed on the sleeping volunteer. Compared to A. arabiensis, significant mortality was recorded for A. funestus that were caught inside Olyset nets within 48 hrs of monitoring (P<0.001). Conclusions: These findings demonstrate the ability of A. funestus and A. arabiensis mosquitoes to penetrate the human-occupied treated and untreated bednets. Despite this ability, mosquitoes that penetrated the bednet succumbed to death within two days.

A statistical calibration tool for methods used to sample outdoor-biting mosquitoes

Background

Improved methods for sampling outdoor-biting mosquitoes are urgently needed to improve surveillance of vector-borne diseases. Such tools could potentially replace the human landing catch (HLC), which, despite being the most direct option for measuring human exposures, raises significant ethical and logistical concerns. Several alternatives are under development, but detailed evaluation still requires common frameworks for calibration relative to HLC. The aim of this study was to develop and validate a statistical framework for predicting human-biting rates from different exposure-free alternatives.

Methods

We obtained mosquito abundance data (Anopheles arabiensis, Anopheles funestus and Culex spp.) from a year-long Tanzanian study comparing six outdoor traps [Suna Trap (SUN), BG Sentinel (BGS), M-Trap (MTR), M-Trap + CDC (MTRC), Ifakara Tent Trap-C (ITT-C) and Mosquito Magnet-X Trap (MMX)] and HLC. Generalised linear models were developed within a Bayesian framework to investigate associations between the traps and HLC, taking intra- and inter-specific density dependence into account. The best model was used to create a calibration tool for predicting HLC-equivalents.

Results

For An. arabiensis, SUN catches had the strongest correlation with HLC (R² = 19.4), followed by BGS (R² = 17.2) and MTRC (R² = 13.1) catches. The least correlated catch was MMX (R² = 2.5). For An. funestus, BGS had the strongest correlation with the HLC (R² = 53.4), followed by MTRC (R² = 37.4) and MTR (R² = 37.4). For Culex mosquitoes, the traps most highly correlated with the HLC were MTR (R² = 45.4) and MTRC (R² = 44.2). Density dependence, both between and within species, influenced the performance of only BGS traps. An interactive Shiny App calibration tool was developed for this and similar applications.

Conclusion

We successfully developed a calibration tool to assess the performance of different traps for assessing outdoor-biting risk, and established a valuable framework for estimating human exposures based on the trap catches. The performance of candidate traps varied between mosquito taxa; thus, there was no single optimum. Although all the traps tested underestimated the HLC-derived exposures, it was possible to mathematically define their representativeness of the true biting risk, with or without density dependence. The results of this study emphasise the need to aim for a consistent and representative sampling approach, as opposed to simply seeking traps that catch the most mosquitoes.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05403-7.

Persistently high proportions of plasmodium-infected Anopheles funestus mosquitoes in two villages in the Kilombero valley, South-Eastern Tanzania

Background

In south-eastern Tanzania where insecticide-treated nets have been widely used for >20 years, malaria transmission has greatly reduced but remains highly heterogenous over small distances. This study investigated the seasonal prevalence of Plasmodium sporozoite infections in the two main malaria vector species, Anopheles funestus and Anopheles arabiensis for 34 months, starting January 2018 to November 2020.

Methods

Adult mosquitoes were collected using CDC-light traps and Prokopack aspirators inside local houses in Igumbiro and Sululu villages, where earlier surveys had found very high densities of An. funestus. Collected females were sorted by taxa, and the samples examined using ELISA assays for detecting Plasmodium circumsporozoite protein in their salivary glands.

Results

Of 7859 An. funestus tested, 4.6% (n = 365) were positive for Pf sporozoites in the salivary glands. On the contrary, only 0.4% (n = 9) of the 2382 An. arabiensis tested were positive. The sporozoite prevalence did not vary significantly between the villages or seasons. Similarly, the proportions of parous females of either species were not significantly different between the two villages (p > 0.05) but was slightly higher in An. funestus (0.50) than in An. arabiensis (0.42). Analysis of the 2020 data determined that An. funestus contributed 97.7% of all malaria transmitted in households in these two villages.

Conclusions

In contexts where individual vector species mediate most of the pathogen transmission, it may be most appropriate to pursue a species-focused approach to better understand the ecology of the dominant vectors and target them with effective interventions to suppress transmission. Despite the ongoing efforts on tackling malaria in the two study villages, there is still persistently high Plasmodium infection prevalence in local populations of An. funestus, which now carry ~97% of all malaria infections and mediates intense year-round transmission. Further reduction in malaria burden in these or other similar settings requires effective targeting of An. funestus.

The Centres for Disease Control light trap (CDC-LT) and the human decoy trap (HDT) compared to the human landing catch (HLC) for measuring Anopheles biting in rural Tanzania

Background

Vector mosquito biting intensity is an important measure to understand malaria transmission. Human landing catch (HLC) is an effective but labour-intensive, expensive, and potentially hazardous entomological surveillance tool. The Centres for Disease Control light trap (CDC-LT) and the human decoy trap (HDT) are exposure-free alternatives. This study compared the CDC-LT and HDT against HLC for measuring Anopheles biting in rural Tanzania and assessed their suitability as HLC proxies.

Methods

Indoor mosquito surveys using HLC and CDC-LT and outdoor surveys using HLC and HDT were conducted in 2017 and in 2019 in Ulanga, Tanzania in 19 villages, with one trap/house/night. Species composition, sporozoite rates and density/trap/night were compared. Aggregating the data by village and month, the Bland–Altman approach was used to assess agreement between trap types.

Results

Overall, 66,807 Anopheles funestus and 14,606 Anopheles arabiensis adult females were caught with 6,013 CDC-LT, 339 indoor-HLC, 136 HDT and 195 outdoor-HLC collections. Indoors, CDC-LT caught fewer An. arabiensis (Adjusted rate ratio [Adj.RR] = 0.35, 95% confidence interval [CI]: 0.27–0.46, p < 0.001) and An. funestus (Adj.RR = 0.63, 95%CI: 0.51–0.79, p < 0.001) than HLC per trap/night. Outdoors, HDT caught fewer An. arabiensis (Adj.RR = 0.04, 95%CI: 0.01–0.14, p < 0.001) and An. funestus (Adj.RR = 0.10, 95%CI: 0.07–0.15, p < 0.001) than HLC. The bias and variability in number of mosquitoes caught by the different traps were dependent on mosquito densities. The relative efficacies of both CDC-LT and HDT in comparison to HLC declined with increased mosquito abundance. The variability in the ratios was substantial for low HLC counts and decreased as mosquito abundance increased. The numbers of sporozoite positive mosquitoes were low for all traps.

Conclusions

CDC-LT can be suitable for comparing mosquito populations between study arms or over time if accuracy in the absolute biting rate, compared to HLC, is not required. CDC-LT is useful for estimating sporozoite rates because large numbers of traps can be deployed to collect adequate mosquito samples. The present design of the HDT is not amenable for use in large-scale entomological surveys. Use of HLC remains important for estimating human exposure to mosquitoes as part of estimating the entomological inoculation rate (EIR).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04192-9.

Outdoor biting and pyrethroid resistance as potential drivers of persistent malaria transmission in Zanzibar

BACKGROUND

Low-level of malaria transmission persist in Zanzibar despite high coverage of core vector control interventions. This study was carried out in hot-spot sites to better understand entomological factors that may contribute to residual malaria transmission in Zanzibar.

METHODS

A total of 135 households were randomly selected from six sites and consented to participate with 20-25 households per site. Mosquito vector surveillance was carried out indoors and outdoors from 6:00 pm-7:00 am using miniaturized double net trap (DN-Mini™). Additional collections were done indoors using mouth aspirators to retrieve resting mosquitoes from wall and ceiling surfaces, and outdoors using resting bucket and pit traps. All collected mosquitoes were morphologically and genetically (PCR) analysed in the laboratory. All collected anopheline and blood-fed mosquitoes were analysed for sporozoite infection and blood meal host preferences by Circumsporozoite Protein ELISA and blood meal ELISA, respectively. The differences between indoor and outdoor mosquito biting rates were analysed using generalized linear mixed models. Levels of resistance to commonly used insecticides were quantified by WHO susceptibility tests.

RESULTS

Out of 704 malaria vectors collected across 135 households, PCR analysis shows that 98.60% were Anopheles arabiensis, 0.6% Anopheles merus and 0.6% Anopheles gambiae sensu stricto. Sporozoite ELISA analysis indicates that all mosquitoes were negative for the malaria parasite. The results show that more An. arabiensis were collected outdoor (~ 85%) compared to indoor (~ 15%). Furthermore, large numbers of An. arabiensis were caught in outdoor resting sites, where the pit trap (67.2%) collected more mosquitoes compared to the outdoor DN-Mini trap (32.8%). Nearly two-thirds (60.7%) of blood-fed mosquitoes had obtained blood meals from non-human hosts. Mosquitoes displayed non-uniform susceptibility status and resistance intensity among the tested insecticides across the study sites to all WHO recommended insecticides across the study sites.

CONCLUSION

This study suggests that in contexts such as Zanzibar, testing of novel techniques to complement indoor protection and targeting outdoor biting and/or resting mosquitoes, may be warranted to complement existing interventions and contribute to malaria elimination efforts. The study highlights the need to implement novel interventions and/or adaptations of strategies that can target outdoors biting mosquitoes.

Using Bayesian state-space models to understand the population dynamics of the dominant malaria vector, Anopheles funestus in rural Tanzania

Background

It is often assumed that the population dynamics of the malaria vector Anopheles funestus, its role in malaria transmission and the way it responds to interventions are similar to the more elaborately characterized Anopheles gambiae. However, An. funestus has several unique ecological features that could generate distinct transmission dynamics and responsiveness to interventions. The objectives of this work were to develop a model which will: (1) reconstruct the population dynamics, survival, and fecundity of wild An. funestus populations in southern Tanzania, (2) quantify impacts of density dependence on the dynamics, and (3) assess seasonal fluctuations in An. funestus demography. Through quantifying the population dynamics of An. funestus, this model will enable analysis of how their stability and response to interventions may differ from that of An. gambiae sensu lato.

Methods

A Bayesian State Space Model (SSM) based on mosquito life history was fit to time series data on the abundance of female An. funestus sensu stricto collected over 2 years in southern Tanzania. Prior values of fitness and demography were incorporated from empirical data on larval development, adult survival and fecundity from laboratory-reared first generation progeny of wild caught An. funestus. The model was structured to allow larval and adult fitness traits to vary seasonally in response to environmental covariates (i.e. temperature and rainfall), and for density dependency in larvae. The effects of density dependence and seasonality were measured through counterfactual examination of model fit with or without these covariates.

Results

The model accurately reconstructed the seasonal population dynamics of An. funestus and generated biologically-plausible values of their survival larval, development and fecundity in the wild. This model suggests that An. funestus survival and fecundity annual pattern was highly variable across the year, but did not show consistent seasonal trends either rainfall or temperature. While the model fit was somewhat improved by inclusion of density dependence, this was a relatively minor effect and suggests that this process is not as important for An. funestus as it is for An. gambiae populations.

Conclusion

The model's ability to accurately reconstruct the dynamics and demography of An. funestus could potentially be useful in simulating the response of these populations to vector control techniques deployed separately or in combination. The observed and simulated dynamics also suggests that An. funestus could be playing a role in year-round malaria transmission, with any apparent seasonality attributed to other vector species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04189-4.

Using ecological observations to improve malaria control in areas where Anopheles funestus is the dominant vector

The most important malaria vectors in sub-Saharan Africa are Anopheles gambiae, Anopheles arabiensis, Anopheles funestus, and Anopheles coluzzii. Of these, An. funestus presently dominates in many settings in east and southern Africa. While research on this vector species has been impeded by difficulties in creating laboratory colonies, available evidence suggests it has certain ecological vulnerabilities that could be strategically exploited to greatly reduce malaria transmission in areas where it dominates. This paper examines the major life-history traits of An. funestus, its aquatic and adult ecologies, and its responsiveness to key interventions. It then outlines a plausible strategy for reducing malaria transmission by the vector and sustaining the gains over the medium to long term. To illustrate the propositions, the article uses data from south-eastern Tanzania where An. funestus mediates over 85% of malaria transmission events and is highly resistant to key public health insecticides, notably pyrethroids. Both male and female An. funestus rest indoors and the females frequently feed on humans indoors, although moderate to high degrees of zoophagy can occur in areas with large livestock populations. There are also a few reports of outdoor-biting by the species, highlighting a broader range of behavioural phenotypes that can be considered when designing new interventions to improve vector control. In comparison to other African malaria vectors, An. funestus distinctively prefers permanent and semi-permanent aquatic habitats, including river streams, ponds, swamps, and spring-fed pools. The species is therefore well-adapted to sustain its populations even during dry months and can support year-round malaria transmission. These ecological features suggest that highly effective control of An. funestus could be achieved primarily through strategic combinations of species-targeted larval source management and high quality insecticide-based methods targeting adult mosquitoes in shelters. If done consistently, such an integrated strategy has the potential to drastically reduce local populations of An. funestus and significantly reduce malaria transmission in areas where this vector species dominates. To sustain the gains, the programmes should be complemented with gradual environmental improvements such as house modification to maintain biting exposure at a bare minimum, as well as continuous engagements of the resident communities and other stakeholders.

The effect of blood feeding on insecticide resistance intensity and adult longevity in the major malaria vector Anopheles funestus (Diptera: Culicidae)

Insecticide-based vector control is key to the reduction and elimination of malaria. Although insecticide resistance is common in malaria vector populations, the operational implications are often unclear. High intensity pyrethroid resistance in the major malaria vector Anopheles funestus has been linked to control failure in Southern Africa. The aim of this study was to assess linkages between mosquito age, blood feeding and the intensity of pyrethroid resistance in two An. funestus laboratory strains that originate from southern Mozambique, namely the moderately pyrethroid resistant FUMOZ and the highly resistant FUMOZ-R. Resistance tended to decline with age. This effect was significantly mitigated by blood feeding and was most apparent in cohorts that received multiple blood meals. In the absence of insecticide exposure, blood feeding tended to increase longevity of An. funestus females and, following insecticide exposure, enhanced their levels of deltamethrin resistance, even in older age groups. These effects were more marked in FUMOZ-R compared to FUMOZ. In terms of programmatic decision-making, these data suggest that it would be useful to assess the level and intensity of resistance in older female cohorts wherever possible, notwithstanding the standard protocols for resistance testing using age-standardised samples.

The effect of light and ventilation on house entry by Anopheles arabiensis sampled using light traps in Tanzania: an experimental hut study

Background

In sub-Saharan Africa, house design and ventilation affects the number of malaria mosquito vectors entering houses. This study hypothesized that indoor light from a CDC-light trap, visible from outside a hut, would increase entry of Anopheles arabiensis, an important malaria vector, and examined whether ventilation modifies this effect.

Methods

Four inhabited experimental huts, each situated within a large chamber, were used to assess how light and ventilation affect the number of hut-entering mosquitoes in Tanzania. Each night, 300 female laboratory-reared An. arabiensis were released inside each chamber for 72 nights. Nightly mosquito collections were made using light traps placed indoors. Temperature and carbon dioxide concentrations were measured using data loggers. Treatments and sleepers were rotated between huts using a randomized block design.

Results

When indoor light was visible outside the huts, there was an 84% increase in the odds of collecting mosquitoes indoors (Odds ratio, OR = 1.84, 95% confidence intervals, 95% CI 1.74–1.95, p < 0.001) compared with when it was not. Although the odds of collecting mosquitoes in huts with closed eaves (OR = 0.54, 95% CI 0.41–0.72, p < 0.001) was less than those with open eaves, few mosquitoes entered either type of well-ventilated hut. The odds of collecting mosquitoes was 99% less in well-ventilated huts, compared with poorly-ventilated traditional huts (OR = 0.01, 95% CI 0.01–0.03, p < 0.001). In well-ventilated huts, indoor temperatures were 1.3 °C (95% CI 0.9–1.7, p < 0.001) cooler, with lower carbon dioxide (CO2) levels (mean difference = 97 ppm, 77.8–116.2, p < 0.001) than in poorly-ventilated huts.

Conclusion

Although light visible from outside a hut increased mosquito house entry, good natural ventilation reduces indoor carbon dioxide concentrations, a major mosquito attractant, thereby reducing mosquito-hut entry.

Temporally consistent predominance and distribution of secondary malaria vectors in the Anopheles community of the upper Zambezi floodplain

Regional optimisation of malaria vector control approaches requires detailed understanding both of the species composition of Anopheles mosquito communities, and how they vary over spatial and temporal scales. Knowledge of vector community dynamics is particularly important in settings where ecohydrological conditions fluctuate seasonally and inter-annually, such as the Barotse floodplain of the upper Zambezi river. DNA barcoding of anopheline larvae sampled in the 2019 wet season revealed the predominance of secondary vector species, with An. coustani comprising > 80% of sampled larvae and distributed ubiquitously across all ecological zones. Extensive larval sampling, plus a smaller survey of adult mosquitoes, identified geographic clusters of primary vectors, but represented only 2% of anopheline larvae. Comparisons with larval surveys in 2017/2018 and a contemporaneous independent 5-year dataset from adult trapping corroborated this paucity of primary vectors across years, and the consistent numerical dominance of An. coustani and other secondary vectors in both dry and wet seasons, despite substantial inter-annual variation in hydrological conditions. This marked temporal consistency of spatial distribution and anopheline community composition presents an opportunity to target predominant secondary vectors outdoors. Larval source management should be considered, alongside prevalent indoor-based approaches, amongst a diversification of vector control approaches to more effectively combat residual malaria transmission.

The ability of Anopheles funestus and A. arabiensis to penetrate LLINs and its effect on their mortality

Background: Variation in mosquito body size and the ability to penetrate long-lasting insecticide-treated nets (LLINs) remains unknown. This study evaluated the ability of Anopheles funestus and A. arabiensis to penetrate commercially available treated and untreated bednets and how this behaviour affects mosquito mortality. Methods: Three types of LLINs; DawaPlus 2.0, PermaNet 2.0, Olyset 2.0, and untreated (Safi Net) were tested inside a semi-field system. One hundred 3-5-day-old female A. funestus and A. arabiensis were released in a chamber with a sleeping adult volunteer under a treated or untreated bednet. Mosquitoes that penetrated inside the nets were collected every two hours using a mouth aspirator. Live mosquitoes were put in paper cups, fed on glucose ad libitum and their mortality rate was monitored for 48 h. Results: The ability of A. funestus to penetrate treated and untreated bednets was significantly higher than for A. arabiensis for all three LLIN net types ( P<0.001). For both species the penetration rate was higher for untreated bednets than treated ones except for the Olyset net. Regardless of the assessed mosquito species, all the mosquitoes that penetrated the net, successfully blood-fed on the sleeping volunteer. Compared to A. arabiensis, significant mortality was recorded for A. funestus that were caught inside Olyset nets within 48 hrs of monitoring ( P<0.001). Conclusions: These findings demonstrate the ability of A. funestus and A. arabiensis mosquitoes to penetrate the human-occupied treated and untreated bednets. Despite this ability, mosquitoes that penetrated the bednet succumbed to death within two days.

Wild populations of malaria vectors can mate both inside and outside human dwellings

BACKGROUND

Wild populations of Anopheles mosquitoes are generally thought to mate outdoors in swarms, although once colonized, they also mate readily inside laboratory cages. This study investigated whether the malaria vectors Anopheles funestus and Anopheles arabiensis can also naturally mate inside human dwellings.

METHOD

Mosquitoes were sampled from three volunteer-occupied experimental huts in a rural Tanzanian village at 6:00 p.m. each evening, after which the huts were completely sealed and sampling was repeated at 11:00 p.m and 6 a.m. the next morning to compare the proportions of inseminated females. Similarly timed collections were done inside local unsealed village houses. Lastly, wild-caught larvae and pupae were introduced inside or outside experimental huts constructed inside two semi-field screened chambers. The huts were then sealed and fitted with exit traps, allowing mosquito egress but not entry. Mating was assessed in subsequent days by sampling and dissecting emergent adults caught indoors, outdoors and in exit traps.

RESULTS

Proportions of inseminated females inside the experimental huts in the village increased from approximately  60% at 6 p.m. to approximately 90% the following morning despite no new mosquitoes entering the huts after 6 p.m. Insemination in the local homes increased from approximately 78% to approximately 93% over the same time points. In the semi-field observations of wild-caught captive mosquitoes, the proportions of inseminated An. funestus were 20.9% (95% confidence interval [CI]: ± 2.8) outdoors, 25.2% (95% CI: ± 3.4) indoors and 16.8% (± 8.3) in exit traps, while the proportions of inseminated An. arabiensis were 42.3% (95% CI: ± 5.5) outdoors, 47.4% (95% CI: ± 4.7) indoors and 37.1% (CI: ± 6.8) in exit traps.

CONCLUSION

Wild populations of An. funestus and An. arabiensis in these study villages can mate both inside and outside human dwellings. Most of the mating clearly happens before the mosquitoes enter houses, but additional mating happens indoors. The ecological significance of such indoor mating remains to be determined. The observed insemination inside the experimental huts fitted with exit traps and in the unsealed village houses suggests that the indoor mating happens voluntarily even under unrestricted egress. These findings may inspire improved vector control, such as by targeting males indoors, and potentially inform alternative methods for colonizing strongly eurygamic Anopheles species (e.g. An. funestus) inside laboratories or semi-field chambers.

Urban malaria in sub-Saharan Africa: dynamic of the vectorial system and the entomological inoculation rate

Sub-Saharan Africa is registering one of the highest urban population growth across the world. It is estimated that over 75% of the population in this region will be living in urban settings by 2050. However, it is not known how this rapid urbanization will affect vector populations and disease transmission. The present study summarizes findings from studies conducted in urban settings between the 1970s and 2020 to assess the effects of urbanization on the entomological inoculation rate pattern and anopheline species distribution. Different online databases such as PubMed, ResearchGate, Google Scholar, Google were screened. A total of 90 publications were selected out of 1527. Besides, over 200 additional publications were consulted to collate information on anopheline breeding habitats and species distribution in urban settings. The study confirms high malaria transmission in rural compared to urban settings. The study also suggests that there had been an increase in malaria transmission in most cities after 2003, which could also be associated with an increase in sampling, resources and reporting. Species of the Anopheles gambiae complex were the predominant vectors in most urban settings. Anopheline larvae were reported to have adapted to different aquatic habitats. The study provides updated information on the distribution of the vector population and the dynamic of malaria transmission in urban settings. The study also highlights the need for implementing integrated control strategies in urban settings.

Transfluthrin eave-positioned targeted insecticide (EPTI) reduces human landing rate (HLR) of pyrethroid resistant and susceptible malaria vectors in a semi-field simulated peridomestic space

Background

Volatile pyrethroids (VPs) are proven to reduce human–vector contact for mosquito vectors. With increasing resistance to pyrethroids in mosquitoes, the efficacy of VPs, such as transfluthrin, may be compromised. Therefore, experiments were conducted to determine if the efficacy of transfluthrin eave-positioned targeted insecticide (EPTI) depends on the resistance status of malaria vectors.

Methods

Ribbons treated with 5.25 g transfluthrin or untreated controls were used around the eaves of an experimental hut as EPTI inside a semi-field system. Mosquito strains with different levels of pyrethroid resistance were released simultaneously, recaptured by means of human landing catches (HLCs) and monitored for 24-h mortality. Technical-grade (TG) transfluthrin was used, followed by emulsifiable concentrate (EC) transfluthrin and additional mosquito strains. Generalized linear mixed models with binomial distribution were used to determine the impact of transfluthrin and mosquito strain on mosquito landing rates and 24-h mortality.

Results

EPTI treated with 5.25 g of either TG or EC transfluthrin significantly reduced HLR of all susceptible and resistant Anopheles mosquitoes (Odds Ratio (OR) ranging from 0.14 (95% Confidence Interval (CI) [0.11–0.17], P < 0.001) to 0.57, (CI [0.42–0.78] P < 0.001). Both TG and EC EPTI had less impact on landing for the resistant Anopheles arabiensis (Mbita strain) compared to the susceptible Anopheles gambiae (Ifakara strain) (OR 1.50 [95% CI 1.18–1.91] P < 0.001) and (OR 1.67 [95% CI 1.29–2.17] P < 0.001), respectively. The EC EPTI also had less impact on the resistant An. arabiensis (Kingani strain) (OR 2.29 [95% CI 1.78–2.94] P < 0.001) compared to the control however the TG EPTI was equally effective against the resistant Kingani strain and susceptible Ifakara strain (OR 1.03 [95% CI 0.82–1.32] P = 0.75). Finally the EC EPTI was equally effective against the susceptible An. gambiae (Kisumu strain) and the resistant An. gambiae (Kisumu-kdr strain) (OR 0.98 [95% CI 0.74–1.30] P = 0.90).

Conclusions

Transfluthrin-treated EPTI could be useful in areas with pyrethroid-resistant mosquitoes, but it remains unclear whether stronger resistance to pyrethroids will undermine the efficacy of transfluthrin. At this dosage, transfluthrin EPTI cannot be used to kill exposed mosquitoes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03880-2.

An increasing role of pyrethroid-resistant Anopheles funestus in malaria transmission in the Lake Zone, Tanzania

Anopheles funestus is playing an increasing role in malaria transmission in parts of sub-Saharan Africa, where An. gambiae s.s. has been effectively controlled by long-lasting insecticidal nets. We investigated vector population bionomics, insecticide resistance and malaria transmission dynamics in 86 study clusters in North-West Tanzania. An. funestus s.l. represented 94.5% (4740/5016) of all vectors and was responsible for the majority of malaria transmission (96.5%), with a sporozoite rate of 3.4% and average monthly entomological inoculation rate (EIR) of 4.57 per house. Micro-geographical heterogeneity in species composition, abundance and transmission was observed across the study district in relation to key ecological differences between northern and southern clusters, with significantly higher densities, proportions and EIR of An. funestus s.l. collected from the South. An. gambiae s.l. (5.5%) density, principally An. arabiensis (81.1%) and An. gambiae s.s. (18.9%), was much lower and closely correlated with seasonal rainfall. Both An. funestus s.l. and An. gambiae s.l. were similarly resistant to alpha-cypermethrin and permethrin. Overexpression of CYP9K1, CYP6P3, CYP6P4 and CYP6M2 and high L1014S-kdr mutation frequency were detected in An. gambiae s.s. populations. Study findings highlight the urgent need for novel vector control tools to tackle persistent malaria transmission in the Lake Region of Tanzania.

Key Characteristics of Residual Malaria Transmission in Two Districts in South-Eastern Tanzania-Implications for Improved Control

After 2 decades of using insecticide-treated nets (ITNs) and improved case management, malaria burden in the historically-holoendemic Kilombero valley in Tanzania has significantly declined. We review key characteristics of the residual transmission and recommend options for improvement. Transmission has declined by >10-fold since 2000 but remains heterogeneous over small distances. Following the crash of Anopheles gambiae, which coincided with ITN scale-up around 2005-2012, Anopheles funestus now dominates malaria transmission. While most infections still occur indoors, substantial biting happens outdoors and before bed-time. There is widespread resistance to pyrethroids and carbamates; An. funestus being particularly strongly-resistant. In short and medium-term, these challenges could be addressed using high-quality indoor residual spraying with nonpyrethroids, or ITNs incorporating synergists. Supplementary tools, eg, spatial-repellents may expand protection outdoors. However, sustainable control requires resilience-building approaches, particularly improved housing and larval-source management to suppress mosquitoes, stronger health systems guaranteeing case-detection and treatment, greater community-engagement and expanded health education.

Efficacy of interceptor® G2, a long-lasting insecticide mixture net treated with chlorfenapyr and alpha-cypermethrin against Anopheles funestus: experimental hut trials in north-eastern Tanzania

Background

The effectiveness of long-lasting insecticidal nets (LLIN), the primary method for preventing malaria in Africa, is compromised by evolution and spread of pyrethroid resistance. Further gains require new insecticides with novel modes of action. Chlorfenapyr is a pyrrole insecticide that disrupts mitochrondrial function and confers no cross-resistance to neurotoxic insecticides. Interceptor® G2 LN (IG2) is an insecticide-mixture LLIN, which combines wash-resistant formulations of chlorfenapyr and the pyrethroid alpha-cypermethrin. The objective was to determine IG2 efficacy under controlled household-like conditions for personal protection and control of wild, pyrethroid-resistant Anopheles funestus mosquitoes.

Methods

Experimental hut trials tested IG2 efficacy against two positive controls—a chlorfenapyr-treated net and a standard alpha-cypermethrin LLIN, Interceptor LN (IG1)—consistent with World Health Organization (WHO) evaluation guidelines. Mosquito mortality, blood-feeding inhibition, personal protection, repellency and insecticide-induced exiting were recorded after zero and 20 washing cycles. The trial was repeated and analysed using multivariate and meta-analysis.

Results

In the two trials held in NE Tanzania, An. funestus mortality was 2.27 (risk ratio 95% CI 1.13–4.56) times greater with unwashed Interceptor G2 than with unwashed Interceptor LN (p = 0.012). There was no significant loss in mortality with IG2 between 0 and 20 washes (1.04, 95% CI 0.83–1.30, p = 0.73). Comparison with chlorfenapyr treated net indicated that most mortality was induced by the chlorfenapyr component of IG2 (0.96, CI 0.74–1.23), while comparison with Interceptor LN indicated blood-feeding was inhibited by the pyrethroid component of IG2 (IG2: 0.70, CI 0.44–1.11 vs IG1: 0.61, CI 0.39–0.97). Both insecticide components contributed to exiting from the huts but the contributions were heterogeneous between trials (heterogeneity Q = 36, P = 0.02). WHO susceptibility tests with pyrethroid papers recorded 44% survival in An. funestus.

Conclusions

The high mortality recorded by IG2 against pyrethroid-resistant An. funestus provides first field evidence of high efficacy against this primary, anthropophilic, malaria vector.

Fitness characteristics of the malaria vector Anopheles funestus during an attempted laboratory colonization

BACKGROUND

The malaria vector Anopheles funestus is increasingly recognized as a dominant vector of residual transmission in many African settings. Efforts to better understand its biology and control are significantly impeded by the difficulties of colonizing it under laboratory conditions. To identify key bottlenecks in colonization, this study compared the development and fitness characteristics of wild An. funestus from Tanzania (FUTAZ) and their F₁ offspring during colonization attempts. The demography and reproductive success of wild FUTAZ offspring were compared to that of individuals from one of the only An. funestus strains that has been successfully colonized (FUMOZ, from Mozambique) under similar laboratory conditions.

METHODS

Wild An. funestus (FUTAZ) were collected from three Tanzanian villages and maintained inside an insectary at 70-85% RH, 25-27 °C and 12 h:12 h photoperiod. Eggs from these females were used to establish three replicate F₁ laboratory generations. Larval development, survival, fecundity, mating success, percentage pupation and wing length were measured in the F₁ -FUTAZ offspring and compared with wild FUTAZ and FUMOZ mosquitoes.

RESULTS

Wild FUTAZ laid fewer eggs (64.1; 95% CI [63.2, 65.0]) than FUMOZ females (76.1; 95% CI [73.3, 79.1]). Survival of F₁-FUTAZ larvae under laboratory conditions was low, with an egg-to-pupae conversion rate of only 5.9% compared to 27.4% in FUMOZ. The median lifespan of F₁-FUTAZ females (32 days) and males (33 days) was lower than FUMOZ (52 and 49 for females and males respectively). The proportion of female F₁-FUTAZ inseminated under laboratory conditions (9%) was considerably lower than either FUMOZ (72%) or wild-caught FUTAZ females (92%). This resulted in nearly zero viable F₂-FUTAZ eggs produced. Wild FUTAZ wings appear to be larger compared to the lab reared F₁-FUTAZ and FUMOZ.

CONCLUSIONS

This study indicates that poor larval survival, mating success, low fecundity and shorter survival under laboratory conditions all contribute to difficulties in colonizing of An. funestus. Future studies should focus on enhancing these aspects of An. funestus fitness in the laboratory, with the biggest barrier likely to be poor mating.

Fine-scale distribution of malaria mosquitoes biting or resting outside human dwellings in three low-altitude Tanzanian villages

BACKGROUND

While malaria transmission in Africa still happens primarily inside houses, there is a substantial proportion of Anopheles mosquitoes that bite or rest outdoors. This situation may compromise the performance of indoor insecticidal interventions such as insecticide-treated nets (ITNs). This study investigated the distribution of malaria mosquitoes biting or resting outside dwellings in three low-altitude villages in south-eastern Tanzania. The likelihood of malaria infections outdoors was also assessed.

METHODS

Nightly trapping was done outdoors for 12 months to collect resting mosquitoes (using resting bucket traps) and host-seeking mosquitoes (using odour-baited Suna® traps). The mosquitoes were sorted by species and physiological states. Pooled samples of Anopheles were tested to estimate proportions infected with Plasmodium falciparum parasites, estimate proportions carrying human blood as opposed to other vertebrate blood and identify sibling species in the Anopheles gambiae complex and An. funestus group. Environmental and anthropogenic factors were observed and recorded within 100 meters from each trapping positions. Generalised additive models were used to investigate relationships between these variables and vector densities, produce predictive maps of expected abundance and compare outcomes within and between villages.

RESULTS

A high degree of fine-scale heterogeneity in Anopheles densities was observed between and within villages. Water bodies covered with vegetation were associated with 22% higher densities of An. arabiensis and 51% lower densities of An. funestus. Increasing densities of houses and people outdoors were both associated with reduced densities of An. arabiensis and An. funestus. Vector densities were highest around the end of the rainy season and beginning of the dry seasons. More than half (14) 58.3% of blood-fed An. arabiensis had bovine blood, (6) 25% had human blood. None of the Anopheles mosquitoes caught outdoors was found infected with malaria parasites.

CONCLUSION

Outdoor densities of both host-seeking and resting Anopheles mosquitoes had significant heterogeneities between and within villages, and were influenced by multiple environmental and anthropogenic factors. Despite the high Anopheles densities outside dwellings, the substantial proportion of non-human blood-meals and absence of malaria-infected mosquitoes after 12 months of nightly trapping suggests very low-levels of outdoor malaria transmission in these villages.

Predicting the impact of outdoor vector control interventions on malaria transmission intensity from semi-field studies

BACKGROUND

Semi-field experiments with human landing catch (HLC) measure as the outcome are an important step in the development of novel vector control interventions against outdoor transmission of malaria since they provide good estimates of personal protection. However, it is often infeasible to determine whether the reduction in HLC counts is due to mosquito mortality or repellency, especially considering that spatial repellents based on volatile pyrethroids might induce both. Due to the vastly different impact of repellency and mortality on transmission, the community-level impact of spatial repellents can not be estimated from such semi-field experiments.

METHODS

We present a new stochastic model that is able to estimate for any product inhibiting outdoor biting, its repelling effect versus its killing and disarming (preventing host-seeking until the next night) effects, based only on time-stratified HLC data from controlled semi-field experiments. For parameter inference, a Bayesian hierarchical model is used to account for nightly variation of semi-field experimental conditions. We estimate the impact of the products on the vectorial capacity of the given Anopheles species using an existing mathematical model. With this methodology, we analysed data from recent semi-field studies in Kenya and Tanzania on the impact of transfluthrin-treated eave ribbons, the odour-baited Suna trap and their combination (push-pull system) on HLC of Anopheles arabiensis in the peridomestic area.

RESULTS

Complementing previous analyses of personal protection, we found that the transfluthrin-treated eave ribbons act mainly by killing or disarming mosquitoes. Depending on the actual ratio of disarming versus killing, the vectorial capacity of An. arabiensis is reduced by 41 to 96% at 70% coverage with the transfluthrin-treated eave ribbons and by 38 to 82% at the same coverage with the push-pull system, under the assumption of a similar impact on biting indoors compared to outdoors.

CONCLUSIONS

The results of this analysis of semi-field data suggest that transfluthrin-treated eave ribbons are a promising tool against malaria transmission by An. arabiensis in the peridomestic area, since they provide both personal and community protection. Our modelling framework can estimate the community-level impact of any tool intervening during the mosquito host-seeking state using data from only semi-field experiments with time-stratified HLC.

Using pastoralist community knowledge to locate and treat dry-season mosquito breeding habitats with pyriproxyfen to control Anopheles gambiae s.l. and Anopheles funestus s.l. in rural Tanzania

Fundamentally, larviciding with pyriproxyfen (PPF) has potential to complement Long Lasting Insecticide Nets (LLINs) and indoor residual sprays (IRS) in settings where resistance to pyrethroids and residual malaria transmission exist. In this study, we evaluated the field effectiveness of larviciding using PPF to reduce dry season productivity of mosquito breeding habitats that were located by pastoralists within the study area. Using pastoralist knowledge, dry season breeding habitats in Mofu village rural Tanzania were located and monitored for larval productivity for a period of 8 months before PPF intervention. During the intervention, six out of twelve breeding habitats were treated with Sumilarv 0.5G PPF granules. The impact of deposited PPF was monitored by recording emergence inhibition of larvae collected from treated habitats compared to the appropriate control group for a period of three months and half post-intervention. During baseline, the average proportion (+SD) of adult emerged was similar between two clusters, with (0.89 + 0.22) for the control cluster and (0.93 + 0.16) for the treatment cluster of breeding habitats. Following treatment with PPF, the average proportion (+SD) of adult emerged in the treated breeding habitats was significantly low (0.096 + 0.22) compared to adults that emerged from larvae in the untreated habitats (0.99 + 0.22) (p < 0.0001). Of all emerged adults, approximately 94% were An. gambiae s.l. and the remaining 6% were An. funestus s.l. This is the first study demonstrating the usefulness of engaging pastoralist community to locate and identify hard to find mosquito breeding habitats. Reduced productivity of the targeted habitats with PPF offers prospect of implementing PPF larviciding in dry season when habitats are few and permanent to control mosquito population in rural settings.

Field evaluation of Veeralin, an alpha-cypermethrin + PBO long-lasting insecticidal net, against natural populations of Anopheles funestus in experimental huts in Muheza, Tanzania

The success of long-lasting insecticidal nets (LLIN) as the primary method for preventing malaria is threatened by pyrethroid resistance in Anopheles vectors. New generation long-lasting nets incorporating PBO synergist (piperonyl butoxide) with pyrethroid are designed to control insecticide-resistant mosquitoes. The efficacy of Veeralin® PBO LLINs was evaluated in experimental huts against wild free-flying pyrethroid-resistant Anopheles funestus (s.l.). Mosquito mortality, blood-feeding inhibition and personal protection were compared between untreated nets, standard LLINs and PBO/pyrethroid combination nets. Blood-feeding rates recorded with 20-times washed Veeralin were not significantly different from those with 20-times washed PermaNet 3.0 LLIN, a WHO Pre-Qualification Team (PQT) approved PBO/pyrethroid LLIN. This provides evidence that Veeralin LLIN provides similar blood-feeding inhibition to the standard approved LLIN and thus meets WHO PQT criteria for blood-feeding. Results show significantly higher mortality for Veeralin PBO LLINs against pyrethroid-resistant Anopheles funestus (s.l.) compared to DuraNet, a WHO PQT approved standard pyrethroid-only LLIN, both when unwashed and washed 20 times. The improved efficacy over a standard pyrethroid-only LLIN can be attributed to the effect of PBO in the Veeralin LLIN, hence meeting the Vector Control Advisory Group (VCAG) criteria for a resistance breaking LLIN.

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Long-lasting insecticidal nets with PBO play a key role in blood-feeding and mortality of mosquitoes.

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Incorporating PBO in LLIN enhanced the efficacy of LLINs against wild populations of Anopheles mosquitoes.

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Understanding the impact of LLINs ​+ ​PBO on mortality, repellency, knockdown could be an asset in vector control toolbox.

Reduced human-biting preferences of the African malaria vectors Anopheles arabiensis and Anopheles gambiae in an urban context: controlled, competitive host-preference experiments in Tanzania

BACKGROUND

Host preference is a critical determinant of human exposure to vector-borne infections and the impact of vector control interventions. Widespread use of long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS) across sub-Saharan Africa, which protect humans against mosquitoes, may select for altered host preference traits of malaria vectors over the long term. Here, the host preferences of Anopheles arabiensis and Anopheles gambiae sensu stricto (s.s.) were experimentally assessed in the field, using direct host-preference assays in two distinct ecological settings in Tanzania.

METHODS

Eight Ifakara Tent Trap (ITT), four baited with humans and four with bovine calves, were simultaneously used to catch malaria vectors in open field sites in urban and rural Tanzania. The numbers of mosquitoes collected in human-baited traps versus calf-baited traps were used to estimate human feeding preference for each site's vector species.

RESULTS

The estimated proportion [95% confidence interval (CI)] of mosquitoes attacking humans rather than cattle was 0.60 [0.40, 0.77] for An. arabiensis in the rural setting and 0.61 [0.32, 0.85] for An. gambiae s.s. in the urban setting, indicating no preference for either host in both cases (P = 0.32 and 0.46, respectively) and no difference in preference between the two (Odds Ratio (OR) [95%] = 0.95 [0.30, 3.01], P = 0.924). However, only a quarter of An. arabiensis in the urban setting attacked humans (0.25 [0.09, 0.53]), indicating a preference for cattle that approached significance (P = 0.08). Indeed, urban An. arabiensis were less likely to attack humans rather than cattle when compared to the same species in the rural setting (OR [95%] = 0.21 [0.05, 0.91], P = 0.037).

CONCLUSION

Urban An. arabiensis had a stronger preference for cattle than the rural population and urban An. gambiae s.s. showed no clear preference for either humans or cattle. In the urban setting, both species exhibited stronger tendencies to attack cattle than previous studies of the same species in rural contexts. Cattle keeping may, therefore, particularly limit the impact of human-targeted vector control interventions in Dar es Salaam and perhaps in other African towns and cities.

Comparative assessment of insecticide resistance phenotypes in two major malaria vectors, Anopheles funestus and Anopheles arabiensis in south-eastern Tanzania

BACKGROUND

Long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS) have greatly reduced malaria transmission in sub-Saharan Africa, but are threatened by insecticide resistance. In south-eastern Tanzania, pyrethroid-resistant Anopheles funestus are now implicated in > 80% of malaria infections, even in villages where the species occurs at lower densities than the other vector, Anopheles arabiensis. This study compared the insecticide resistance phenotypes between the two malaria vectors in an area where pyrethroid-LLINs are widely used.

METHODS

The study used the World Health Organization (WHO) assays with 1×, 5× and 10× insecticide doses to assess levels of resistance, followed by synergist bioassays to understand possible mechanisms of the observed resistance phenotypes. The tests involved adult mosquitoes collected from three villages across two districts in south-eastern Tanzania and included four insecticide classes.

FINDINGS

At baseline doses (1×), both species were resistant to the two candidate pyrethroids (permethrin and deltamethrin), but susceptible to the organophosphate (pirimiphos-methyl). Anopheles funestus, but not An. arabiensis was also resistant to the carbamate (bendiocarb). Both species were resistant to DDT in all villages except in one village where An. arabiensis was susceptible. Anopheles funestus showed strong resistance to pyrethroids, surviving the 5× and 10× doses, while An. arabiensis reverted to susceptibility at the 5× dose. Pre-exposure to the synergist, piperonyl butoxide (PBO), enhanced the potency of the pyrethroids against both species and resulted in full susceptibility of An. arabiensis (> 98% mortality). However, for An. funestus from two villages, permethrin-associated mortalities after pre-exposure to PBO only exceeded 90% but not 98%.

CONCLUSIONS

In south-eastern Tanzania, where An. funestus dominates malaria transmission, the species also has much stronger resistance to pyrethroids than its counterpart, An. arabiensis, and can survive more classes of insecticides. The pyrethroid resistance in both species appears to be mostly metabolic and may be partially addressed using synergists, e.g. PBO. These findings may explain the continued persistence and dominance of An. funestus despite widespread use of pyrethroid-treated LLINs, and inform new intervention choices for such settings. In short and medium-term, these may include PBO-based LLINs or improved IRS with compounds to which the vectors are still susceptible.

Assessing the acoustic behaviour of Anopheles gambiae (s.l.) dsxF mutants: implications for vector control

BACKGROUND

Release of gene-drive mutants to suppress Anopheles mosquito reproduction is a promising method of malaria control. However, many scientific, regulatory and ethical questions remain before transgenic mosquitoes can be utilised in the field. At a behavioural level, gene-drive carrying mutants should be at least as sexually attractive as the wildtype populations they compete against, with a key element of Anopheles copulation being acoustic courtship. We analysed sound emissions and acoustic preference in a doublesex mutant previously used to collapse Anopheles gambiae (s.l.) cages.

METHODS

Anopheles rely on flight tones produced by the beating of their wings for acoustic mating communication. We assessed the impact of disrupting a female-specific isoform of the doublesex gene (dsxF) on the wing beat frequency (WBF; measured as flight tone) of males (XY) and females (XX) in homozygous dsxF^- mutants (dsxF^-/-), heterozygous dsxF^- carriers (dsxF^+/-) and G3 dsxF⁺ controls (dsxF^+/+). To exclude non-genetic influences, we controlled for temperature and wing length. We used a phonotaxis assay to test the acoustic preferences of mutant and control mosquitoes.

RESULTS

A previous study showed an altered phenotype only for dsxF^-/- females, who appear intersex, suggesting that the female-specific dsxF allele is haplosufficient. We identified significant, dose-dependent increases in the WBF of both dsxF^-/- and dsxF^+/- females compared to dsxF^+/+ females. All female WBFs remained significantly lower than male equivalents, though. Males showed stronger phonotactic responses to the WBFs of control dsxF^+/+ females than to those of dsxF^+/- and dsxF^-/- females. We found no evidence of phonotaxis in any female genotype. No male genotypes displayed any deviations from controls.

CONCLUSIONS

A prerequisite for anopheline copulation is the phonotactic attraction of males towards female flight tones within mating swarms. Reductions in mutant acoustic attractiveness diminish their mating efficiency and thus the efficacy of population control efforts. Caged population assessments may not successfully reproduce natural mating scenarios. We propose to amend existing testing protocols to better reflect competition between mutants and target populations. Our findings confirm that dsxF disruption has no effect on males; for some phenotypic traits, such as female WBFs, the effects of dsxF appear dose-dependent rather than haplosufficient.

Passive Outdoor Host Seeking Device (POHD): Designing and Evaluation against Outdoor Biting Malaria Vectors

Odor-baited devices are increasingly needed to compliment long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) for control of residual malaria transmission. However, the odor-baited devices developed so far are bulky, dependent on the source of electricity and carbon dioxide (CO₂), and they are logistically unsuitable for scaling up in surveillance and control of malaria vectors. We designed a passive and portable outdoor host seeking device (POHD) and preliminarily evaluated suitable components against Anopheles arabiensis that maintains residual malaria transmission. Experiments were conducted using semifield reared An. arabiensis within the semifield system at Ifakara Health Institute (IHI) in southeastern Tanzania. These mosquitoes were exposed to Suna traps® baited with BG lures or source of light and augmented with carbon dioxide (CO₂) in view of identifying best attractants necessary to improve attractiveness of designed POHD. Two Suna traps® were hanged at the corner but outside the experimental hut in a diagonal line and rotated between four corners to control for the effect of position and wind direction on mosquito catches. Furthermore, mosquitoes were also exposed to either a bendiocarb-treated or bendiocarb-untreated POHD baited with Mbita blend, Ifakara blend, and worn socks and augmented with warmth (i.e., 1.5 liter bottle of warm water) inside an experimental hut or a screened rectangular box. This study demonstrated that mosquitoes were more strongly attracted to Suna trap® baited with BG lures and CO₂ relative to those traps baited with a source of light and CO₂. The POHD baited with synthetic blends attracted and killed greater proportion of An. arabiensis compared with POHD baited with worn socks. Efficacy of the POHD was unaffected by source of warmth, and it was reduced by about 50% when the device was tested inside a screened rectangular box relative to closed experimental hut. Overall, this study demonstrates that the POHD baited with synthetic blends (Mbita and Ifakara blends) and bendiocarb can effectively attract and kill outdoor biting malaria vector species. Such POHD baited with synthetic blends may require the source of CO₂ to enhance attractiveness to mosquitoes. Further trials are, therefore, ongoing to evaluate attractiveness of improved design of POHD baited with slow-release formulation of synthetic blends and sustainable source of CO₂ to malaria vectors under semifield and natural environments.

Aquatic habitats of the malaria vector Anopheles funestus in rural south-eastern Tanzania

BACKGROUND

In rural south-eastern Tanzania, Anopheles funestus is a major malaria vector, and has been implicated in nearly 90% of all infective bites. Unfortunately, little is known about the natural ecological requirements and survival strategies of this mosquito species.

METHODS

Potential mosquito aquatic habitats were systematically searched along 1000 m transects from the centres of six villages in south-eastern Tanzania. All water bodies were geo-referenced, characterized and examined for presence of Anopheles larvae using standard 350 mLs dippers or 10 L buckets. Larvae were collected for rearing, and the emergent adults identified to confirm habitats containing An. funestus.

RESULTS

One hundred and eleven habitats were identified and assessed from the first five villages (all < 300 m altitude). Of these, 36 (32.4%) had An. funestus co-occurring with other mosquito species. Another 47 (42.3%) had other Anopheles species and/or culicines, but not An. funestus, and 28 (25.2%) had no mosquitoes. There were three main habitat types occupied by An. funestus, namely: (a) small spring-fed pools with well-defined perimeters (36.1%), (b) medium-sized natural ponds retaining water most of the year (16.7%), and (c) slow-moving waters along river tributaries (47.2%). The habitats generally had clear waters with emergent surface vegetation, depths > 0.5 m and distances < 100 m from human dwellings. They were permanent or semi-permanent, retaining water most of the year. Water temperatures ranged from 25.2 to 28.8 °C, pH from 6.5 to 6.7, turbidity from 26.6 to 54.8 NTU and total dissolved solids from 60.5 to 80.3 mg/L. In the sixth village (altitude > 400 m), very high densities of An. funestus were found along rivers with slow-moving clear waters and emergent vegetation.

CONCLUSION

This study has documented the diversity and key characteristics of aquatic habitats of An. funestus across villages in south-eastern Tanzania, and will form an important basis for further studies to improve malaria control. The observations suggest that An. funestus habitats in the area can indeed be described as fixed, few and findable based on their unique characteristics. Future studies should investigate the potential of targeting these habitats with larviciding or larval source management to complement malaria control efforts in areas dominated by this vector species.

Semifield Evaluation of Improved Passive Outdoor Host Seeking Device (POHD) for Outdoor Control of Anopheles arabiensis Mosquitoes

Despite the considerable progress made so far, the effectiveness and mass application of odour-baited outdoor mosquito control devices in pipelines is limited by several factors. These include the design and size of the devices, optimal placement of attractive blends, and nature of materials into which the blends are impregnated. The primary aim of this study was to manipulate these factors to improve the attractiveness of our recently developed passive outdoor host seeking device (POHD) to outdoor biting Anopheles arabiensis. Specifically, the study aimed to determine optimal placement of odour blends and killing bioactives in POHD for maximum attraction and killing of An. arabiensis and to assess the effects of blend types, formulation, and residual activity on attractiveness of the POHD to An. arabiensis. The POHDs baited with attractive blends, carbon dioxide (CO₂), and bendiocarb-treated electrostatic netting were placed either towards the top or bottom openings, and other modifications were exposed to An. arabiensis under the semifield system at Ifakara Health Institute (IHI). Each night, a total of 100 starved female, 3–7-day-old, semifield reared An. arabiensis mosquitoes were released, collected the next morning (alive or dead), counted, and recorded. Live mosquitoes were maintained in the semifield insectary and monitored for 24 hours mortality. Each treatment combination of the POHD was tested in three replicates. Overall, the results indicated that the proportion of mosquitoes attracted to and killed in the POHD varied with position of attractants and killing agent (bendiocarb). The POHD with bottom placed attractants and bendiocarb attracted and killed higher proportion of mosquitoes compared to the POHD with top placed attractants and bendiocarb. The highest mortalities were observed when the POHD was baited with a combination of attractive blends and CO₂. Moreover, the residual activity of attractive blends applied inside POHD varied with type and formulation of attractive blend. The POHD packed with Mbita and Ifakara blend in microencapsulated pellets (granules) attracted higher proportion of mosquitoes than that baited with soaked nylon-strip formulation of either blends. Interestingly, POHD baited with Mbita blend in microencapsulated pellets (granules) formulation attracted and killed higher proportion of mosquitoes (>90%) than that baited with Ifakara blend even 9 months after application. Conclusively, the POHD remained effective for a relatively longer period of time when baited with bottom placed synthetic blends and CO₂ combination, thus warranting further trials under real life situations.

Methods and indicators for measuring patterns of human exposure to malaria vectors

BACKGROUND

Effective targeting and evaluation of interventions that protect against adult malaria vectors requires an understanding of how gaps in personal protection arise. An improved understanding of human and mosquito behaviour, and how they overlap in time and space, is critical to estimating the impact of insecticide-treated nets (ITNs) and determining when and where supplemental personal protection tools are needed. Methods for weighting estimates of human exposure to biting Anopheles mosquitoes according to where people spend their time were first developed over half a century ago. However, crude indoor and outdoor biting rates are still commonly interpreted as indicative of human-vector contact patterns without any adjustment for human behaviour or the personal protection effects of ITNs.

MAIN TEXT

A small number of human behavioural variables capturing the distribution of human populations indoors and outdoors, whether they are awake or asleep, and if and when they use an ITN over the course of the night, can enable a more accurate representation of human biting exposure patterns. However, to date no clear guidance is available on what data should be collected, what indicators should be reported, or how they should be calculated. This article presents an integrated perspective on relevant indicators of human-vector interactions, the critical entomological and human behavioural data elements required to quantify human-vector interactions, and recommendations for collecting and analysing such data.

CONCLUSIONS

If collected and used consistently, this information can contribute to an improved understanding of how malaria transmission persists in the context of current intervention tools, how exposure patterns may change as new vector control tools are introduced, and the potential impact and limitations of these tools. This article is intended to consolidate understanding around work on this topic to date and provide a consistent framework for building upon it. Additional work is needed to address remaining questions, including further development and validation of methods for entomological and human behavioural data collection and analysis.

The Use of Granular Cyclopentanone as Alternative to Artificial Source of Carbon Dioxide in Improved Passive Outdoor Host Seeking Device (POHD)

Reliable sources of CO2 that are relatively cheap, obtainable, and easy to sustain are immediately required for scaling up of odor-baited mosquito surveillance and control devices. Several odor-baited devices are in the pipeline; however, their scale-up against residual malaria transmission, particularly in resource poor areas, is limited by the unavailability of reliable sources of CO2 and reliance on electrical power sources among other factors. We evaluated the use of granular cyclopentanone as an alternative to artificial or yeast fermentation-produced CO2 in passive outdoor host seeking device (POHD). Experiments were conducted against semifield reared An. arabiensis within the semifield system (SFS) at Ifakara Health Institute. Mosquitoes were tested against odor-baited POHDs augmented with yeast fermentation-produced CO2, granular cyclopentanone, attractive blends (Mbita or Ifakara), or their combinations. An insecticide, bendiocarb, was a killing agent used as a proxy for marking the mosquitoes visit the POHDs. Relative attractiveness of different treatment combinations was compared based on the proportion of dead mosquitoes that visited the POHD. The POHD augmented with granules of cyclopentanone alone was attractive to An. arabiensis as much as, or more than, POHDs augmented with yeast fermentation-produced CO2. The POHD baited with CO2 attracted more mosquitoes than those POHDs baited with synthetic blends alone; when these blends are combined with CO2, they attracted more mosquitoes than individual blends. More importantly, such POHDs baited with cyclopentanone attracted far greater proportion of mosquitoes than the POHD baited with either Mbita or Ifakara blend alone. The granular cyclopentanone strongly enhanced/potentiated the attractiveness of POHD baited with Mbita blends against mosquitoes compared to that of POHD baited with Ifakara blend. Moreover, the granular cyclopentanone retained its residual activity against An. arabiensis for up to 2 months after application particularly when used in combination with Mbita blend. In conclusion, this study demonstrates that cyclopentanone granules have the potential to substitute sources of CO2 in outdoor-based surveillance and control devices, thus warranting evaluation of such alternative under realistic field conditions.