Human behaviour directs household-level exposure to malaria vectors in Bandarban, Bangladesh

BACKGROUND

Bangladesh has reduced malaria incidence and mortality by over 75% between 2010 and 2020. Widespread long-lasting insecticidal net (LLIN) distribution and use is one of the measures responsible for this success. Recalcitrant malaria hotspots within the Chittagong Hill Tracts districts suggest important drivers of malaria risk may remain uncharacterized.

METHODS

Towards understanding how household-level human behaviour impacts exposure to mosquitoes, parallel human landing catches and human behavioural observations were conducted in four households for 6 months (May-October) over the rainy season in the Bandarban District. Analysis quantifies spatiotemporal human behaviour-adjusted exposure to Anopheles with and without LLINs.

RESULTS

This small-scale operational study demonstrates that human spatial and temporal presence along with LLIN use drives exposure to Anopheles. Though the four households had both outdoor and indoor exposure, especially in the evening (1800-2000 h) and early morning (0400-0500 h), data points to household-based heterogeneity in malaria exposure even with similar LLIN access.

CONCLUSION

Incorporating human behaviour into exposure estimates can be used to understand the efficacy and limitations of local vector control strategies and identify gaps in protection, as well as where present intervention strategies may be optimized.

Zoonotic malaria transmission and land use change in Southeast Asia: what is known about the vectors

Zoonotic Plasmodium infections in humans in many Southeast Asian countries have been increasing, including in countries approaching elimination of human-only malaria transmission. Most simian malarias in humans are caused by Plasmodium knowlesi, but recent research shows that humans are at risk of many different simian Plasmodium species. In Southeast Asia, simian Plasmodium species are mainly transmitted by mosquitoes in the Anopheles leucosphyrus and Anopheles dirus complexes. Although there is some evidence of species outside the Leucosphyrus Group transmitting simian Plasmodium species, these await confirmation of transmission to humans. The vectors of monkey malarias are mostly found in forests and forest fringes, where they readily bite long-tailed and pig-tailed macaques (the natural reservoir hosts) and humans. How changing land-uses influence zoonotic malaria vectors is still poorly understood. Fragmentation of forests from logging, agriculture and other human activities is associated with increased zoonotic Plasmodium vector exposure. This is thought to occur through altered macaque and mosquito distributions and behaviours, and importantly, increased proximity of humans, macaques, and mosquito vectors. Underlying the increase in vector densities is the issue that the land-use change and human activities create more oviposition sites and, in correlation, increases availably of human blood hosts. The current understanding of zoonotic malaria vector species is largely based on a small number of studies in geographically restricted areas. What is known about the vectors is limited: the data is strongest for distribution and density with only weak evidence for a limited number of species in the Leucosphyrus Group for resting habits, insecticide resistance, blood feeding habits and larval habitats. More data are needed on vector diversity and bionomics in additional geographic areas to understand both the impacts on transmission of anthropogenic land-use change and how this significant disease in humans might be controlled.

Behavioural plasticity of Anopheles coluzzii and Anopheles arabiensis undermines LLIN community protective effect in a Sudanese-savannah village in Burkina Faso

BACKGROUND

Despite the overall major impact of long-lasting insecticide treated nets (LLINs) in eliciting individual and collective protection to malaria infections, some sub-Saharan countries, including Burkina Faso, still carry a disproportionately high share of the global malaria burden. This study aims to analyse the possible entomological bases of LLIN limited impact, focusing on a LLIN-protected village in the Plateau Central region of Burkina Faso.

METHODS

Human landing catches (HLCs) were carried out in 2015 for 12 nights both indoors and outdoors at different time windows during the highest biting activity phase for Anopheles gambiae (s.l.). Collected specimens were morphologically and molecularly identified and processed for Plasmodium detection and L1014F insecticide-resistance allele genotyping.

RESULTS

Almost 2000 unfed An. gambiae (s.l.) (54% Anopheles coluzzii and 44% Anopheles arabiensis) females landing on human volunteers were collected, corresponding to a median number of 23.5 females/person/hour. No significant differences were observed in median numbers of mosquitoes collected indoors and outdoors, nor between sporozoite rates in An. coluzzii (6.1%) and An. arabiensis (5.5%). The estimated median hourly entomological inoculation rate (EIR) on volunteers was 1.4 infective bites/person/hour. Results do not show evidence of the biting peak during night hours typical for An. gambiae (s.l.) in the absence of bednet protection. The frequency of the L1014F resistant allele (n = 285) was 66% in An. coluzzii and 38% in An. arabiensis.

CONCLUSIONS

The observed biting rate and sporozoite rates are in line with the literature data available for An. gambiae (s.l.) in the same geographical area before LLIN implementation and highlight high levels of malaria transmission in the study village. Homogeneous biting rate throughout the night and lack of preference for indoor-biting activity, suggest the capacity of both An. coluzzii and An. arabiensis to adjust their host-seeking behaviour to bite humans despite bednet protection, accounting for the maintenance of high rates of mosquito infectivity and malaria transmission. These results, despite being limited to a local situation in Burkina Faso, represent a paradigmatic example of how high densities and behavioural plasticity in the vector populations may contribute to explaining the limited impact of LLINs on malaria transmission in holo-endemic Sudanese savannah areas in West Africa.

Transmission risk beyond the village: entomological and human factors contributing to residual malaria transmission in an area approaching malaria elimination on the Thailand-Myanmar border

Background

A mixed methods study was conducted to look at the magnitude of residual malaria transmission (RMT) and factors contributing to low (< 1% prevalence), but sustained transmission in rural communities on the Thai–Myanmar border.

Methods

A cross-sectional behaviour and net survey, observational surveys and entomological collections in both villages and forested farm huts frequented by community members for subsistence farming practices were conducted.

Results

Community members frequently stayed overnight at subsistence farm huts or in the forest. Entomological collections showed higher biting rates of primary vectors in forested farm hut sites and in a more forested village setting compared to a village with clustered housing and better infrastructure. Despite high levels of outdoor biting, biting exposure occurred predominantly indoors, particularly for non-users of long-lasting insecticidal nets (LLINs). Risk of biting exposure was exacerbated by sub-optimal coverage of LLINs, particularly in subsistence farm huts and in the forest. Furthermore, early waking hours when people had left the safety of their nets coincided with peaks in biting in later morning hours.

Conclusions

Entomological and epidemiological findings suggest drivers and modulators of sustained infection prevalence in the area to be: higher mosquito abundance in forested areas where LLINs were used less frequently or could not be used; late sleeping and waking times coinciding with peak biting hours; feeding preferences of Anopheles taking them away from contact with LLIN and indoor residual spraying (IRS), e.g. exophagy and zoophagy; non-use of LLIN and use of damaged/torn LLIN; high population movement across the border and into forested areas thereby increasing risk of exposure, decreasing use of protection and limiting access to healthcare; and, Plasmodium vivax predominance resulting in relapse(s) of previous infection. The findings highlight gaps in current intervention coverage beyond the village setting.

Electronic supplementary material

The online version of this article (10.1186/s12936-019-2852-5) contains supplementary material, which is available to authorized users.

Patterns of anopheline feeding/resting behaviour and Plasmodium infections in North Cameroon, 2011-2014: implications for malaria control

Background

Effective malaria control relies on evidence-based interventions. Anopheline behaviour and Plasmodium infections were investigated in North Cameroon, following long-lasting insecticidal net (LLIN) distribution in 2010.

Methods

During four consecutive years from 2011 to 2014, adult mosquitoes were collected indoors, outdoors and in exit traps across 38 locations in the Garoua, Pitoa and Mayo-Oulo health districts. Anophelines were morphologically and molecularly identified, then analysed for blood meal origins and Plasmodium falciparum circumsporozoite protein (Pf-CSP). Blood from children under 5 years-old using LLINs was examined for Plasmodium infections.

Results

Overall, 9376 anophelines belonging to 14 species/sibling species were recorded. Anopheles gambiae (s.l.) [An. arabiensis (73.3%), An. coluzzii (17.6%) and An. gambiae (s.s.) (9.1%)] was predominant (72%), followed by An. funestus (s.l.) (20.5%) and An. rufipes (6.5%). The recorded blood meals were mainly from humans (28%), cattle (15.6%) and sheep (11.6%) or mixed (45%). Pf-CSP rates were higher indoors (3.2–5.4%) versus outdoors (0.8–2.0%), and increased yearly (χ² < 18, df = 10, P < 0.03). Malaria prevalence in children under 5 years-old, in households using LLINs was 30% (924/3088).

Conclusions

The present study revealed the variability of malaria vector resting and feeding behaviour, and the persistence of Plasmodium infections regardless the use of LLINs. Supplementary interventions to LLINs are therefore needed to sustain malaria prevention in North Cameroon.

Avian malaria alters the dynamics of blood feeding in Culex pipiens mosquitoes

BACKGROUND

Some Plasmodium species have the ability to modify the behaviour of their mosquito vectors. This is thought to be an adaptive strategy that maximizes the parasite's transmission.

METHODS

The effect of Plasmodium relictum infections on the blood feeding behaviour of Culex pipiens quinquefasciatus mosquitoes was monitored.

RESULTS

Plasmodium infections did not alter the proportion of blood fed mosquitoes but they did affect the dynamics and the size of the blood meal. Sporozoite-infected mosquitoes completed their blood meal 1.3 times later than uninfected mosquitoes and ended up with smaller blood meals.

CONCLUSION

The potential adaptive nature of this manipulation of mosquito behaviour is discussed in the light of previous studies on other malaria models.

Host Decoy Trap (HDT) with cattle odour is highly effective for collection of exophagic malaria vectors

BACKGROUND

As currently implemented, malaria vector surveillance in sub-Saharan Africa targets endophagic and endophilic mosquitoes, leaving exophagic (outdoor blood-feeding) mosquitoes underrepresented. We evaluated the recently developed host decoy trap (HDT) and compared it to the gold standard, human landing catch (HLC), in a 3 × 3 Latin square study design outdoors in western Kenya. HLCs are considered to represent the natural range of Anopheles biting-behaviour compared to other sampling tools, and therefore, in principle, provide the most reliable profile of the biting population transmitting malaria. The HDT incorporates the main host stimuli that attract blood-meal seeking mosquitoes and can be baited with the odours of live hosts.

RESULTS

Numbers and species diversity of trapped mosquitoes varied significantly between HLCs and HDTs baited with human (HDT-H) or cattle (HDT-C) odour, revealing important differences in behaviour of Anopheles species. In the main study in Kisian, the HDT-C collected a nightly mean of 43.2 (95% CI: 26.7-69.8) Anopheles, compared to 5.8 (95% CI: 4.1-8.2) in HLC, while HDT-H collected 0.97 (95% CI: 0.4-2.1), significantly fewer than the HLC. Significantly higher proportions of An. arabiensis were caught in HDT-Cs (0.94 ± 0.01; SE) and HDT-Hs (0.76 ± 0.09; SE) than in HLCs (0.45 ± 0.05; SE) per trapping night. The proportion of An. gambiae (s.s.) was highest in HLC (0.55 ± 0.05; SE) followed by HDT-H (0.20 ± 0.09; SE) and least in HDT-C (0.06 ± 0.01; SE). An unbaited HDT placed beside locales where cattle are usually corralled overnight caught mostly An. arabiensis with proportions of 0.97 ± 0.02 and 0.80 ± 0.2 relative to the total anopheline catch in the presence and absence of cattle, respectively. A mean of 10.4 (95% CI: 2.0-55.0) Anopheles/night were trapped near cattle, compared to 0.4 (95% CI: 0.1-1.7) in unbaited HDT away from hosts.

CONCLUSIONS

The capability of HDTs to combine host odours, heat and visual stimuli to simulate a host provides the basis of a system to sample human- and cattle-biting mosquitoes. HDT-C is particularly effective for collecting An. arabiensis outdoors. The HDT offers the prospect of a system to monitor and potentially control An. arabiensis and other outdoor-biting mosquitoes more effectively.

Age- and bite-structured models for vector-borne diseases

The biology and behaviour of biting insects is a vitally important aspect in the spread of vector-borne diseases. This paper aims to determine, through the use of mathematical models, what effect incorporating vector senescence and realistic feeding patterns has on disease. A novel model is developed to enable the effects of age- and bite-structure to be examined in detail. This original PDE framework extends previous age-structured models into a further dimension to give a new insight into the role of vector biting and its interaction with vector mortality and spread of disease. Through the PDE model, the roles of the vector death and bite rates are examined in a way which is impossible under the traditional ODE formulation. It is demonstrated that incorporating more realistic functions for vector biting and mortality in a model may give rise to different dynamics than those seen under a more simple ODE formulation. The numerical results indicate that the efficacy of control methods that increase vector mortality may not be as great as predicted under a standard host-vector model, whereas other controls including treatment of humans may be more effective than previously thought.