Insecticide resistance intensity in Anopheles gambiae (s.l.) from five malaria epidemiological zones in Kenya

Insecticide-based malaria vector-control interventions have been economically important in reducing malaria prevalence. However, insecticide resistance now threatens the continued efficacy of insecticides in malaria vector control. Monitoring insecticide resistance in mosquito populations is needed to guide the implementation of effective insecticide resistance management (IRM) strategies. Thus the study assessed the levels and intensity of insecticide resistance in Anopheles gambiae (sensu lato) in five malaria epidemiological zones of Kenya, which are subjected to different vector control interventions. Anopheles gambiae (s.l.) mosquito larvae were sampled from Teso in Busia County in the lake malaria-endemic zone, Kwale in Kwale County in the coastal malaria-endemic zone, Kakuma in Turkana County, a malaria epidemic zone, Mwea in Kirinyaga County in the seasonal malaria transmission zone of Central Kenya and Thika in Kiambu County in the low-risk malaria zones. The mosquito larvae were reared into adults, and the Centre for Disease Control (CDC) bottle DD intensity of resistance assays were conducted at 2× , 5× , and 10× the discriminating doses (DD); the WHO guidelines were used to evaluate the percentage knockdown or mortality of the adult vectors. Anopheles gambiae (s.l.) mosquitoes from all the malaria epidemiological zones showed resistance to deltamethrin and pirimiphos-methyl, while mosquitoes from most of the zones were resistant to alpha-cypermethrin. However, the mosquitoes were susceptible to the other insecticides tested, i.e. chlorfenapyr and clothianidin. In cases where resistance was found, the resistance intensity ranged from low to moderate levels. Anopheles arabiensis was the most prevalent species in all the sites except in Busia County, where An. gambiae (sensu stricto) was the majority. The results of this study showed widespread insecticide resistance in An. gambiae (s.l.) to commonly used insecticides in different malaria epidemiological zones in Kenya. Routine surveillance of insecticide resistance through monitoring and subsequent management in the zones of occurrence is a reliable component of evidence-based policy decision-making for mitigating malaria transmission using insecticide-based vector control interventions.