Distribution patterns, dispersal and population genetics of anopheline mosquitoes along the kenyan coast

Abstract

Studies on the distribution patterns, dispersal and population genetics of Anopheles gambiae (Diptera: Culicidae) and Anopheles funestus (Diptera: Culicidae) were conducted at two sites, i.e Jaribuni and Mtepeni in Kilifi, along the Kenyan Coast. Longitudinal sampling of mosquito populations was conducted during the period 2002 - 2003. Day resting indoor collections and all night human biting catches revealed the presence of An. funestus, An. gambiae s.s, An. squamosus, An. coustani. An. nili and An. pharoensis. An. gambiae was the most predominant specie at the Mtepeni site representing 91.8% of the total anophelines captured, with the least being An. funestus which accounted for to 6.7% of the total. At the Jaribuni site, An. funestus represented 85% of the total anophelines compared to 14.6% An. gambiae. Sporozoite ELISA tests on these species revealed no significant differences in the sporozoite rates between An. gambiae and An. funestus from Jaribuni during 2001. In Mtepeni, sporozoite rates were significantly different (p=0.02) between An. gambiae and An. funestus in 2001 and 2002. Variations in Anopheles population density were associated with changes in rainfall, temperature and relative humidity. At Jaribuni, average relative humidity and daily rainfall were significantly associated with An. gambiae abundance. An. funestus abundance was significantly associated with daily rainfall, and maximum and minimum temperatures. At the Mtepeni site, significant associations were observed between An. gambiae and rainfall An. funestus and minumum temperatures (P =0.04) and An. funestus and maximum temperatures (P =0.01) Mark-release-recapture (MRR) experiments were conducted with emergent An. gambiae s.l and An. funestus species to determine the vector dispersal, survival and population sizes yielded recapture rates of 24.6% and 4.33% at Jaribuni and Mtepeni respectively. Mean population size estimates for all Anopheles was estimated at 16,871 for Jaribuni, and 854 for Mtepeni. Daily survival probabilities were 0.96 for An. funestus and 0.95 for An. gambiae at Jaribuni, and 0.95 for An gambiae at Mtepeni. The highest mean distance of dispersal from the release point was 393 meters for An. gambiae and 384 meters for An. funestus at both sites. The population size estimates observed, coupled with the high estimates of survival probability of An. gambiae and An. funestus, facilitate the continuous transmission of malaria along the Kenyan coast. Population genetics analysis using microsatellite DNA was conducted on samples of female An. gambiae from Jaribuni and Mtepeni; and An. funestus from Jaribuni. Eleven microsatellite markers were used to investigate Anopheles population genetic structure, gene flow and effective population sizes. These studies demonstrated that allelic composition and frequencies varied between the dry an wet seasons, suggesting the effect of temporal variations in environmental conditions on the genetic structure of vector populations. Higher inbreeding, confirmed by significant linkage disquilibrium was observed at both Jaribuni and Mtepeni. Effective populations sizes ranged between 11,427-33,067 for An. gambiae in Jaribuni; 4,595-17,968 for An. gambiae in Mtepeni, and 1,859-4,484 for An. funestus in Jaribuni. Overall, observations from this study suggest that Anopheles populations along the Kenyan coast consist of small locally inbred populations bearing a few genetic similarities spread cross seasons. Estimates of effective population size indicate that even though population sizes drop considerably during the dry season, mosquitoes along the coast do not suffer severe population bottlenecks during the dry season, hence a small population of adults with a high survival probability sustains malaria transmission during the dry season. From this study, a better understanding of some aspects of malaria transmission patterns observed along the Kenya coast was derived and the information on population dynamics, population size, dispersal, survival and population genetics presented in the findings from this study will be very important to malaria control programme officers planning malaria control interventions based on vector control.