Ecological and physiological adaptations of malaria vectors in a semi-arid area in Baringo district, Kenya

Abstract

Longitudinal entomological sampling was executed to study physiological and ecological adaptations of malaria vectors at two semi-arid villages (Kamarimar and Tirion) in Kenya. Adult mosquitoes were sampled using CDC light traps and pyrethrum spray catch collections. Mosquito larvae were sampled using standard dipping methods. Parity rates, length of gonotophic cycle duration and insemination rates of female mosquitoes were assessed. Host choice and sporozoite rates were determined by ELISAs. Sub-species of the Anopheles gambiae complex were identified by PCR. Out of 16,683 mosquitoes collected in both villages An. gambiae si was most predominant followed by An. pharoensis , An. funestus and An. coustani. An. arabiensis was the most predominant member of the An. gambiae complex. Only 13.77% of all adults were blood-fed and 0.02% were Plasmodium falciparum-infected. Some 30.44% of the mosquitoes had fed on humans; 28.46% and 42% had fed on bovines and goats, respectively. Larval productivity was low all year. During the wet season, larvae occupied all habitats but more so in pan dams. All hoof prints, stream beds, tyre tracks, concrete tanks and ditches were rain-fed and produced larvae for short periods only. Habitat stability was positively associated with larval density. Duration of gonotrophic cycles varied significantly between wet and dry seasons. Christopher stage I incidence differed significantly between dry and wet seasons. Prevalence of stage II, III, IV and V did not vary with season. There was no significant difference in insemination frequency between seasons. Breeding of malaria vectors was predominantly driven by human factors. An. arabiensis and An. funestus bred at low levels in permanent breeding sites throughout the year. Permanent water sources available during the dry season served as innocula that provided “larval seed” to fresh habitats formed during the rainy season. The vectors fed mainly on humans and to a lesser but substantial extent on goats and bovines. Malaria transmission was undetectable through most of the year but occurred at low-level in a few wet months. The duration of gonotrophic cycles was longer during the wet than the dry season indicating that these vectors adapt to hot dry weather by ovipositing more frequently. Shortened gonotrophic cycles during peak dry seasons apparently compensated for the low vector densities.