Spatial distribution and habitat characterisation of Anopheles larvae along the Kenyan coast

Background & objectives: A study was conducted to characterise larval habitats and to determine spatialheterogeneity of the Anopheles mosquito larvae. The study was conducted from May to June 1999 innine villages along the Kenyan coast.Methods: Aquatic habitats were sampled by use of standard dipping technique. The habitats werecharacterised based on size, pH, distance to the nearest house, coverage of canopy, surface debris, algaeand emergent plants, turbidity, substrate, and habitat type.Results: A total of 110 aquatic habitats like stream pools (n = 10); puddles (n = 65); tire tracks (n =5); ponds (n = 5) and swamps (n = 25) were sampled in nine villages located in three districts of theKenyan coast. A total of 7,263 Anopheles mosquito larvae were collected, 63.9% were early instarsand 36.1% were late instars. Morphological identification of the III and IV instar larvae by use ofmicroscopy yielded 90.66% (n = 2,377) Anopheles gambiae Complex, 0.88% (n = 23) An. funestus,An. coustani 7.63% (n = 200), An. rivulorum 0.42% (n = 11), An. pharoensis 0.19% (n = 5), An.swahilicus 0.08% (n = 2), An. wilsoni 0.04% (n = 1) and 0.11% (n = 3) were unidentified. A subset ofthe An. gambiae Complex larvae identified morphologically, was further analysed using rDNA-PCRtechnique resulting in 68.22% (n = 1,290) An. gambiae s.s., 7.93% (n = 150) An. arabiensis and 23.85%(n = 451) An. merus. Multiple logistic regression model showed that emergent plants (p = 0.019), andfloating debris (p = 0.038) were the best predictors of An. gambiae larval abundance in these habitats.Interpretation & conclusion: Habitat type, floating debris and emergent plants were found to be thekey factors determining the presence of Anopheles larvae in the habitats. For effective larval control,the type of habitat should be considered and most productive habitat type be given a priority in themosquito abatement programm

Shifts in malaria vector species composition and transmission dynamics along the Kenyan coast over the past 20 years

BACKGROUND: Over the past 20 years, numerous studies have investigated the ecology and behaviour of malaria vectors and Plasmodium falciparum malaria transmission on the coast of Kenya. Substantial progress has been made to control vector populations and reduce high malaria prevalence and severe disease. The goal of this paper was to examine trends over the past 20 years in Anopheles species composition, density, blood-feeding behaviour, and P. falciparum sporozoite transmission along the coast of Kenya. METHODS: Using data collected from 1990 to 2010, vector density, species composition, blood-feeding patterns, and malaria transmission intensity was examined along the Kenyan coast. Mosquitoes were identified to species, based on morphological characteristics and DNA extracted from Anopheles gambiae for amplification. Using negative binomial generalized estimating equations, mosquito abundance over the period were modelled while adjusting for season. A multiple logistic regression model was used to analyse the sporozoite rates. RESULTS: Results show that in some areas along the Kenyan coast, Anopheles arabiensis and Anopheles merus have replaced An. gambiae sensu stricto (s.s.) and Anopheles funestus as the major mosquito species. Further, there has been a shift from human to animal feeding for both An. gambiae sensu lato (s.l.) (99% to 16%) and An. funestus (100% to 3%), and P. falciparum sporozoite rates have significantly declined over the last 20 years, with the lowest sporozoite rates being observed in 2007 (0.19%) and 2008 (0.34%). There has been, on average, a significant reduction in the abundance of An. gambiae s.l. over the years (IRR = 0.94, 95% CI 0.90–0.98), with the density standing at low levels of an average 0.006 mosquitoes/house in the year 2010. CONCLUSION: Reductions in the densities of the major malaria vectors and a shift from human to animal feeding have contributed to the decreased burden of malaria along the Kenyan coast. Vector species composition remains heterogeneous but in many areas An. arabiensis has replaced An. gambiae as the major malaria vector. This has important implications for malaria epidemiology and control given that this vector predominately rests and feeds on humans outdoors. Strategies for vector control need to continue focusing on tools for protecting residents inside houses but additionally employ outdoor control tools because these are essential for further reducing the levels of malaria transmission

Mosquito species abundance and diversity in Malindi, Kenya and their potential implication in pathogen transmission

Mosquitoes (Diptera: Culicidae) are important vectors of human disease-causing pathogens. Mosquitoes are found both in rural and urban areas. Deteriorating infrastructure, poor access to health, water and sanitation services, increasing population density, and widespread poverty contribute to conditions that modify the environment, which directly influences the risk of disease within the urban and peri-urban ecosystem. The objective of this study was to evaluate the mosquito vector abundance and diversity in urban, peri-urban, and rural strata in Malindi along the Kenya coast. The study was conducted in the coastal district of Malindi between January and December 2005. Three strata were selected which were described as urban, peri-urban, and rural. Sampling was done during the wet and dry seasons. Sampling in the wet season was done in the months of April and June to cover the long rainy season and in November and December to cover the short rainy season, while the dry season was between January and March and September and October. Adult mosquito collection was done using Pyrethrum Spray Collection (PSC) and Centers for Disease Control and Prevention (CDC) light traps inside houses and specimens were identified morphologically. In the three strata (urban, peri-urban, and rural), 78.5% of the total mosquito (n = 7,775) were collected using PSC while 18.1% (n = 1,795) were collected using the CDC light traps. Using oviposition traps, mosquito eggs were collected and reared in the insectary which yielded 329 adults of which 83.8% (n = 276) were Aedes aegypti and 16.2% (n = 53) were Culex quinquefasciatus. The mosquito distribution in the three sites varied significantly in each collection site. Anopheles gambiae, Anopheles funestus and Anopheles coustani were predominant in the rural stratum while C. quinquefasciatus was mostly found in urban and peri-urban strata. However, using PSC and CDC light trap collection techniques, A. aegypti was only found in urban strata. In the three strata, mosquitoes were mainly found in high numbers during the wet season. Further, A. gambiae, C. quinquefasciatus, and A. aegypti mosquitoes were found occurring together inside the houses. This in turn exposes the inhabitants to an array of mosquito-borne diseases including malaria, bancroftian filariasis, and arboviruses (dengue fever, Yellow fever, Rift Valley fever, Chikungunya fever, and West Nile Virus). In conclusion, our findings provide useful information for the design of integrated mosquito and disease control programs in East African environments

Blood-meal analysis for anopheline mosquitoes sampled along the Kenyan coast

A total of 1,480 Anopheles gambiae sensu lato and 439 An. funestus, collected from 30 sites along the Kenyan coast, were tested by direct enzyme-linked immunosorbent assay (ELISA) for blood-meal identification. Overall, the enzyme-linked immunosorbent assay (ELISA) identified 92 and 87% of the samples tested in An. gambiae s.l. and An. funestus, respectively. Of these, human IgG was detected in 98.97% (n = 1,347) of An. gambiae s.l. and 99.48% (n = 379) of An. funestus. Only 14 (1.03%) of the An. gambiae s.l. had fed on other vertebrate hosts tested, which were bovines, chickens, and goats. Additionally, only 2 An. funestus had fed on goats. In all the 28 sites that had bloodfed mosquitoes, An. gambiae s.l. had a human blood index greater than 0.9. Twenty-five of these sites had a human blood index greater than 0.9 for An. funestus, while the other 3 sites had no bloodfed mosquitoes. The An. gambiae s.l. were tested by polymerase chain reaction (PCR) for species identification. A total of 338 were An. gambiae s.s., 79 were An. arabiensis, and 12 were An. merus. The human blood index was 0.96, 0.91, and 1.0 for An. gambiae s.s., An. arabiensis, and An. merus, respectively. The Plasmodium falciparum sporozoite infection rates were 6.2% for species in the An. gambiae complex and 3.7% for An. funestus. These results emphasize that An. funestus and members of the An. gambiae complex on the Kenyan coast are highly anthropophilic, with nearly all specimens feeding on humans during every blood meal. The results further demonstrated active transmission of P. falciparum sporozoites by the primary vector species. This study suggests that the use of insecticide-treated nets will be effective for controlling biting mosquitoes inside houses along the coast of Kenya

Anopheles gambiae

This study investigated whether Anopheles gambiae s.l. and Anopheles funestus Giles mosquito populations were distributed randomly among houses on the coast of Kenya. Sample means and variances of mosquitoes were estimated from bimonthly pyrethrum spray collections at 30 villages from July 1997 through May 1998. In total, 5,476 An. gambiae s.l. and 3,461 An. funestus were collected. The number of An. gambiae s.l. collected was highest in November/December and lowest in May. The number of An. funestus collected was highest during September/October and lowest during May. As the density of mosquitoes decreased, there was a tendency toward randomness in the distribution. The proportion of An. gambiae s.l. and An. funestus mosquitoes collected per house for each sampling period also showed patterns of clustering, with 80% of An. gambiae s.l. collected from <30% of the houses and 80% of An. funestus collected from <20% of the total houses. The total number of mosquitoes collected from any one house ranged from 0 to 121 for An. gambiae s.l. and from 0 to 152 for An. funestus. This coupled with the results of the variance to mean ratio plots suggests extensive clustering in the distribution of An. gambiae s.l. and An. funestus mosquito populations throughout the year along the coast of Kenya

Estimating Dispersal and Survival of Anopheles gambiae and Anopheles funestus Along the Kenyan Coast by Using Mark–Release–Recapture Methods

Mark-release-recapture (MRR) experiments were conducted with emerging Anopheles gambiae s.l. and Anopheles funestus Giles at Jaribuni and Mtepeni in Kilifi, along the Kenyan Coast. Of 739 and 1,246 Anopheles released at Jaribuni and Mtepeni, 24.6 and 4.33% were recaptured, respectively. The daily survival probability was 0.96 for An. funestus and 0.95 for An. gambiae in Jaribuni and 0.83 and 0.95, respectively, in Mtepeni. The maximum flight distance recorded was 661 m. The high survival probability of An. gambiae and An. funestus estimated accounts for the continuous transmission of malaria along the Kenyan coast. This study also shows that the release of young, emergent female Anopheles improves the recapture rates and may be a better approach to MRR studies

Estimating Dispersal and Survival of Anopheles gambiae

Mark-release-recapture (MRR) experiments were conducted with emerging Anopheles gambiae s.l. and Anopheles funestus Giles at Jaribuni and Mtepeni in Kilifi, along the Kenyan Coast. Of 739 and 1,246 Anopheles released at Jaribuni and Mtepeni, 24.6 and 4.33% were recaptured, respectively. The daily survival probability was 0.96 for An. funestus and 0.95 for An. gambiae in Jaribuni and 0.83 and 0.95, respectively, in Mtepeni. The maximum flight distance recorded was 661 m. The high survival probability of An. gambiae and An. funestus estimated accounts for the continuous transmission of malaria along the Kenyan coast. This study also shows that the release of young, emergent female Anopheles improves the recapture rates and may be a better approach to MRR studies