Impact of Seasonal Changes on Malaria and Rift Valley Fever Vector Ecology and Infection Status in Baringo County, Kenya

Abstract

The seriousness of vector borne diseases is expected to be altered with ongoing climate change. This is because the vectors responsible for transmission of vector borne diseases (VBDs) are climate sensitive. The current study sought to find out the impact of seasonal variations on rift valley fever (RVF) and malaria vector bionomics in Baringo County. Immature and adult mosquitoes were collected once every month from 24 sites distributed in four ecological zones based on elevation. Larvae were collected from breeding habitats using a dipper and a pipette in 2014 to 2016. Indoor adult mosquitoes were collected by pyrethrum spray collection (PSC) while outdoor adult mosquitoes were collected using Centers for Disease Control and Prevention (CDC) light traps. Both larvae and adults were morphologically identified using taxonomic keys. Anopheles gambiae complex mosquitoes were further genetically identified to sub species level by polymerase chain reaction (PCR). Known vectors of malaria were screened for Plasmodium falciparum parasites by enzyme linked immunosorbent assay (ELISA). Larval species diversity was significantly different (p<0.000) with highest diversity in the cold dry season (H=1.99) and lowest in the long rain season (H=1.59). Out of the nine types of habitats sampled, ditches had the highest mean of anopheline larvae (16.6 per 20 dips) while concrete tanks had the highest mean of culicine larvae (333.7 per 20 dips). Swampy habitats were preferred by both anopheline (12.4 per 20 dips) and culicine (23.5 per 20 dips) larvae. Malaria vectors were more abundant indoors (80.8%) than outdoors (19.2%) while Rift Valley fever (RVF) vectors (culicines) were more abundant outdoors than indoors. Vector abundance was highest in mud wall houses and in houses with open eaves. Most vectors were collected in the lowland (82.5 %) followed by the riverine zone (14.6 %) and these were the only two zones where Anopheles funestus was found. Larger numbers of mosquitoes (3,629) were collected in the drier seasons than wet seasons (2,141). Out of the 635 An. gambiae mosquitoes that were identified to sub species level, 626 (98.6%) were An. arabiensis while 9 (1.4%) were An. gambiae s. s. Plasmodium falciparum infections were detected in mosquitoes collected from the lowland and riverine zones only during dry season. Sporozoite rates were higher in An. funestus (10.0%) than in An. arabiensis (0.35%) though the latter species was most abundant and constituted 93.4% of tested mosquitoes. The dry season presented greater risk for malaria transmission when mosquito species diversity, abundance and sporozoite rates were high. Anopheles arabiensis was the dominant species, but results implicate An. funestus as a key malaria vector in Baringo County. Abundance of outdoor secondary malaria vectors and RVF vectors calls for multiple control strategies that target outdoor mosquitoes. Thus Baringo County Government should implement integrated mosquito control strategies to target both indoor and outdoor mosquitoes in the lowland and riverine zones; where vector abundance and infection rates were highest. Intervention measures should not be limited to the wet seasons only. Simple house modifications such as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves tosuch as screening of eaves to hinder mosquito entry are recommended as part of integrated approaaches to vector control.