Mosquito species diversity and abundance in relation to land use in a riceland agroecosystem in Mwea, Kenya

Local tree cover predicts mosquito species richness and disease vector presence in a tropical countryside landscape

Context

Land use change drives both biodiversity loss and zoonotic disease transmission in tropical countryside landscapes. Developing solutions for protecting countryside biodiversity, public health, and livelihoods requires understanding the scales at which habitat characteristics such as land cover shape biodiversity, especially for arthropods that transmit pathogens. Evidence increasingly shows that species richness for many taxa correlates with local tree cover.

Objectives

We investigated whether mosquito species richness, community composition, and presence of disease vector species responded to land use and tree cover - and if so, whether at spatial scales similar to other taxa.

Methods

We paired a field survey of mosquito communities in agricultural, residential, and forested lands in rural southern Costa Rica with remotely sensed tree cover data. We compared mosquito community responses to tree cover surrounding survey sites measured across scales, and analyzed community responses to land use and environmental gradients.

Results

Tree cover was positively correlated with mosquito species richness, and negatively correlated with the presence of the common invasive dengue vector Aedes albopictus, particularly at small spatial scales of 80 - 200m. Land use predicted community composition and Ae. albopictus presence. Environmental gradients of tree cover, temperature, and elevation explained 7% of species turnover among survey sites.

Conclusions

The results suggest that preservation and expansion of tree cover at local scales can protect biodiversity for a wide range of taxa, including arthropods, and also confer protection against disease vector occurrence. The identified spatial range of tree cover benefits can inform land management for conservation and public health protection.

Local tree cover predicts mosquito species richness and disease vector presence in a tropical countryside landscape

Context

Land use change drives both biodiversity loss and zoonotic disease transmission in tropical countryside landscapes. Developing solutions for protecting countryside biodiversity, public health, and livelihoods requires understanding the scales at which habitat characteristics such as land cover shape biodiversity, especially for arthropods that transmit pathogens. Evidence increasingly shows that species richness for many taxa correlates with local tree cover.

Objectives

We investigated whether mosquito species richness, community composition, and presence of disease vector species responded to land use and tree cover - and if so, whether at spatial scales similar to other taxa.

Methods

We paired a field survey of mosquito communities in agricultural, residential, and forested lands in rural southern Costa Rica with remotely sensed tree cover data. We compared mosquito community responses to tree cover surrounding survey sites measured across scales, and analyzed community responses to land use and environmental gradients.

Results

Tree cover was positively correlated with mosquito species richness, and negatively correlated with the presence of the common invasive dengue vector Aedes albopictus , particularly at small spatial scales of 80 - 200m. Land use predicted community composition and Ae. albopictus presence. Environmental gradients of tree cover, temperature, and elevation explained 7% of species turnover among survey sites.

Conclusions

The results suggest that preservation and expansion of tree cover at local scales can protect biodiversity for a wide range of taxa, including arthropods, and also confer protection against disease vector occurrence. The identified spatial range of tree cover benefits can inform land management for conservation and public health protection.

Effects of seasonality and developed land cover on Culex mosquito abundance and microbiome diversity

The vectorial capacity of mosquitoes, which influences the dynamics of vector-borne disease transmission, is intricately linked to mosquito abundance and the composition and diversity of their associated microbiomes. However, the influence of environmental factors on mosquito populations and microbiome diversity remains underexplored. Here we examined the effects of seasonality and developed land cover on Culex mosquito abundance and bacterial diversity. Biweekly field sampling of female Culex mosquitoes was conducted using gravid and CDC light traps, spanning summer to autumn across varying developed land cover levels in two urban areas in Central Illinois. Mosquito abundance was assessed by the number of mosquitoes captured per trap night and compared across seasons and developed levels. The mean mosquito abundance for gravid and light traps was 12.96 ± 2.15 and 7.67 ± 1.44, respectively. Notably, higher levels of developed land cover exhibited higher Culex abundance than the low level for light traps, but no significant difference was found between summer and early autumn. In gravid traps, no significant differences were detected across seasons or developed levels. Microbial analysis of the mosquito microbiome revealed that Proteobacteria and Wolbachia, with a mean relative abundance of 80.77 and 52.66% respectively, were identified as the most dominant bacterial phylum and genus. Their relative abundance remained consistent across seasons and developed land cover levels, with negligible variations. Alpha diversity, as measured by observed species, Chao1, Shannon, and Simpson, showed slightly higher values in early-autumn compared to late-summer. A notable pattern of bacterial diversity, as indicated by all four diversity indexes, is evident across varying levels of land development. Significantly, high or intermediate developed levels consistently showed reduced alpha diversity when compared to the lower level. This underscores the pronounced impact of anthropogenic ecological disturbances in shaping mosquito microbiomes. Beta diversity analysis revealed no significant dissimilarities in bacterial community composition across seasons and developed levels, although some separation was noted among different levels of developed land cover. These findings highlight the significant role of environmental factors in shaping mosquito abundance and their associated microbiomes, with potential implications for the vectorial capacity in the transmission of vector-borne diseases.

The influence of manure-based organic fertilisers on the oviposition behaviour of Anopheles arabiensis

The rice agroecosystem provides suitable breeding habitat for many malaria vector species, and rice-adjacent communities are consequently exposed to a greater malaria transmission risk than non-rice-associated communities. As part of efforts to expand rice production in Africa, sustainable and climate-adapted practices such as the System of Rice Intensification (SRI) are being promoted. SRI encourages the use of organic fertilisers (OFs) such as cow and chicken dung, as opposed to inorganic industrially produced fertilisers, due to their lower resource cost, apparent benefit to the rice agroecosystem and as a means to reduce the greenhouse gas emissions associated with the production of industrial fertilisers. However, the impact of OFs on mosquito fauna is not well documented and may have knock-on consequences on malaria transmission risk. Here, we demonstrate, using dual choice egg count assays, that both cow and chicken dung modulate the oviposition behaviour of Anopheles arabiensis, a major malaria vector in Sub-Saharan Africa. A significantly reduced proportion of eggs were laid in water treated with either cow or chicken dung compared to untreated water, with higher dung concentrations resulting in further reduced proportions. When presented in competition, significantly fewer eggs were laid in water treated with chicken dung than with cow dung. Moreover, there was no evidence of egg retention in any experiment, including in no-choice experiments where only dung-containing dishes were available. These results suggest both cow and chicken dung may act as oviposition deterrents to malaria vector species and that the application of manure-based OFs in rice agriculture may modulate the oviposition behaviour of An. gambiae s.l. within agroecosystems. Quantification of the ammonia present in dung-infused water showed higher concentrations were present in the chicken dung infusion, which may be one contributing factor to the difference in observed deterrence between the two dung types. Deterrence of mosquito oviposition in OF-treated farms may potentially affect the overall production of malaria vectors within rice fields and their contribution to local malaria transmission.

Manure and mosquitoes: life history traits of two malaria vector species enhanced by larval exposure to cow dung, whilst chicken dung has a strong negative effect

BACKGROUND

Malaria vectors have a strong ecological association with rice agroecosystems, which can provide abundant aquatic habitats for larval development. Climate-adapted rice cultivation practices, such as the System of Rice Intensification (SRI), are gaining popularity in malaria-endemic countries seeking to expand rice production; however, the potential impact of these practices on vector populations has not been well characterised. In particular, SRI encourages the use of organic fertilisers (OFs), such as animal manures, as low-cost and environmentally friendly alternatives to industrially produced inorganic fertilisers. We therefore set out to understand the effects of two common manure-based OFs on the life history traits of two major African malaria vectors, Anopheles arabiensis and Anopheles gambiae sensu stricto (s.s.).

METHODS

Larvae of An. arabiensis and An. gambiae s.s. were reared from first instar to emergence in water containing either cow or chicken dung at one of four concentrations (0.25, 0.5, 0.75, and 1.0 g/100 ml), or in a clean water control. Their life history traits were recorded, including survival, development rate, adult production, and adult wing length.

RESULTS

Exposure to cow dung significantly increased the development rate of An. gambiae s.s. independent of concentration, but did not affect the overall survival and adult production of either species. Chicken dung, however, significantly reduced survival and adult production in both species, with a greater effect as concentration increased. Interestingly, An. arabiensis exhibited a relative tolerance to the lowest chicken dung concentration, in that survival was unaffected and adult production was not reduced to the same extent as in An. gambiae s.s. The effects of chicken dung on development rate were less clear in both species owing to high larval mortality overall, though there was some indication that it may reduce development rate. Adult wing lengths in males and females increased with higher concentrations of both cow and chicken dung.

CONCLUSIONS

Our findings suggest that manure-based OFs significantly alter the life history traits of An. gambiae s.s. and An. arabiensis. In both species, exposure to cow dung may improve fitness, whereas exposure to chicken dung may reduce it. These findings have implications for understanding vector population dynamics in rice agroecosystems and may inform the use of OFs in SRI, and rice agriculture more widely, to avoid their adverse effects in enhancing vector fitness.

Malaria vector bionomics in Taita-Taveta County, coastal Kenya

Background

Estimation of the composition and densities of mosquito species populations is crucial for monitoring the epidemiology of mosquito-borne diseases and provide information on local vectors to public health officials and policy-makers. The aim of this study was to evaluate malaria vector bionomics in ecologically distinct sites in Taita-Taveta County, Kenya.

Methods

Adult mosquitoes were collected using backpack aspirators and paired indoor/outdoor CDC light traps in 10 randomly selected households in six villages with distinct ecologies over a study period of 3 years. All Anopheles mosquitoes were morphotyped, and sibling species of Anopheles gambiae sensu lato (An. gambiae s.l.) were identified and separated by PCR analysis of extracted ribosomal DNA. All female anophelines were tested for sporozoite infectivity, with engorged females screened for blood-meal sources using the enzyme-linked immunosorbent assay technique. A subsample of those testing positive and those testing negative for Plasmodium in the ELISA were subjected to PCR assay.

Results

A total of eight different Anopheles species were collected both indoors and outdoors. Anopheles gambiae s.l. (82.6%, n = 5252) was the predominant species sensu lato, followed by Anopheles coustani sensu lato (An. coustani s.l.; (10.5%, n = 666) and Anopheles funestus sensu lato (An. funestus s.l.; 5.6%, n = 357). A subset of 683 mosquito samples representing An. gambiae s.l. (n = 580, approx. 11.0%) and An. funestus s.l. (n = 103, approx. 28.9%) were identified by molecular diagnostic assays into sibling species. The An. gambiae s.l. complex was composed of Anopheles arabiensis (62.5%, n = 363/580), An. gambiae sensu stricto (An. gambiae s.s.; 0.7%, n = 4/580), Anopheles merus (0.7%, n = 4/580) and Anopheles quadriannulatus (0.2%, n = 1/580), with the remaining samples (35.5%, n = 206/580) unamplified. Anopheles funestus s.l. was composed of An. rivulorum (14.6%, n = 15/103) and An. leesoni (11.6%, n = 12/103); the remaining samples were unamplified (73.8%, n = 76/103). A total of 981 samples were subjected to PCR analysis for malaria parasite detection; of these 16 (1.6%) were confirmed to be positive for Plasmodium falciparum. The overall human blood index was 0.13 (32/238).

Conclusions

Anopheles gambiae, An. funestus and An. coustani are key malaria vectors in the Taveta region of Kenya, showing concurrent indoor and outdoor transmission. All of the vectors tested showed a higher propensity for bovine and goat blood than for human blood.

Graphical Abstract

Malaria vector bionomics and transmission in irrigated and non-irrigated sites in western Kenya

Irrigation not only helps to improve food security but also creates numerous water bodies for mosquito production. This study assessed the effect of irrigation on malaria vector bionomics and transmission in a semi-arid site with ongoing malaria vector control program. The effectiveness of CDC light traps in the surveillance of malaria vectors was also evaluated relative to the human landing catches (HLCs) method. Adult mosquitoes were sampled in two study sites representing irrigated and non-irrigated agroecosystems in western Kenya using a variety of trapping methods. The mosquito samples were identified to species and assayed for host blood meal source and Plasmodium spp. sporozoite infection using polymerase chain reaction. Anopheles arabiensis was the dominant malaria vector in the two study sites and occurred in significantly higher densities in irrigated study site compared to the non-irrigated study site. The difference in indoor resting density of An. arabiensis during the dry and wet seasons was not significant. Other species, including An. funestus, An. coustani, and An. pharoensis, were collected. The An. funestus indoor resting density was 0.23 in irrigated study site while almost none of this species was collected in the non-irrigated study site. The human blood index (HBI) for An. arabiensis in the irrigated study site was 3.44% and significantly higher than 0.00% for the non-irrigated study site. In the irrigated study site, the HBI of An. arabiensis was 3.90% and 5.20% indoor and outdoor, respectively. The HBI of An. funestus was 49.43% and significantly higher compared to 3.44% for An. arabiensis in the irrigated study site. The annual entomologic inoculation rate for An. arabiensis in the irrigated study site was 0.41 and 0.30 infective bites/person/year indoor and outdoor, respectively, whereas no transmission was observed in the non-irrigated study site. The CDC light trap performed consistently with HLC in terms of vector density. These findings demonstrate that irrigated agriculture may increase the risk of malaria transmission in irrigated areas compared to the non-irrigated areas and highlight the need to complement the existing malaria vector interventions with novel tools targeting the larvae and both indoor and outdoor biting vector populations.

Diversity and behavioral activity of Anopheles mosquitoes on the slopes of Mount Cameroon

BACKGROUND

Malaria remains endemic in Cameroon, with heterogeneous transmission related to eco-climatic variations, vector diversity and spatial distribution. The intensification of malaria prevention and control through the free distribution of insecticide-treated nets in recent years may have altered the composition, geographic distribution and natural infection rate of Anopheles species, with implications for malaria transmission dynamics. The present study seeks to assess the vectorial diversity, dynamics and infectivity across different seasons and altitudes in relationship to parasite prevalence around the slopes of Mount Cameroon, southwestern region.

METHOD

Mosquitoes were sampled (indoors and outdoors) in 11 eco-epidemiological settings at low (18-197 m), intermediate (371-584 m) and high (740-1067 m) altitude by nightly human landing catches. The mosquitoes were identified morphologically and Anopheles gambiae sibling species identified by PCR. Parity status was ascertained by examining the ovaries and the entomological inoculation rates (EIR) determined by Plasmodium falciparum circumsporozoite antigen ELISA of the head-thorax. The prevalence of Plasmodium infection across target communities was assessed using rapid diagnostic tests.

RESULTS

A total of 7327 (18.0 mosquitoes/trap/night) mosquitoes were trapped, mainly during the rainy season (5678, 77.5%) and at low altitude (3669, 50.1%). Anopheles spp. (5079, 69.3%) was the most abundant genera and An. gambiae complex (2691, 36.7%) the major vector, varying with altitude (χ² = 183.87, df = 8, P < 0.001) and season (χ² = 28.14, df = 4, P < 0.001). Only An. gambiae (s.s.) was identified following molecular analysis of An. gambiae complex siblings. The overall biting peak for An. gambiae complex was 2-3 a.m. Anopheles cinctus was the most abundant secondary vector in the area. The average EIR in the area was 2.08 infective bites per person per night (ib/p/n), higher at low (2.45 ib/p/n) than at intermediate altitude (1.39 ib/p/n) and during the rainy (1.76 ib/p/n) compared to the dry season (0.34 ib/p/n). Anopheles funestus was most infectious overall (28.1%, 16/57) while An. gambiae had the highest inoculation rates averaging 1.33 ib/p/n. Most Anopheles species across all altitudes and seasons were parous, highest in communities with the highest proportion of malaria parasite infections.

CONCLUSION

Anopheles gambiae (s.s.) remains the major malaria vector in the area and An. cinctus possibly a secondary vector of the disease in the slopes of Mt. Cameroon. The seasonal and altitudinal effects on the distribution of these mosquitoes may have implications for the transmission of malaria and its control strategies in the area. Regular monitoring of the bionomics of local Anopheles vector species and targeted control interventions in the 'hotspots' is necessary to curb the prevalence of the infection and incidence of disease.

Modeling Mosquitoes and their Potential Odonate Predators Under Different Land Uses

To efficiently face the accelerated landscape transformation and its consequences in restructuring biotic communities and ecosystem services, one first question is which regional systems deserve prioritization for empirical assessments and interventive strategies. For the particular case of vector-borne disease control, we should consider generalist predators exhibiting differential responses to land-use change, as is the case of odonate insects. Thus, our aim was to infer land uses in Mexico where odonates (i.e., damselflies and dragonflies) might have some potential to predate mosquitoes of medical relevance. The study area included the hydrological basins of central Mexico. We modelled 167 species of odonates, four species of mosquitoes, and 51 land-use categories. Inferring spatial co-occurrence patterns from data mining and complex networks, we identified: (1) the ecological network of odonates and mosquitoes and (2) the land uses shared by these two groups. We inferred that 34% of odonate species co-occur with mosquitoes of medical relevance mainly in some preserved-mountain mesophyll cloud forest, high evergreen rainforest, and low tropical dry forest-but also in highly modified-human settlements, irrigation-based and pastures crop fields-land uses with strong human presence. Our findings highlight the relevance of community-regional studies for understanding the public health consequences of landscape change.

The human-baited host decoy trap (HDT) is an efficient sampling device for exophagic Anopheles arabiensis within irrigated lands in southern Malawi

Irrigation schemes provide an ideal habitat for Anopheles mosquitoes particularly during the dry season. Reliable estimates of outdoor host-seeking behaviour are needed to assess the impact of vector control options and this is particularly the case for Anopheles arabiensis which displays a wide range of behaviours that circumvent traditional indoor-insecticide based control. In this study we compared the sampling efficiency of the host decoy trap (HDT) with the human landing catch (HLC) and Suna trap in a repeated Latin square design in two villages (Lengwe and Mwanza) on an irrigated sugar estate in southern Malawi. Over the course of 18 trapping nights, we caught 379 female Anopheles, the majority of which were identified as An. arabiensis. Across both villages, there was no detectable difference in Anopheles catch between the HDT compared with the HLC (RR = 0.85, P = 0.508). The overall sensitivity of the HLC was greater than the Suna trap regardless of mosquito density (Lengwe, α = 2.75, 95% credible interval: 2.03-3.73; Mwanza, α = 3.38, 95% credible interval: 1.50-9.30) whereas the sensitivity of the HDT was only greater than the Suna trap when mosquito numbers were high (Lengwe, α = 2.63, 95% credible interval: 2.00-3.85).We conclude that the HDT is an effective sampling device for outdoor host seeking An. arabiensis in southern Malawi. The presence of An. arabiensis in irrigated lands during the dry season poses a challenge for ongoing indoor vector control efforts.

Malaria transmission and prevalence in rice-growing versus non-rice-growing villages in Africa: a systematic review and meta-analysis

BACKGROUND

Rice fields in Africa are major breeding sites for malaria vectors. However, when reviewed in the 1990s, in settings where transmission was relatively intense, there was no tendency for malaria indices to be higher in villages with irrigated rice fields than in those without. Subsequently, intervention coverage in sub-Saharan Africa has been massively scaled up and malaria infection prevalence has halved. We re-examined this rice-malaria relationship to assess whether, with lower malaria transmission, malaria risk is greater in rice-growing than in non-rice-growing areas.

METHODS

For this systematic review and meta-analysis, we searched EMBASE, Global Health, PubMed, Scopus, and Web of Science to identify observational studies published between Jan 1, 1900, and Sept 18, 2020. Studies were considered eligible if they were observational studies (cross-sectional, case-control, or cohort) comparing epidemiological or entomological outcomes of interest between people living in rice-growing and non-rice-growing rural communities in sub-Saharan Africa. Studies with pregnant women, displaced people, and military personnel as participants were excluded because they were considered not representative of a typical community. Data were extracted with use of a standardised data extraction form. The primary outcomes were parasite prevalence (P falciparum parasite rate age-standardised to 2-10-year-olds, calculated from total numbers of participants and number of infections [confirmed by microscopy or rapid diagnostic test] in each group) and clinical malaria incidence (number of diagnoses [fever with Plasmodium parasitaemia confirmed by microscopy or rapid diagnostic test] per 1000 person-days in each group). We did random-effects meta-analyses to estimate the pooled risk ratio (RR) for malaria parasite prevalence and incidence rate ratio (IRR) for clinical malaria in rice-growing versus non-rice-growing villages. RRs were compared in studies conducted before and after 2003 (chosen to mark the start of the mass scale-up of antimalaria interventions). This study is registered with PROSPERO (CRD42020204936).

FINDINGS

Of the 2913 unique studies identified and screened, 53 studies (including 113 160 participants across 14 African countries) were eligible for inclusion. In studies done before 2003, malaria parasite prevalence was not significantly different in rice-growing versus non-rice-growing villages (pooled RR 0·82 [95% CI 0·63-1·06]; 16 studies, 99 574 participants); however, in post-2003 studies, prevalence was significantly higher in rice-growing versus non-rice growing villages (1·73 [1·01-2·96]; seven studies, 14 002 participants). Clinical malaria incidence was not associated with residence in rice-growing versus non-rice-growing areas (IRR 0·75 [95% CI 0·47-1·18], four studies, 77 890). Potential limitations of this study include its basis on observational studies (with evidence quality rated as very low according to the GRADE approach), as well as its omission for the effects of seasonality and type of rice being cultivated. Risk of bias and inconsistencies was relatively serious, with I2 greater than 90% indicating considerable heterogeneity.

INTERPRETATION

Irrigated rice-growing communities in sub-Saharan Africa are exposed to greater malaria risk, as well as more mosquitoes. As increasing rice production and eliminating malaria are two major development goals in Africa, there is an urgent need to improve methods for growing rice without producing mosquitoes.

FUNDING

Wellcome Trust Our Planet Our Health programme, CGIAR Agriculture for Nutrition and Health.

Population genetics of Anopheles arabiensis, the primary malaria vector in the Republic of Sudan

Background

Anopheles arabiensis is a member of Anopheles gambiae complex and the main malaria vector in Sudan. There is insufficient population genetics data available on An. arabiensis for an understanding of vector population structure and genetics, which are important for the malaria vector control programmes in this country. The objective of this investigation is to study the population structure, gene flow and isolation by distance among An. arabiensis populations for developing control strategies.

Methods

Mosquitoes were collected from six sites located in three different states in Sudan, Khartoum, Kassala and Sennar, using pyrethrum spray catch of indoor resting mosquitoes. Anopheline mosquitoes were identified morphologically and based on species specific nucleotide sequences in the ribosomal DNA intergenic spacers (IGS). Seven published An. gambiae microsatellite loci primers were used to amplify the DNA of An. arabiensis samples.

Results

PCR confirmed that An. arabiensis was the main malaria vector found in the six localities. Of the seven microsatellite loci utilized, six were found to be highly polymorphic across populations, with high allelic richness and heterozygosity with the remaining one being monomorphic. Deviation from Hardy–Weinberg expectations were found in 21 out of 42 tests in the six populations due to heterozygote deficiency. Bayesian clustering analysis revealed two gene pools, grouping samples into two population clusters; one includes four and the other includes two populations. The clusters were not grouped according to the three states but were instead an admixture. The genetic distances between pairs of populations ranged from 0.06 to 0.24. Significant F_ST was observed between all pairwise analyses of An. arabiensis populations. The Kassala state population indicated high genetic differentiation (F_ST ranged from 0.17 to 0.24) from other populations, including one which is also located in the same state. High gene flow (Nm = 1.6–8.2) was detected among populations within respective clusters but limited between clusters particularly with respect to Kassala state. There was evidence of a bottleneck event in one of the populations (Al Haj Yousif site). No isolation by distance pattern was detected among populations.

Conclusions

This study revealed low levels of population differentiation with high gene flow among the An. arabiensis populations investigated in Sudan, with the exception of Kassala state.

Abundance and Distribution of Malaria Vectors in Various Aquatic Habitats and Land Use Types in Kakamega County, Highlands of Western Kenya

Background

Management of malaria transmission relies heavily on vector control. Implementation and sustenance of effective control measures require regular monitoring of malaria vector occurrences, species abundance and distribution. The study assessed mosquito larval species composition, distribution and productivity in Kakamega County, western Kenya.

Methods

A cross-sectional survey of Anopheline larvae was conducted in various aquatic habitats and land use types in Kakamega County, highlands of western Kenya between the month of March and June 2019.

Results

One thousand, five hundred and seventy six aquatic habitats were sampled in various land use types. The mean densities of An. gambiae s.l (46.2), An. funestus (5.3), An. coustani (1.7), An. implexus (0.13) and An. squamosus (2.0) were observed in fish ponds, burrow pits, drainage ditches, and tire tracks, respectively. High mean densities of An. gambiae s.l was reported in farmland (20.4) while high mean abundance of An. funestus s.l (8.2) and An. coustani s.l (4.0) were observed in artificial forests.

Conclusion

The study revealed that the productivity of anopheles larvae varied across various habitat types and land use types. Therefore, treatment of potential breeding sites should be considered as an additional strategy for malaria vector control in Kakamega County, western Kenya.

Urban Ecology of Arboviral Mosquito Vectors Along the Kenyan Coast

The purpose of this study was to determine the ecology of the common arboviral mosquito vectors in Mombasa, Kilifi and Malindi urban areas of coastal Kenya. Mosquito larvae were collected using standard dippers and pipettes. Egg survivorship in dry soil was evaluated by collecting soil samples from dry potential larval developmental sites, re-hydrating them for hatching and rearing of the eventual larvae to adults. Adult mosquitoes were collected with CDC light traps and BG-Sentinel traps. All blood-fed females were tested for bloodmeal origin. Mosquitoes were screened for arboviruses using RT-qPCR. Overall, the predominant species were Culex quinquefasciatus (Say) 72.4% (n = 2,364) and Aedes aegypti (L.), 25.7%, (n = 838). A total of 415 larval developmental sites were identified indoors (n = 317) and outdoors (n = 98). The most productive larval developmental sites, both indoors and outdoors, were assorted small containers, water tanks, drainages, drums, and jerricans. Overall, 62% (n = 18) of the soil samples collected were positive for larvae which were used as a proxy to measure the presence of eggs. The mosquitoes fed on humans (29.8%) and chickens (3.7%). Of 259 mosquitoes tested for viral infection, 11.6% were positive for Flavivirus only. The most productive larval developmental sites for arboviral vectors indoors were small containers, water tanks, jerricans, and drums whereas small containers, water tanks, drainage channels, buckets, tires, and water troughs were the productive larval developmental sites outdoors.

Diversity of mosquito (Diptera: Culicidae) vectors in a heterogeneous landscape endemic for arboviruses

BACKGROUND

In Brazil and several countries in the Americas, where dengue, chikungunya and Zika are cocirculating, there is a need to understand how different mosquito species relate to landscape and humans. Mosquito ecology and distribution, especially at finer spatial scales, are key factors to study since the relationship of mosquito communities to their habitats might have important consequences in the risk of disease transmission to humans. The aim of this study was to evaluate the diversity of resident culicids along heterogeneous landscapes in different endemic cities for dengue, chikungunya and Zika.

METHODS

Fourteen collection sites were randomly selected in six landscapes characterized as urban, periurban and rural along two endemic metropolitan cities in Rio de Janeiro, Brazil: Itaboraí and Cachoeiras de Macacu, Rio de Janeiro. In each site, adult mosquito collections were performed using different traps and backpack aspiration. Collections took place during the rainy and dry seasons of 2015 and 2016. To measure diversity in each landscape, we generated species accumulation curves and used different indexes: rarefied species richness, Chao1-bc and ACE-1. Mosquito habitat segregation along different land use types was measured with a partial canonical correspondence analysis (pCCA). Generalized linear mixed models (GLMM) were used to model the probability of occurrence of the most abundant species along an Urban-Forest gradient.

RESULTS

A total of 13,462 adult mosquitoes from 10 genera and 41 species were collected. The most abundant species were Culex quinquefasciatus Say, 1823, Aedes aegypti (Linnaeus, 1762) and Aedes albopictus (Skuse, 1895). There was a significant association between rarefied species richness and landscape, with higher richness in the Rural landscape. The number of observed species was matched only in the Urban landscape. Most species showed segregation along an Urban-Forest gradient, and the great majority were associated with forested habitats. We were able to fit prediction models for six mosquito species.

DISCUSSION

The paper discusses the impact of human activities on landscape and its effects on mosquito populations, focusing on the segregation of different known vector species and their proximity to human altered environments. Most of these species are known arbovirus vectors and knowledge of their distribution are key elements that health authorities should take into account when planning arbovirus surveillance and vector control activities.

Surveillance of Culicine Mosquitoes in Six Villages of Taita-Taveta County, Kenya, With Host Determinations From Blood-Fed Females

Culicine mosquitoes are vectors of human disease-causing pathogens like filarial worms and several arthropod-borne viruses (arboviruses). Currently, there has been an increase in emerging and re-emerging vector-borne diseases along coastal Kenya, which has been of major concern in public health. This study aimed at determining culicine mosquito species abundance, diversity and their host feeding preferences in Taita-Taveta County, Coastal Kenya. Entomological sampling was done during the long-wet season (March and May) and long dry season (June to October) 2016-2018. Mosquito sampling was done using CDC light traps and Backpack aspiration for indoor and outdoor environments. All culicine mosquitoes collected were identified morphologically and categorized according to their physiological status. Blood fed culicine mosquitoes were tested for bloodmeal sources using ELISA. In total, 3,278 culicine mosquitoes were collected, of which 738 (22.5 %) were found indoors and 2,540, (77.5 %) outdoors. The mosquitoes consisted of 18 species belonging to four genera: Aedes (7), Culex (8), Mansonia (2), and Coquillettidia (1). Overall, there was high mosquito species diversity (H) in outdoors (H = 2.4339) than in indoors (H = 2.2523), whereas even distribution (EH) was higher in indoors (EH = 0.9064) than outdoors (EH = 0.8266). Majorly the bloodmeals identified were from multiple host sources with (51.6%), single hosts (41.3%), and unidentified (7.2%). This study has demonstrated a high diversity of culicine mosquitoes with relaxed feeding tendencies. These mosquitoes are contributing to mosquito biting nuisance and the likelihood of exposure of populations to diseases of public health.

Minimal tillage and intermittent flooding farming systems show a potential reduction in the proliferation of Anopheles mosquito larvae in a rice field in Malanville, Northern Benin

Background

Irrigation systems have been identified as one of the factors promoting malaria disease around agricultural farms in sub-Saharan Africa. However, if improved water management strategy is adopted during rice cultivation, it may help to reduce malaria cases among human population living around rice fields. This study aimed to assess the impact of the different irrigation practices on malaria transmission, as well as to evaluate the water management system that will best mitigate malaria transmission in Malanville, Benin.

Methods

Knowledge, Attitude and Practice (KAP) study was conducted on 104 households staying on and around the rice fields in Malanville. The study focused on the frequency of mosquito bites and preventive measures against malaria as well as soil preparation and rice planting methods. Mosquito larvae density was assessed in different water management system: continuous flooding (CF) or intermittent flooding (IF), deep tillage (DT) or minimal tillage (MT) and normal levelling (NL) or abnormal levelling (AL) in an experimental hut set-up. Larvae were collected using dipping methods and their density was determined.

Results

Three tillage systems, which include the use of tiller, plow and hoe, were identified on the rice field. Continuous flooding was the only irrigation system used by farmers. Retrospective data from Malanville Health Centre revealed higher malaria cases during rice production season, which was also confirmed by field participants. The density of Anopheles larvae was reduced by 80.8%, 30.8% and 40.7% (P = 0.000) during transplanting, tillering and maturation periods, respectively with intermittent flooding compared to continuous flooding. In addition, a clear reduction of larva density was observed with both intermittent flooding systems applied to minimal tillage (MT + IF + NL) and intermittent flooding applied to deep tillage (DT + IF + AL), showing that intermittent flooding could reduce the abundance of malaria vector in rice fields.

Conclusion

Recommending intermittent flooding technology for rice cultivation may not only be useful for water management but could also be an intentional strategy to control mosquitoes vector-borne diseases around rice farms.

Hand, foot, and mouth disease in Changsha City, China, 2009-2017: a new method to analyse the epidemiological characteristics of the disease

Objectives: By adopting a new method, this study aimed to analyse the epidemiological characteristics of hand, foot, and mouth disease (HFMD) in nine districts and counties (cities) of Changsha City, China, from 2009 to 2017.Methods: The reported HFMD cases were collected in Changsha from 2009 to 2017. The traditional descriptive method and a new method (index system) including six indices (richness index N, Simpson diversity index D, Shannon diversity index H, Berger-Parker dominance index d, Shannon evenness index E, and Morisita-Horn similarity index C) were used to describe the epidemiological characteristics of HFMD in Changsha.Results: There were 214155 HFMD reported in Changsha during the study period. The incidence of the disease was higher in even-numbered years (2010, 2012, 2014, and 2016) than in uneven-numbered years (2009, 2011, 2013, 2015, and 2017), with two peaks in May to June and October to November every year. The age of onset was mainly from 0 to 5 years old, and the death was mainly from 0 to 2 years old. According to occupational classification, districts and counties (cities) had a high degree of similarity of the composition of HFMD, and there was no regional difference.Conclusions: Changsha had a yearly increasing trend of HFMD from 2009 to 2017, and the key population for prevention and control was children aged in 0-5 years old. Seasonal distribution of high incidence and peak incidence were occurred in even-numbered years. The sub-regions of the city shared moderate diversity and high similarity of occupational distribution of HFMD.

Diversity and Molecular Characterization of Mosquitoes (Diptera: Culicidae) in Selected Ecological Regions in Kenya

Mosquitoes play a predominant role as leading agents in the spread of vector-borne diseases and the consequent mortality in humans. Despite reports on increase of new and recurrent mosquito borne-disease outbreaks such as chikungunya, dengue fever and Rift Valley fever in Kenya, little is known about the genetic characteristics and diversity of the vector species that have been incriminated in transmission of disease pathogens. In this study,  mosquito species were collected from Kisumu city, Kilifi town and Nairobi city and we determined their genetic diversity and phylogenetic relationships. PCR was used to amplify the partial cytochrome oxidase subunit 1 (CO1) gene of mosquito samples. Molecular-genetic and phylogenetic analysis of the partial cytochrome oxidase subunit 1 (CO1) gene were employed to identify their relationship with known mosquito species. Fourteen (14) haplotypes belonging to genus Aedes, nine (9) haplotypes belonging to genus Anopheles and twelve (12) haplotypes belonging to genus Culex were identified in this study. Findings from this study revealed a potentially new haplotype belonging to Anopheles genus and reported the first molecular characterization of Aedes cumminsii in Kenya. Sequence results revealed variation in mosquito species from Kilifi, Kisumu and Nairobi. Since vector competence varies greatly across species as well as species-complexes and is strongly associated with specific behavioural adaptations, proper species identification is important for vector control programs.

Biodiversity Pattern of Mosquitoes in Southeastern Senegal, Epidemiological Implication in Arbovirus and Malaria Transmission

The composition, density, diversity, and temporal distribution of mosquito species and the influence of temperature, relative humidity, and rainfall on these data were investigated in 50 sites across five land cover classes (forest, savannah, barren, village, and agriculture) in southeastern Senegal. Mosquitoes were collected monthly in each site between June 2009 and March 2011, with three people collecting mosquitoes landing on their legs for one to four consecutive days. In total, 81,219 specimens, belonging to 60 species and 7 genera, were collected. The most abundant species were Aedes furcifer (Edwards) (Diptera: Culicidae) (20.7%), Ae. vittatus (Bigot) (19.5%), Ae. dalzieli (Theobald) (14.7%), and Ae. luteocephalus (Newstead) (13.7%). Ae. dalzieli, Ae. furcifer, Ae. vittatus, Ae. luteocephalus, Ae. taylori Edwards, Ae. africanus (Theobald), Ae. minutus (Theobald), Anopheles coustani Laveran, Culex quinquefasciatus Say, and Mansonia uniformis (Theobald) comprised ≥10% of the total collection, in at least one land cover. The lowest species richness and Brillouin diversity index (HB = 1.55) were observed in the forest-canopy. The urban-indoor fauna showed the highest dissimilarity with other land covers and was most similar to the urban-outdoor fauna following Jaccard and Morisita index. Mosquito abundance peaked in June and October 2009 and July and October 2010. The highest species density was recorded in October. The maximum temperature was correlated positively with mean temperature and negatively with rainfall and relative humidity. Rainfall showed a positive correlation with mosquito abundance and species density. These data will be useful for understanding the transmission of arboviruses and human malaria in the region.

Receptivity to malaria in the China-Myanmar border in Yingjiang County, Yunnan Province, China

Background

The re-establishment of malaria has become an important public health issue in and out of China, and receptivity to this disease is key to its re-emergence. Yingjiang is one of the few counties with locally acquired malaria cases in the China–Myanmar border in China. This study aimed to understand receptivity to malaria in Yingjiang County, China, from June to October 2016.

Methods

Light-traps were employed to capture the mosquitoes in 17 villages in eight towns which were categorized into four elevation levels: level 1, 0–599 m; level 2, 600–1199 m; level 3, 1200–1799 m; and level 4, > 1800 m. Species richness, diversity, dominance and evenness were used to picture the community structure. Similarity in species composition was compared between different elevation levels. Data of seasonal abundance of mosquitoes, human biting rate, density of light-trap-captured adult mosquitoes and larvae, parous rate, and height distribution (density) of Anopheles minimus and Anopheles sinensis were collected in two towns (Na Bang and Ping Yuan) each month from June to October, 2016.

Results

Over the study period, 10,053 Anopheles mosquitoes were collected from the eight towns, and 15 Anopheles species were identified, the most-common of which were An. sinensis (75.4%), Anopheles kunmingensis (15.6%), and An. minimus (3.5%). Anopheles minimus was the major malaria vector in low-elevation areas (< 600 m, i.e., Na Bang town), and An. sinensis in medium-elevation areas (600–1200 m, i.e., Ping Yuan town). In Na Bang, the peak human-biting rate of An. minimus at the inner and outer sites of the village occurred in June and August 2016, with 5/bait/night and 15/bait/night, respectively. In Ping Yuan, the peak human-biting rate of An. sinensis was in August, with 9/bait/night at the inner site and 21/bait/night at the outer site. The two towns exhibited seasonal abundance with high density of the two adult vectors: The peak density of An. minimus was in June and that of An. sinensis was in August. Meanwhile, the peak larval density of An. minimus was in July, but that of An. sinensis decreased during the investigation season; the slightly acidic water suited the growth of these vectors. The parous rates of An. sinensis and An. minimus were 90.46 and 93.33%, respectively.

Conclusions

The Anopheles community was spread across different elevation levels. Its structure was complex and stable during the entire epidemic season in low-elevation areas at the border. The high human-biting rates, adult and larval densities, and parous rates of the two Anopheles vectors reveal an exceedingly high receptivity to malaria in the China–Myanmar border in Yingjiang County.

Indoor residual spraying with micro-encapsulated pirimiphos-methyl (Actellic® 300CS) against malaria vectors in the Lake Victoria basin, Tanzania

BACKGROUND

The indoor residual spraying programme for malaria vectors control was implemented in four districts of the Lake Victoria basin of Tanzania namely Ukerewe, Sengerema, Rorya andSerengeti. Entomological monitoring activities were implemented in one sentinel village in each district to evaluate the efficacy of pirimiphos-methyl 300 CS sprayed on different wall surfaces and its impact against malaria vectors post-IRS intervention.

METHODS

The residual decay rate of p-methyl 300 CS applied at a target dosage of 1g a.i./m2 on thesprayed wall surfaces was monitored for a period of 43 weeks post-IRSusing the WHO cone wall bioassay method. The bioassays were performed by exposing 2-5 days old unfed susceptible female Anopheles gambiae s.s. (Kisumu strain) to sprayed wall surfaces for a period of 30 minutes. In each sentinel village, mosquito collection was carried out by trained community mosquito collectors. Monthly mosquito collections were carried out from 6.00pm to 6.00am using CDC light traps and clay pot methods for indoors host seekingand outdoors resting mosquitoes respectively. Six traps (2 CDC light traps and 4 clay pots) were set per sentinel village per night for28 consecutive days in a moon. PCR and ELISA were used for mosquito species identification and sporozoite detection, respectively.

RESULTS

Based on the WHOPES recommendation, insecticides should have a minimum efficacy of ≥ 80% mosquito mortality at 24 hours post exposure on the sprayed wall surfaces to be considered effective. In this study, p-methyl 300 CS was demonstrated to have a long residual efficacy of 21-43 weeks post-IRS on mud, cement, painted and wood wall surfaces. Numberof anopheline mosquitoes decreased post-IRS interventions in all sentinel villages. The highest numbers ofanopheline mosquitoes were collected in November-December, 38-43 weeks post-IRS. A total of 270 female anopheline mosquitoes were analyzed by PCR; out of which 236 (87.4%) were An. gambiae s.l. and 34 (12.6%) were An. funestus group. Of the 236 An. gambiae s.l.identified 12.6% (n = 34) were An. gambiae s.s. and 68.6% (n = 162) were An. arabiensis. Ofthe 34 An. funestus group indentified 91.2% (n = 31) were An. parensis and 8.8% (n = 3) were An. rivulorum. The overall Plasmodium falciparum sporozoite rate was 0.7% (n = 2,098).

CONCLUSIONS

Pirimiphos-methyl 300 CS was found to be effective for IRS in the Lake Victoria basin,Tanzania. P-methyl 300 CShas a long residual efficacy on sprayed wall surfaces and therefore it is effective in controlling principal malaria vectors of An. gambiae s.l and An. funestus which rest on wall surfaces after and before feeding.

Malaria impact of large dams at different eco-epidemiological settings in Ethiopia

BACKGROUND

Dams are important to ensure food security and promote economic development in sub-Saharan Africa. However, a poor understanding of the negative public health consequences from issues such as malaria could affect their intended advantages. This study aims to compare the malaria situation across elevation and proximity to dams. Such information may contribute to better understand how dams affect malaria in different eco-epidemiological settings.

METHODS

Larval and adult mosquitoes were collected from dam and non-dam villages around the Kesem (lowland), Koka (midland), and Koga (highland) dams in Ethiopia between October 2013 and July 2014. Determination of blood meal sources and detection of Plasmodium falciparum sporozoites was done using enzyme-linked immunosorbent assay (ELISA). Five years of monthly malaria case data (2010-2014) were also collected from health centers in the study villages.

RESULTS

Mean monthly malaria incidence was two- and ten-fold higher in the lowland dam village than in midland and highland dam villages, respectively. The total surface area of anopheline breeding habitats and the mean larval density was significantly higher in the lowland dam village compared with the midland and highland dam villages. Similarly, the mean monthly malaria incidence and anopheline larval density was generally higher in the dam villages than in the non-dam villages in all the three dam settings. Anopheles arabiensis, Anopheles pharoensis, and Anopheles funestus s.l. were the most common species, largely collected from lowland and midland dam villages. Larvae of these species were mainly found in reservoir shoreline puddles and irrigation canals. The mean adult anopheline density was significantly higher in the lowland dam village than in the midland and highland dam villages. The annual entomological inoculation rate (EIR) of An. arabiensis, An. funestus s.l., and An. pharoensis in the lowland dam village was 129.8, 47.8, and 33.3 infective bites per person per annum, respectively. The annual EIR of An. arabiensis and An. pharoensis was 6.3 and 3.2 times higher in the lowland dam village than in the midland dam village.

CONCLUSIONS

This study found that the presence of dams intensifies malaria transmission in lowland and midland ecological settings. Dam and irrigation management practices that could reduce vector abundance and malaria transmission need to be developed for these regions.

Rice volatiles lure gravid malaria mosquitoes, Anopheles arabiensis

Mosquito oviposition site selection is essential for vector population dynamics and malaria epidemiology. Irrigated rice cultivations provide ideal larval habitats for malaria mosquitoes, which has resulted in increased prevalence of the malaria vector, Anopheles arabiensis, in sub-Saharan Africa. The nature and origin of the cues regulating this behaviour are only now being elucidated. We show that gravid Anopheles arabiensis are attracted and oviposit in response to the odour present in the air surrounding rice. Furthermore, we identify a synthetic rice odour blend, using electrophysiological and chemical analyses, which elicits attraction and oviposition in laboratory assays, as well as attraction of free-flying gravid mosquitoes under semi-field conditions. This research highlights the intimate link between malaria vectors and agriculture. The identified volatile cues provide important substrates for the development of novel and cost-effective control measures that target female malaria mosquitoes, irrespective of indoor or outdoor feeding and resting patterns.

Agricultural chemicals: life changer for mosquito vectors in agricultural landscapes?

BACKGROUND

Although many mosquito species develop within agricultural landscapes where they are potentially exposed to agricultural chemicals (fertilizers and pesticides), the effects of these chemicals on mosquito biology remain poorly understood. This study investigated the effects of sublethal concentrations of four agricultural chemicals on the life history traits of Anopheles arabiensis and Culex quinquefasciatus mosquitoes.

METHODS

Field and laboratory experiments were conducted to examine how sublethal concentrations of four agricultural chemicals: an insecticide (cypermethrin), a herbicide (glyphosate), and two nitrogenous fertilizers (ammonium sulfate and diammonium phosphate) alter oviposition site selection, emergence rates, development time, adult body size, and longevity of An. arabiensis and Cx. quinquefasciatus.

RESULTS

Both mosquito species had preference to oviposit in fertilizer treatments relative to pesticide treatments. Emergence rates for An. arabiensis were significantly higher in the control and ammonium sulfate treatments compared to cypermethrin treatment, while emergence rates for Cx. quinquefasciatus were significantly higher in the diammonium phosphate treatment compared to glyphosate and cypermethrin treatments. For both mosquito species, individuals from the ammonium sulfate and diammonium phosphate treatments took significantly longer time to develop compared to those from cypermethrin and glyphosate treatments. Although not always significant, males and females of both mosquito species tended to be smaller in the ammonium sulfate and diammonium phosphate treatments compared to cypermethrin and glyphosate treatments. There was no significant effect of the agrochemical treatments on the longevity of either mosquito species.

CONCLUSIONS

These results demonstrate that the widespread use of agricultural chemicals to enhance crop production can have unexpected effects on the spatial distribution and abundance of mosquito vectors of malaria and lymphatic filariasis.

Increased malaria transmission around irrigation schemes in Ethiopia and the potential of canal water management for malaria vector control

Background

Irrigation schemes have been blamed for the increase in malaria in many parts of sub-Saharan Africa. However, proper water management could help mitigate malaria around irrigation schemes in this region. This study investigates the link between irrigation and malaria in Central Ethiopia.

Methods

Larval and adult mosquitoes were collected fortnightly between November 2009 and October 2010 from two irrigated and two non-irrigated (control) villages in the Ziway area, Central Ethiopia. Daily canal water releases were recorded during the study period and bi-weekly correlation analysis was done to determine relationships between canal water releases and larval/adult vector densities. Blood meal sources (bovine vs human) and malaria sporozoite infection were tested using enzyme-linked immunosorbent assay (ELISA). Monthly malaria data were also collected from central health centre of the study villages.

Results

Monthly malaria incidence was over six-fold higher in the irrigated villages than the non-irrigated villages. The number of anopheline breeding habitats was 3.6 times higher in the irrigated villages than the non-irrigated villages and the most common Anopheles mosquito breeding habitats were waterlogged field puddles, leakage pools from irrigation canals and poorly functioning irrigation canals. Larval and adult anopheline densities were seven- and nine-fold higher in the irrigated villages than in the non-irrigated villages, respectively, during the study period. Anopheles arabiensis was the predominant species in the study area. Plasmodium falciparum sporozoite rates of An. arabiensis and Anopheles pharoensis were significantly higher in the irrigated villages than the non-irrigated villages. The annual entomological inoculation rate (EIR) calculated for the irrigated and non-irrigated villages were 34.8 and 0.25 P. falciparum infective bites per person per year, respectively. A strong positive correlation was found between bi-weekly anopheline larval density and canal water releases. Similarly, there was a strong positive correlation between bi-weekly vector density and canal water releases lagged by two weeks. Furthermore, monthly malaria incidence was strongly correlated with monthly vector density lagged by a month in the irrigated villages.

Conclusion

The present study revealed that the irrigation schemes resulted in intensified malaria transmission due to poor canal water management. Proper canal water management could reduce vector abundance and malaria transmission in the irrigated villages.

Nutrient enrichment affects immature mosquito abundance and species composition in field-based mesocosms in the coastal plain of Georgia

This study is the first to examine the relationship between nutrient enrichment and oviposition preference as measured by larval mosquito abundance using field-based mesocosms adjacent to wetlands in the Gulf Coastal Plain of Georgia. Agricultural wetlands in this region are known to have higher levels of nutrients and suspended sediments compared with forested wetlands, and previous studies have shown differences in mosquito communities in agricultural and forested wetlands. The purpose of this study was to determine whether nutrient enrichment contributes to the differences in mosquito oviposition preferences as reflected by larval mosquito presence and abundance in wetlands. We predicted that adding nutrients to mesocosms similar to concentrations in agricultural wetlands would favor responses of mosquito species often associated with nutrient-rich agricultural wetlands. Results indicated vegetation height, vegetation stem density, and phosphate levels were greater in fertilized mesocosms compared with nonfertilized mesocosms. The abundance of Aedes albopictus (Skuse) and Culex restuans Theobald was greater in mesocosms treated with fertilizer compared with nonfertilized mesocosms. Nutrient enrichment influenced oviposition and larval abundance of some mosquitoes of medical concern. This study provides evidence that can be used to predict the influence of nutrient enrichment on distribution and abundance of disease vectors and other nuisance mosquito species within an agricultural landscape.

Mosquito vector diversity across habitats in central Thailand endemic for dengue and other arthropod-borne diseases

Recent years have seen the greatest ecological disturbances of our times, with global human expansion, species and habitat loss, climate change, and the emergence of new and previously-known infectious diseases. Biodiversity loss affects infectious disease risk by disrupting normal relationships between hosts and pathogens. Mosquito-borne pathogens respond to changing dynamics on multiple transmission levels and appear to increase in disturbed systems, yet current knowledge of mosquito diversity and the relative abundance of vectors as a function of habitat change is limited. We characterize mosquito communities across habitats with differing levels of anthropogenic ecological disturbance in central Thailand. During the 2008 rainy season, adult mosquito collections from 24 sites, representing 6 habitat types ranging from forest to urban, yielded 62,126 intact female mosquitoes (83,325 total mosquitoes) that were assigned to 109 taxa. Female mosquito abundance was highest in rice fields and lowest in forests. Diversity indices and rarefied species richness estimates indicate the mosquito fauna was more diverse in rural and less diverse in rice field habitats, while extrapolated estimates of true richness (Chao1 and ACE) indicated higher diversity in the forest and fragmented forest habitats and lower diversity in the urban. Culex sp. (Vishnui subgroup) was the most common taxon found overall and the most frequent in fragmented forest, rice field, rural, and suburban habitats. The distributions of species of medical importance differed significantly across habitat types and were always lowest in the intact, forest habitat. The relative abundance of key vector species, Aedes aegypti and Culex quinquefasciatus, was negatively correlated with diversity, suggesting that direct species interactions and/or habitat-mediated factors differentially affecting invasive disease vectors may be important mechanisms linking biodiversity loss to human health. Our results are an important first step for understanding the dynamics of mosquito vector distributions under changing environmental features across landscapes of Thailand.

Ecology and behavior of Anopheles arabiensis in relation to agricultural practices in central Kenya

Ecological changes associated with anthropogenic ecosystem disturbances can influence human risk of exposure to malaria and other vector-borne infectious diseases. This study in Mwea, Kenya, investigated the pattern of insecticide use in irrigated and nonirrigated agroecosystems and association with the density, survival, and blood-feeding behavior of the malaria vector Anopheles arabiensis. The parity rates of adult An. arabiensis from randomly selected houses were determined by examining their ovaries for tracheal distension, and polymerase chain reaction was used to identify the host blood meals. In addition, structured questionnaires were used to generate data on insecticide use. Anopheles arabiensis densities were highest in irrigated rice agroecosystems, intermediate in irrigated French beans agroecosystems, and lowest in the nonirrigated agroecosystem. Anopheles arabiensis adult survivorship was significantly lower in irrigated rice agroecosystems than in irrigated French beans agroecosystems. The human blood index (HBI) was significantly higher in the nonirrigated agroecosystem compared to irrigated agroecosystems. Moreover, there was marked variation in HBI among villages in irrigated agroecosystems with significantly lower HBI in Kangichiri and Mathangauta compared to Kiuria, Karima, and Kangai. The proportion of mosquitoes with mixed blood meals varied among villages ranging from 0.25 in Kangichiri to 0.83 in Kiuria. Sumithion, dimethoate, and alpha cypermethrin were the most commonly used insecticides. The 1st was used mostly in irrigated rice agroecosystems, and the last 2 were used mostly in irrigated French beans agroecosystems. These findings indicate that agricultural practices may influence the ecology and behavior of malaria vectors and ultimately the risk of malaria transmission.

Quantifying species diversity with a DNA barcoding-based method: Tibetan moth species (Noctuidae) on the Qinghai-Tibetan Plateau

With the ongoing loss of biodiversity, there is a great need for fast and effective ways to assess species richness and diversity: DNA barcoding provides a powerful new tool for this. We investigated this approach by focusing on the Tibetan plateau, which is one of the world's top biodiversity hotspots. There have been few studies of its invertebrates, although they constitute the vast majority of the region's diversity. Here we investigated species diversity of the lepidopteran family Noctuidae, across different environmental gradients, using measurements based on traditional morphology as well as on DNA barcoding. The COI barcode showed an average interspecific K2P distance of 9.45±2.08%, which is about four times larger than the mean intraspecific distance (1.85±3.20%). Using six diversity indices, we did not detect any significant differences in estimated species diversity between measurements based on traditional morphology and on DNA barcoding. Furthermore, we found strong positive correlations between them, indicating that barcode-based measures of species diversity can serve as a good surrogate for morphology-based measures in most situations tested. Eastern communities were found to have significantly higher diversity than Western ones. Among 22 environmental factors tested, we found that three (precipitation of driest month, precipitation of driest quarter, and precipitation of coldest quarter) were significantly correlated with species diversity. Our results indicate that these factors could be the key ecological factors influencing the species diversity of the lepidopteran family Noctuidae on the Tibetan plateau.

The role of Anopheles arabiensis and Anopheles coustani in indoor and outdoor malaria transmission in Taveta District, Kenya

BACKGROUND

The scaling up of malaria vector control efforts in Africa has resulted in changing the malaria vectorial systems across different ecological settings. In view of the ongoing trends in vector population dynamics, abundance, species composition and parasite infectiousness, there is a need to understand vector distribution and their contribution to malaria transmission to facilitate future planning of control strategies. We studied indoor and outdoor malaria transmission dynamics and vector population variability of Anopheles mosquitoes in Taveta district along the Kenyan Coast.

METHODS

Anopheles mosquitoes were collected indoors and outdoors in 4 ecologically different villages using CDC light traps (both indoor and outdoor) and aspiration method (day resting indoors; DRI) methods. Mosquitoes were examined for infection with P. falciparum sporozoites and blood feeding preferences using enzyme linked immunosorbent assay (ELISA). The An. gambiae and An. funestus complexes were identified by PCR technique to determine the sibling species composition.

RESULTS

A total of 4,004 Anopheles mosquitoes were collected consisting of 34.9%% (n = 1,397) An. gambiae s.1., 28.1% (n = 1,124) An. funestus s.l., 33.5% (n = 1,340) An. coustani and 3.6% (n = 143) An. pharoensis. A total of 14,654 culicine mosquitoes were collected, mainly Cx. quinquefasciatus. Of the total Anopheles collected, 3,729 were tested for P. falciparum sporozoite infection. The sporozoite transmission was found to be occurring both indoors and outdoors. The overall sporozoite infectivity was 0.68% (n = 2,486) indoors and 1.29% (n = 1,243) outdoors. Indoor and outdoor sporozoite infectivity and the vectorial systems varied across the 4 ecological villages. Entomological inoculation rates for the 4 villages indicate that there was site-to-site variation. In the 4 villages, Mwarusa had the highest EIRs with An. arabiensis, An. funestus and An. coustani contributing to 23.91, 11.96 and 23.91 infectious bites per person per year ib/p/year respectively. In Kiwalwa and Njoro outdoor EIR was significantly higher than indoors.

CONCLUSIONS

This study shows that malaria transmission is occurring both indoors and outdoors. The main vectors are An. arabiensis, An. funestus and An. coustani indoors while An. coustani is playing a major role in outdoor transmission. Effective malaria control programmes, should therefore include tools that target both indoor and outdoor transmission.

Efficacy of aquatain, a monomolecular film, for the control of malaria vectors in rice paddies

Background

Rice paddies harbour a large variety of organisms including larvae of malaria mosquitoes. These paddies are challenging for mosquito control because their large size, slurry and vegetation make it difficult to effectively apply a control agent. Aquatain, a monomolecular surface film, can be considered a suitable mosquito control agent for such breeding habitats due to its physical properties. The properties allow Aquatain to self-spread over a water surface and affect multiple stages of the mosquito life cycle.

Methodology/Principal Findings

A trial based on a pre-test/post-test control group design evaluated the potential of Aquatain as a mosquito control agent at Ahero rice irrigation scheme in Kenya. After Aquatain application at a dose of 2 ml/m² on rice paddies, early stage anopheline larvae were reduced by 36%, and late stage anopheline larvae by 16%. However, even at a lower dose of 1 ml/m² there was a 93.2% reduction in emergence of anopheline adults and 69.5% reduction in emergence of culicine adults. No pupation was observed in treated buckets that were part of a field bio-assay carried out parallel to the trial. Aquatain application saved nearly 1.7 L of water in six days from a water surface of 0.2 m² under field conditions. Aquatain had no negative effect on rice plants as well as on a variety of non-target organisms, except backswimmers.

Conclusions/Significance

We demonstrated that Aquatain is an effective agent for the control of anopheline and culicine mosquitoes in irrigated rice paddies. The agent reduced densities of aquatic larval stages and, more importantly, strongly impacted the emergence of adult mosquitoes. Aquatain also reduced water loss due to evaporation. No negative impacts were found on either abundance of non-target organisms, or growth and development of rice plants. Aquatain, therefore, appears a suitable mosquito control tool for use in rice agro-ecosystems.

The dominant Anopheles vectors of human malaria in Africa, Europe and the Middle East: occurrence data, distribution maps and bionomic précis

Background

This is the second in a series of three articles documenting the geographical distribution of 41 dominant vector species (DVS) of human malaria. The first paper addressed the DVS of the Americas and the third will consider those of the Asian Pacific Region. Here, the DVS of Africa, Europe and the Middle East are discussed. The continent of Africa experiences the bulk of the global malaria burden due in part to the presence of the An. gambiae complex. Anopheles gambiae is one of four DVS within the An. gambiae complex, the others being An. arabiensis and the coastal An. merus and An. melas. There are a further three, highly anthropophilic DVS in Africa, An. funestus, An. moucheti and An. nili. Conversely, across Europe and the Middle East, malaria transmission is low and frequently absent, despite the presence of six DVS. To help control malaria in Africa and the Middle East, or to identify the risk of its re-emergence in Europe, the contemporary distribution and bionomics of the relevant DVS are needed.

Results

A contemporary database of occurrence data, compiled from the formal literature and other relevant resources, resulted in the collation of information for seven DVS from 44 countries in Africa containing 4234 geo-referenced, independent sites. In Europe and the Middle East, six DVS were identified from 2784 geo-referenced sites across 49 countries. These occurrence data were combined with expert opinion ranges and a suite of environmental and climatic variables of relevance to anopheline ecology to produce predictive distribution maps using the Boosted Regression Tree (BRT) method.

Conclusions

The predicted geographic extent for the following DVS (or species/suspected species complex*) is provided for Africa: Anopheles (Cellia) arabiensis, An. (Cel.) funestus*, An. (Cel.) gambiae, An. (Cel.) melas, An. (Cel.) merus, An. (Cel.) moucheti and An. (Cel.) nili*, and in the European and Middle Eastern Region: An. (Anopheles) atroparvus, An. (Ano.) labranchiae, An. (Ano.) messeae, An. (Ano.) sacharovi, An. (Cel.) sergentii and An. (Cel.) superpictus*. These maps are presented alongside a bionomics summary for each species relevant to its control.

Anopheles larval abundance and diversity in three rice agro-village complexes Mwea irrigation scheme, central Kenya

BACKGROUND

The diversity and abundance of Anopheles larvae has significant influence on the resulting adult mosquito population and hence the dynamics of malaria transmission. Studies were conducted to examine larval habitat dynamics and ecological factors affecting survivorship of aquatic stages of malaria vectors in three agro-ecological settings in Mwea, Kenya.

METHODS

Three villages were selected based on rice husbandry and water management practices. Aquatic habitats in the 3 villages representing planned rice cultivation (Mbui Njeru), unplanned rice cultivation (Kiamachiri) and non-irrigated (Murinduko) agro-ecosystems were sampled every 2 weeks to generate stage-specific estimates of mosquito larval densities, relative abundance and diversity. Records of distance to the nearest homestead, vegetation coverage, surface debris, turbidity, habitat stability, habitat type, rice growth stage, number of rice tillers and percent Azolla cover were taken for each habitat.

RESULTS

Captures of early, late instars and pupae accounted for 78.2%, 10.9% and 10.8% of the total Anopheles immatures sampled (n = 29,252), respectively. There were significant differences in larval abundance between 3 agro-ecosystems. The village with 'planned' rice cultivation had relatively lower Anopheles larval densities compared to the villages where 'unplanned' or non-irrigated. Similarly, species composition and richness was higher in the two villages with either 'unplanned' or limited rice cultivation, an indication of the importance of land use patterns on diversity of larval habitat types. Rice fields and associated canals were the most productive habitat types while water pools and puddles were important for short periods during the rainy season. Multiple logistic regression analysis showed that presence of other invertebrates, percentage Azolla cover, distance to nearest homestead, depth and water turbidity were the best predictors for Anopheles mosquito larval abundance.

CONCLUSION

These results suggest that agricultural practices have significant influence on mosquito species diversity and abundance and that certain habitat characteristics favor production of malaria vectors. These factors should be considered when implementing larval control strategies which should be targeted based on habitat productivity and water management.

Population genetic structure of Anopheles arabiensis (Diptera: Culicidae) in a rice growing area of central Kenya

Studies were conducted to examine the population genetic structure of Anopheles arabiensis (Patton) in Mwea Rice Irrigation Scheme and surrounding areas in Central Kenya, under different agricultural systems. This study was motivated by observed differences in malaria transmission indices of An. arabiensis within the scheme compared with adjacent nonirrigated areas. Agricultural practices can modify local microclimate and influence the number and diversity of larval habitats and in so doing may occasion subpopulation differentiation. Thirty samples from each of the three study sites were genotyped at eight microsatellite loci. Seven microsatellite loci showed high polymorphism but revealed no genetic differentiation (FST = 0.006, P = 0.312) and high gene flow (Nm = 29-101) among the three populations. Genetic bottleneck analysis showed no indication of excess heterozygosity in any of the populations. There was high frequency of rare alleles, suggesting that An. arabiensis in the study area has a high potential of responding to selective pressures from environmental changes and vector control efforts. These findings imply that An. arabiensis in the study area occurs as a single, continuous panmictic population with great ability to adapt to human-imposed selective pressures.

The impact of a small-scale irrigation scheme on malaria transmission in Ziway area, Central Ethiopia

OBJECTIVE

To assess the impact of a small-scale irrigation scheme in Ziway area, a semi-arid area in the Central Ethiopian Rift Valley, on malaria transmission.

METHOD

Parasitological, entomological and socio-economic studies were conducted in a village with and a village without irrigation. Blood smear samples were taken from individuals during the dry and wet seasons of 2005/2006. Socio-economic data were collected from household heads and key agricultural and health informants through interviews and questionnaires. Larval and adult mosquitoes were sampled during the dry and short wet seasons of 2006. Female anopheline mosquitoes were tested by enzyme-linked immunosorbent assay for blood meal sources and sporozoite infections.

RESULTS

Malaria prevalence was higher in the irrigated village (19%, P < 0.05) than the non-irrigated village (16%). In the irrigated village, malaria prevalence was higher in the dry season than in the wet season while the reverse occurred in the non-irrigated village. Households with access to irrigation had larger farm land sizes and higher incomes, but also higher prevalence of malaria. Larval and adult abundance of the malaria vectors, Anopheles arabiensis and Anopheles pharoensis, was higher in the irrigated than in the non-irrigated village throughout the study period. Furthermore, the abundance of An. pharoensis was significantly higher than that of An. arabiensis during the dry irrigated period of the year. Canal leakage pools, irrigated fields and irrigation canals were the major breeding habitats of the two vector mosquitoes. Plasmodium falciparum sporozoite infection rates of 1.18% and 0.66% were determined for An. arabiensis and An. pharoensis in the irrigated village. Peak biting activities of the vectors occurred before 22:00 h, which is a source of concern that the effectiveness of ITNs may be compromised as the mosquitoes feed on blood before people go to bed.

CONCLUSION

Irrigation schemes along the Ethiopian Rift Valley may intensify malaria by increasing the level of prevalence during the dry season. To reduce the intensity of malaria transmission in the small-scale irrigation schemes currently in operation in Ethiopia, year-round source reduction by using proper irrigation water management, coupled with health education, needs to be incorporated into the existing malaria control strategies.

Agriculture and the promotion of insect pests: rice cultivation in river floodplains and malaria vectors in The Gambia

BACKGROUND

Anthropogenic modification of natural habitats can create conditions in which pest species associated with humans can thrive. In order to mitigate for these changes, it is necessary to determine which aspects of human management are associated with the promotion of those pests. Anopheles gambiae, the main Africa malaria vector, often breeds in rice fields. Here the impact of the ancient practice of 'swamp rice' cultivation, on the floodplains of the Gambia River, on the production of anopheline mosquitoes was investigated.

METHODS

Routine surveys were carried out along 500 m transects crossing rice fields from the landward edge of the floodplains to the river during the 2006 rainy season. Aquatic invertebrates were sampled using area samplers and emergence traps and fish sampled using nets. Semi-field experiments were used to investigate whether nutrients used for swamp rice cultivation affected mosquito larval abundance.

RESULTS

At the beginning of the rainy season rice is grown on the landward edge of the floodplain; the first area to flood with fresh water and one rich in cattle dung. Later, rice plants are transplanted close to the river, the last area to dry out on the floodplain. Nearly all larval and adult stages of malaria vectors were collected 0-100 m from the landward edge of the floodplains, where immature rice plants were grown. These paddies contained stagnant freshwater with high quantities of cattle faeces. Semi-field studies demonstrated that cattle faeces nearly doubled the number of anopheline larvae compared with untreated water.

CONCLUSION

Swamp rice cultivation creates ideal breeding sites for malaria vectors. However, only those close to the landward edge harboured vectors. These sites were productive since they were large areas of standing freshwater, rich in nutrients, protected from fish, and situated close to human habitation, where egg-laying mosquitoes from the villages had short distances to fly. The traditional practice of 'swamp rice' cultivation uses different bodies of water on the floodplains to cultivate rice during the rainy season. A consequence of this cultivation is the provizion of ideal conditions for malaria vectors to thrive. As the demand for locally-produced rice grows, increased rice farming will generate great numbers of vectors; emphasizing the need to protect local communities against malaria.

Spatial distribution, blood feeding pattern, and role of Anopheles funestus complex in malaria transmission in central Kenya

Studies were conducted to determine the role of sibling species of Anopheles funestus complex in malaria transmission in three agro-ecosystems in central Kenya. Mosquitoes were sampled indoors and outdoors, and rDNA PCR was successfully used to identify 340 specimens. Anopheles parensis (91.8%), A. funestus (6.8%), and Anopheles leesoni (1.5%) were the three sibling species identified. A. parensis was the dominant species at all study sites, while 22 of 23 A. funestus were collected in the non-irrigated study site. None of the 362 specimens tested was positive for Plasmodium falciparum circumsporozoite proteins by enzyme-linked immunosorbent assay. The most common blood-meal sources (mixed blood meals included) for A. parensis were goat (54.0%), human (47.6%), and bovine (39.7%), while the few A. funestus s.s. samples had fed mostly on humans. The human blood index (HBI) for A. parensis (mixed blood meals included) in the non-irrigated agro-ecosystem was 0.93 and significantly higher than 0.33 in planned rice agro-ecosystem. The few samples of A. funestus s.s. and A. funestus s.l. also showed a trend of higher HBI in the non-irrigated agro-ecosystem. We conclude that agricultural practices have significant influence on distribution and blood feeding behavior of A. funestus complex. Although none of the species was implicated with malaria transmission, these results may partly explain why non-irrigated agro-ecosystems are associated with higher risk of malaria transmission by this species compared to irrigated agro-ecosystems.

Malaria vectors and transmission dynamics in Goulmoun, a rural city in south-western Chad

BACKGROUND

Knowledge of some baseline entomological data such as Entomological Inoculation Rates (EIR) is crucially needed to assess the epidemiological impact of malaria control activities directed either against parasites or vectors. In Chad, most published surveys date back to the 1960's. In this study, anopheline species composition and their relation to malaria transmission were investigated in a dry Sudanian savannas area of Chad.

METHODS

A 12-month longitudinal survey was conducted in the irrigated rice-fields area of Goulmoun in south western Chad. Human landing catches were performed each month from July 2006 to June 2007 in three compounds (indoors and outdoors) and pyrethrum spray collections were conducted in July, August and October 2006 in 10 randomly selected rooms. Mosquitoes belonging to the Anopheles gambiae complex and to the An. funestus group were identified by molecular diagnostic tools. Plasmodium falciparum infection and blood meal sources were detected by ELISA.

RESULTS

Nine anopheline species were collected by the two sampling methods. The most aggressive species were An. arabiensis (51 bites/human/night), An. pharoensis (12.5 b/h/n), An. funestus (1.5 b/h/n) and An. ziemanni (1.3 b/h/n). The circumsporozoite protein rate was 1.4% for An. arabiensis, 1.4% for An. funestus, 0.8% for An. pharoensis and 0.5% for An. ziemanni. Malaria transmission is seasonal, lasting from April to December. However, more than 80% of the total EIR was concentrated in the period from August to October. The overall annual EIR was estimated at 311 bites of infected anophelines/human/year, contributed mostly by An. arabiensis (84.5%) and An. pharoensis (12.2%). Anopheles funestus and An. ziemanni played a minor role. Parasite inoculation occurred mostly after 22:00 hours but around 20% of bites of infected anophelines were distributed earlier in the evening.

CONCLUSION

The present study revealed the implication of An. pharoensis in malaria transmission in the irrigated rice fields of Goulmoun, complementing the major role played by An. arabiensis. The transmission period did not depend upon irrigation. Correct use of insecticide treated nets in this area may be effective for vector control although additional protective measures are needed to prevent pre-bedtime exposure to the bites of infected anophelines.

The utility of mosquito-borne disease as an environmental monitoring tool in tropical ecosystems

The intrinsic link between ecosystem health and human health has been firmly established in the literature and has given rise to the development of new multidisciplinary fields of research such as medical geology. An important practical implication of the ecosystem health approach is the utility of human disease outbreaks as indicators of underlying ecosystem disruption. The use of such a bioindicator is particularly relevant in developing countries where monitoring of traditional environmental and ecological indicators is not routinely undertaken. Mosquito-borne diseases appear to have good potential as bioindicators in tropical regions because the burden of disease is high, the disease ecology has a strong environmental component and intensive surveillance systems are well established. Evidence is reviewed regarding the utility of mosquito-borne disease to detect a range of ecosystem insults including: hydro-geological disruption in soil-water systems (e.g. secondary soil salinisation and waterlogging); escalating agricultural intensification; deforestation; and urbanisation. The evidence suggests that overall, mosquito-borne disease is a specific but insensitive indicator, because human modification of natural ecosystems does not always result in increases in disease incidence and can, in some cases, lead to reductions. Nevertheless, mosquito-borne disease remain useful as bioindicators if utilised as a complement to traditional environmental variables in identifying ecological disturbances; they can then assist in directing interventions that are concurrently beneficial to both human health and ecosystem health.

Spatiotemporal dynamics of immature culicines (subfamily Culicinae) and their larval habitats in Mwea Rice Scheme, Kenya

An ecological study was conducted at three study sites in Mwea Rice Scheme, Kenya to identify the diverse aquatic habitats in which culicine mosquitoes thrived and to explore the best strategies for mosquito control in the area. During the 11-month study period, ten habitat categories and 11 culicine species mainly dominated by Culex quinquefasciatus (72.0%) and Culex annulioris (17.9%) were identified from pupae and late instars larval samples. Two of the 11 culicine species, Ficalbia (Mimomyia) plumosa and Uranotaenia spp., have not been reported previously in the study area. Rurumi had more habitat types than either of the other study sites but the least number of mosquito species. In contrast, Karima had the least number of habitat types but significantly higher density of early instars than the other study sites. The relative abundance of late instars and pupae did not vary significantly among study sites. The contribution of different habitat types to larval production varied markedly between seasons and among study sites. Paddies and canals were perennial contributors of culicine mosquito larvae while the other habitat types were important mainly during the wet season. Some habitat types such as ditches, seeps, marshes, and fishpond were absent in some study sites but of great significance in other study sites. C. quinquefasciatus was positively associated with turbidity at all study sites and also negatively associated with emergent vegetation and distance to the nearest homestead in Karima, emergent vegetation in Kiuria, and other aquatic invertebrates in Rurumi. C. annulioris was positively associated with emergent vegetation at all study sites and also with depth in Kiuria. These findings indicate that besides rice fields and associated habitats, a diversity of other aquatic habitats contribute to culicine adult mosquito production in the study area and that environmental factors that determine the occurrence of a particular mosquito species may vary significantly even among areas of similar land use. This information is critical when designing and implementing mosquito larval control programs.

Land use and mosquito diversity in the Peruvian Amazon

Anthropogenic environmental disturbance is a significant factor driving mosquito community composition. However, researchers subjectively define environmental change creating difficulties for cross-study comparison. To examine the relationship between terrestrial change and mosquito composition, we used remote sensing techniques to define spatially explicit land use categories along a gradient with low (rural), medium (peri-urban), and high (urban) anthropogenic influence in the Peruvian Amazon. We found significant differences in mosquito diversity among land use categories. Our results provide baseline data linking mosquito distribution to land use in the Peruvian Amazon and present an easily replicable method of comparison for future research. Creating standardized methods to measure the impact of human influence on the environment is of particular importance in designing targeted public health policies and in predicting disease risk in rapidly changing environments such as the Amazon.

Diversity of riceland mosquitoes and factors affecting their occurrence and distribution in Mwea, Kenya

Knowledge of mosquito species diversity, occurrence, and distribution is an essential component of vector ecology and a guiding principle to formulation and implementation of integrated vector management programs. A 12-month entomological survey was conducted to determine the diversity of riceland mosquitoes and factors affecting their occurrence and distribution at 3 sites targeted for malaria vector control in Mwea, Kenya. Adult mosquitoes were sampled indoors by pyrethrum spray catch and outdoors by the Centers for Disease Control and Prevention light traps. Mosquitoes were then morphologically identified to species using taxonomic keys. The characteristics of houses sampled for indoor resting mosquitoes, including number of people sleeping in each house the night preceding collection, presence of bed nets, location of the house, size of eaves, wall type, presence of cattle and distance of the house to the cowshed, and proximity to larval habitats, were recorded. Of the 191,378 mosquitoes collected, 95% were identified morphologically to species and comprised 25 species from 5 genera. Common species included Anopheles arabiensis (53.5%), Culex quinquefasciatus (35.5%), An. pharoensis (4.7%), An. coustani (2.5%), and An. funestus (1.6%). Shannon's species diversity and evenness indices did not differ significantly among the 3 study sites. There was a marked house-to-house variation in the average number of mosquitoes captured. The number of people sleeping in the house the night preceding collection, size of eaves, distance to the cowshed, and the nearest larval habitat were significant predictors of occurrence of either or both An. arabiensis and Cx. quinquefasciatus. The peak abundance of An. arabiensis coincided with land preparation and the first few weeks after transplanting of rice seedlings, and that of Cx. quinquefasciatus coincided with land preparation, late stage of rice development, and short rains. After transplanting of rice seedlings, the populations of Cx. quinquefasciatus were collected more outdoors than indoors, suggesting a shift from endophily to exophily. These results demonstrate that irrigated rice cultivation has a strong impact on mosquito species occurrence, distribution, abundance, and behavior, and that certain house characteristics increase the degree of human-vector contact.

Distribution of mosquito larvae within the paddy and its implication in larvicidal application in Mwea rice irrigation scheme, Central Kenya

Distribution of mosquito larvae in inundated rice fields is poorly known despite its profound implications in implementation of vector control programs. Based on oviposition behavior of gravid females and biotic and abiotic conditions of the rice field, distribution of mosquito larvae within the paddy may vary greatly. As a guide to implementation of mosquito vector control program targeting the aquatic stages in the rice fields in Mwea, studies were conducted to determine the distribution of mosquito larvae within the paddy. Twenty-eight cages measuring 50 cm3 were distributed randomly within the paddy during the transplanting stage of the rice growth cycle, and were examined twice per week up to the flowering stage to determine mosquito oviposition pattern. A total of 17,218 mosquito larvae were collected at the periphery and a further 17,570 at the center of the paddy. These comprised 7,461 larvae from the genus Anopheles and 27,327 from genus Culex. The number of pupae collected at the periphery was 1,004 and 1.5 times greater than the number collected at the center. Significantly higher counts of Anopheles larvae were collected at the center (1.00 +/- 0.11) than at the periphery (0.55 +/- 0.05) of the paddy during transplanting stage, but the difference was not significant during the tillering stage. In contrast, significantly higher numbers of Culex larvae were collected from the periphery (3.09 +/- 0.39) than at the center (2.81 +/- 0.24) of the paddy. More pupae were also collected at the center than at the periphery of the paddy. These findings indicate the distribution of Anopheles and Culex larvae in rice fields to be nonrandom; however, for successful achievement of an integrated vector control program targeting the diverse mosquito fauna occurring in rice fields, there is need to target the whole paddy for larvicidal application.

Host choice and multiple blood feeding behaviour of malaria vectors and other anophelines in Mwea rice scheme, Kenya

Background

Studies were conducted between April 2004 and February 2006 to determine the blood-feeding pattern of Anopheles mosquitoes in Mwea Kenya.

Methods

Samples were collected indoors by pyrethrum spay catch and outdoors by Centers for Disease Control light traps and processed for blood meal analysis by an Enzyme-linked Immunosorbent Assay.

Results

A total of 3,333 blood-fed Anopheles mosquitoes representing four Anopheles species were collected and 2,796 of the samples were assayed, with Anopheles arabiensis comprising 76.2% (n = 2,542) followed in decreasing order by Anopheles coustani 8.9% (n = 297), Anopheles pharoensis 8.2% (n = 272) and Anopheles funestus 6.7% (n = 222). All mosquito species had a high preference for bovine (range 56.3–71.4%) over human (range 1.1–23.9%) or goat (0.1–2.2%) blood meals. Some individuals from all the four species were found to contain mixed blood meals. The bovine blood index (BBI) for An. arabiensis was significantly higher for populations collected indoors (71.8%), than populations collected outdoors (41.3%), but the human blood index (HBI) did not differ significantly between the two populations. In contrast, BBI for indoor collected An. funestus (51.4%) was significantly lower than for outdoor collected populations (78.0%) and the HBI was significantly higher indoors (28.7%) than outdoors (2.4%). Anthropophily of An. funestus was lowest within the rice scheme, moderate in unplanned rice agro-ecosystem, and highest within the non-irrigated agro-ecosystem. Anthropophily of An. arabiensis was significantly higher in the non-irrigated agro-ecosystem than in the other agro-ecosystems.

Conclusion

These findings suggest that rice cultivation has an effect on host choice by Anopheles mosquitoes. The study further indicate that zooprophylaxis may be a potential strategy for malaria control, but there is need to assess how domestic animals may influence arboviruses epidemiology before adapting the strategy.

Blood-feeding patterns of Culex quinquefasciatus and other culicines and implications for disease transmission in Mwea rice scheme, Kenya

Studies were conducted in Mwea Rice Scheme, Kenya during the period April 2005 and January 2007 to determine the host-feeding pattern of culicine mosquitoes. Mosquitoes were collected indoors and outdoors and tested for human, bovine, goat, and donkey blood meals by an Enzyme-Linked Immunosorbent Assay. A total of 1,714 blood-engorged samples comprising Culex quinquefasciatus Say (96.1%), Culex annulioris Theobald (1.8%), Culex poicilipes Theobald (0.9%), Aedes cuminsi Theobald (1.0%), Aedes taylori Edwards (0.1%), and Mansonia africana Theobald (0.1%) were tested. Except for A. taylori, in which the single blood meal tested was of bovine origin, the other species fed mostly on both bovine (range 73.3-100%) and goats (range 50-100%). Donkeys were also common hosts for all species (range 19.4-23.5%) except A. taylori and M. africana. C. quinquefasciatus was the only species containing human blood meals (0.04), and indoor collected populations of this species had significantly higher frequency of human blood meals (9.8%) compared with outdoor-collected populations (3.0%). Mixed blood feeding was dominant among culicine species comprising 50.0%, 73.3%, 73.5%, 80.6%, and 94.1% of the samples for M. africana, C. poicilipes, C. quinquefasciatus, C. annulioris, and A. cuminsi, respectively. Ten mixed blood meal combinations including a mixture of all the four hosts were observed in C. quinquefasciatus, compared to one blood meal combination for M. Africana, and two combinations for C. poicilipes, C. annulioris, and A. cuminsi. Mixed bovine and goat blood meal was the most common combination among the five culicine species followed by a mixture of donkey, bovine, and goat blood meals. We conclude that culicine species in Mwea are least likely to be vectors of lymphatic filariasis due to their high "preference" for livestock over human hosts, but they present an increased risk for arbovirus transmission particularly Rift Valley Fever virus, in which domestic animals serve as amplification hosts.

Thorsellia anophelis is the dominant bacterium in a Kenyan population of adult Anopheles gambiae mosquitoes

Anopheles gambiae mosquitoes are not known to harbor endosymbiotic bacteria. Here we show, using nucleic acid-based methods, that 16S rRNA gene sequences specific to a recently described mosquito midgut bacterium, Thorsellia anophelis, is predominant in the midgut of adult An. gambiae s.l. mosquitoes captured in residences in central Kenya, and also occurs in the aquatic rice paddy environment nearby. PCR consistently detected T. anophelis in the surface microlayer of rice paddies, which is also consistent with the surface-feeding behavior of A. gambiae s.l. larvae. Phylogenetic analysis of cloned environmental 16S rRNA genes identified four major Thorsellia lineages, which are closely affiliated to an insect endosymbiont of the genus Arsenophonus. Physiological characterizations support the hypothesis that T. anophelis is well adapted to the female anopheline midgut by utilizing blood and tolerating the alkaline conditions in this environment. The results suggest that aquatically derived bacteria such as T. anophelis can persist through mosquito metamorphosis and become well-established in the adult mosquito midgut.

Malaria in Africa: vector species' niche models and relative risk maps

A central theoretical goal of epidemiology is the construction of spatial models of disease prevalence and risk, including maps for the potential spread of infectious disease. We provide three continent-wide maps representing the relative risk of malaria in Africa based on ecological niche models of vector species and risk analysis at a spatial resolution of 1 arc-minute (9 185 275 cells of approximately 4 sq km). Using a maximum entropy method we construct niche models for 10 malaria vector species based on species occurrence records since 1980, 19 climatic variables, altitude, and land cover data (in 14 classes). For seven vectors (Anopheles coustani, A. funestus, A. melas, A. merus, A. moucheti, A. nili, and A. paludis) these are the first published niche models. We predict that Central Africa has poor habitat for both A. arabiensis and A. gambiae, and that A. quadriannulatus and A. arabiensis have restricted habitats in Southern Africa as claimed by field experts in criticism of previous models. The results of the niche models are incorporated into three relative risk models which assume different ecological interactions between vector species. The “additive” model assumes no interaction; the “minimax” model assumes maximum relative risk due to any vector in a cell; and the “competitive exclusion” model assumes the relative risk that arises from the most suitable vector for a cell. All models include variable anthrophilicity of vectors and spatial variation in human population density. Relative risk maps are produced from these models. All models predict that human population density is the critical factor determining malaria risk. Our method of constructing relative risk maps is equally general. We discuss the limits of the relative risk maps reported here, and the additional data that are required for their improvement. The protocol developed here can be used for any other vector-borne disease.

Remote and field level quantification of vegetation covariates for malaria mapping in three rice agro-village complexes in Central Kenya

Background

We examined algorithms for malaria mapping using the impact of reflectance calibration uncertainties on the accuracies of three vegetation indices (VI)'s derived from QuickBird data in three rice agro-village complexes Mwea, Kenya. We also generated inferential statistics from field sampled vegetation covariates for identifying riceland Anopheles arabiensis during the crop season. All aquatic habitats in the study sites were stratified based on levels of rice stages; flooded, land preparation, post-transplanting, tillering, flowering/maturation and post-harvest/fallow. A set of uncertainty propagation equations were designed to model the propagation of calibration uncertainties using the red channel (band 3: 0.63 to 0.69 μm) and the near infra-red (NIR) channel (band 4: 0.76 to 0.90 μm) to generate the Normalized Difference Vegetation Index (NDVI) and the Soil Adjusted Vegetation Index (SAVI). The Atmospheric Resistant Vegetation Index (ARVI) was also evaluated incorporating the QuickBird blue band (Band 1: 0.45 to 0.52 μm) to normalize atmospheric effects. In order to determine local clustering of riceland habitats Gi*(d) statistics were generated from the ground-based and remotely-sensed ecological databases. Additionally, all riceland habitats were visually examined using the spectral reflectance of vegetation land cover for identification of highly productive riceland Anopheles oviposition sites.

Results

The resultant VI uncertainties did not vary from surface reflectance or atmospheric conditions. Logistic regression analyses of all field sampled covariates revealed emergent vegetation was negatively associated with mosquito larvae at the three study sites. In addition, floating vegetation (-ve) was significantly associated with immature mosquitoes in Rurumi and Kiuria (-ve); while, turbidity was also important in Kiuria. All spatial models exhibit positive autocorrelation; similar numbers of log-counts tend to cluster in geographic space. The spectral reflectance from riceland habitats, examined using the remote and field stratification, revealed post-transplanting and tillering rice stages were most frequently associated with high larval abundance and distribution.

Conclusion

NDVI, SAVI and ARVI generated from QuickBird data and field sampled vegetation covariates modeled cannot identify highly productive riceland An. arabiensis aquatic habitats. However, combining spectral reflectance of riceland habitats from QuickBird and field sampled data can develop and implement an Integrated Vector Management (IVM) program based on larval productivity.

Evaluation of four sampling techniques for surveillance of Culex quinquefasciatus (Diptera: Culicidae) and other mosquitoes in African rice agroecosystems

Field studies were conducted in a rice, Oryza sativa L., agroecosystem in Mwea Kenya to compare the efficiency of CO2-baited Centers for Disease Control (CDC) light traps against nonbaited CDC light traps and gravid traps against oviposition traps in outdoor collection of Culex quinquefasciatus Say (Diptera: Culicidae) and other mosquitoes. Collectively, 21 mosquito species from the genera Culex, Anopheles, Mansonia, Ficalbia, and Aedes were captured during the 10-wk study period. Cx. quinquefasciatus was the predominant species in all trap types with proportions ranging from 57% in the nonbaited CDC light traps to 95% in the gravid traps. Significantly higher numbers of Cx. quinquefasciatus and Culex annulioris Theobald were collected in the CO2-baited CDC light traps than in the nonbaited CDC light traps, but the numbers of other mosquito species, including malaria vectors Anopheles arabiensis Patton and Anopheles funestus Giles did not differ significantly between the two trap types. More Cx. quinquefasciatus females were collected in grass infusion-baited gravid traps than egg rafts of this species in oviposition traps containing the same infusion. Although most mosquitoes captured in CO,-baited and nonbaited CDC light traps were unfed, most of those collected in gravid traps were gravid. From these findings, it is concluded that at least in the rice-growing area of Mwea Kenya, CO2-baited CDC light traps in conjunction with gravid traps can be used in monitoring of Cx. quinquefasciatus both for control and disease surveillance.