Effect of Knockdown Resistance on Plasmodium Falciparum Sporozoite Rates in Malaria Vectors in Western Kenya

Abstract

Malaria, a vector borne disease is still a public health problem in sub-Saharan Africa. In western Kenya, malaria is endemic with the predominant vectors being An. gambiae s.s, An. arabiensis and An. junestus. Pyrethroid-based long lasting insecticidal nets and indoor residual spraying are currently major effective tools employed to control malaria vectors in endemic areas in western Kenya. However, they are currently threatened by insecticide resistance. Malaria vectors exhibit varied geographical distributions and An. gambiae s.s, An. arabiensis and An. funestus are the primary vectors in Busia, Ahero and Bondo respectively. Knockdown resistance has been documented as a mechanism of resistance against pyrethroids. Recently, both LlO14S and Ll014F mutations associated with knockdown resistance have been reported in western Kenya. Whether these mechanisms correlate with sporozoite rates in Anopheles gambiae s.l mosquitoes in western Kenya is unknown. The overall aim of this study was to investigate whether knockdown resistance had impact on malaria transmission. Specifically, this study investigated rate of deltamethrin phenotypic resistance; P.falciparum sporozoite rate and Anopheles densities between wild-caught female An. gambiae s. I and An. funestus mosquitoes; determined whether kdr genotypes (L 10 14S and L 10 14F) frequency has association with P. falciparum sporozoite rate in An. gambiae s.s and An. arabiensis; compared the knockdown resistance allele frequency in larval raised and wild-caught female An. gambiae s. I. Samples were col1ected from Ahero, Bondo and Busia in western Kenya. In the laboratory-based experimental design, larval raised adults and wild caught Anopheles were exposed to 0.05% deltamethrin using the WHO tube assays to test for susceptibility. Conventional PCR and real-time PCR were used for species identification and kdr genotyping respectively. Sporozoite rates were determined using ELISA in all wild-caught female mosquitoes. Of 4,360 wild-caught Anopheles sampled, 32.8% were An. funestus and the rest were An. gambiae s.l. A total of2,186 larvae raised adults An. gambiae s.l. were also used for the study. Independent t-test showed that An. Junestus were more phenotypically resistant compare to An. gambiae s.l. (P<O.OOl). Independent t-test also showed that wild-caught adult mosquitoes were more phenotypically resistant than larval raised ones (P<O.OOI). Independent t-test showed that An. funestus had higher sporozoite infectivity rate than An. gambiae s.l (P<O.OOl). Two-way classification chi-square test of association revealed that there was a significant association between kdr east (L 1014S) and sporozoite rate in An. gambiae s.s (X21=5.49, P=O.032). It also showed that the LlO14S and LIOl4F allele frequencies in larvae raised and wild An. gambiae s.l were not statistically significant. The fmding of a higher level of resistance in wild versus larval raised adults could be alluded to other mutations and mechanisms of resistance. It also suggests that wild mosquitoes should be considered in assessment of phenotypic resistance. The observation of high sporozoite rates in An. funestus in Bondo and Busia compared to An. gambiae s.l indicates that this re-emerged species could be driving malaria transmission in the sub-Counties. The significant association between kdr east (L 1014S) and sporozoite rate in An. gambiae s.s, suggest the epidemiological importance of that marker; it could drive malaria transmission. The insignificant difference observed in kdr (LJ014S and Ll014F) frequencies in wild and larval raised adults which did not reflect in phenotypic resistance suggest that other mechanisms of insecticide resistance might contribute to resistance in wild mosquitoes. The findings of this study will help in insecticide resistance monitoring and assessment programmes