EFFECTS OF PYRETHROID RESISTANCE ON THE FITNESS AND LIFE HISTORY TRAITS OF Anopheles gambiae s.s IN BUNGOMA, WESTERN KENYA

Abstract

Pyrethroid based vector control tools have played a major role in malaria control. However, Anopheles gambiae s.s from Bungoma has shown increased insecticide resistance to pyrethroids. Mosquito life traits such as larval development, gonotrophic cycle, fecundity and longevity are an integral part of the vectorial capacity of malaria vectors that influence malaria transmission. Studies have reported effects of pyrethroid resistance on the life history traits of Aedes mosquitoes but its effects on the biological fitness of An. gambiae a major malaria vector remain largely understudied yet knowledge of these traits is important for improving insecticide resistance management strategies. The main objective of this study was to determine the effect of pyrethroid resistance on An. gambiae fitness as measured by life-history traits. Established colonies of An. gambiae s.s collected from Bungoma were used in the study. The insecticide resistant selected colony was exposed to 0.05% deltamethrin while the unselected colony was not exposed to any insecticide. The susceptible An. gambiae s.s Kisumu laboratory reference strain was used as the control. The study applied an experimental design involving the construction of larval and adult life tables so as to evaluate the fitness and biological traits of the selected and unselected colonies. The larvae and adult mosquitoes used were randomly selected from the insectary. The first larval instars were reared while recording the surviving larvae and their development stage until the adults emerged. The larval development time, pupation rate and adult emergence rate was then determined. Gravid females were individually allowed to lay eggs which were counted and recorded and their gonotrophic cycle measured. The adult longevity was also measured by rearing female mosquitoes in a cage in the malaria sphere then the dead mosquitoes were recorded, recorded daily. The average female adult survival rate, fecundity and longevity was calculated. Turkey HSD test compared the significance of insecticide resistance on development time, gonotrophic cycle, fecundity and longevity. Kaplan-Mier survival analysis determined the effect of insecticide on adult mosquito survivorship. The mean pre-marginal development time for the females in selected colony (12.1 ± 0.3) was significantly longer than that of the unselected colony (9.6 ± 0.2 days; p<0.05). The gonotrophic cycle for the selected colony (6.07 ± 0.83 days) was also significantly longer than that of the unselected colony (4.27 ± 1.14 days; p<0.05). On the contrary the female longevity of the selected (39.68 ± 1.6 days) was significantly longer than that of the unselected colony (29.86 ± 1.1 days; p<0.05). This study showed parathyroid resistance delayed the development, elongated the gonotrophic cycle, reduced fecundity and increased longevity of An. gambiae s.s. The study findings are important in understanding the benefits and demerits of pyrethroid resistance on malaria which could be useful in shaping tailor made resistant management strategies with the target of eliminating malaria..