Abstract:
Background: The strategy for malaria vector control in the context of reducing malaria
morbidity and mortality has been the scale-up of long-lasting insecticidal nets to
universal coverage and indoor residual spraying. This has led to significant decline in
malaria transmission. However, these vector control strategies rely on insecticides which
are threatened by insecticide resistance. In this study the status of pyrethroid resistance in
malaria vectors and it's implication in malaria transmission at the Kenyan Coast was
investigated.
Results: Using World Health Organization diagnostic bioassay, levels of phenotypic
resistance to permethrin and deltamethrin was determined. Anopheles arabiensis showed
high resistance to pyrethroids while Anopheles gambiae sensu stricto (s.s.) and
Anopheles funestus showed low resistance and susceptibility, respectively. Anopheles
gambiae sensu lato (s.l.) mosquitoes were further genotyped for L1014S and L1014F kdr
mutation by real time PCR. An allele frequency of 1.33% for L1014S with no L1014F
was detected. To evaluate the implication of pyrethroid resistance on malaria
transmission, Plasmodium falciparum infection rates in field collected adult mosquitoes
was determined using enzyme linked immunosorbent assay and further, the behaviour of
the vectors was assessed by comparing indoor and outdoor proportions of mosquitoes
collected. Sporozoite infection rate was observed at 4.94 and 2.60% in An. funestus s.l.
and An. gambiae s.l., respectively. A higher density of malaria vectors was collected
outdoor and this also corresponded with high Plasmodium infection rates outdoor.
Conclusions: This study showed phenotypic resistance to pyrethroids and low frequency
of L1014S kdr mutation in An. gambiae s.l. The occurrence of phenotypic resistance with
low levels of kdr frequencies highlights the need to investigate other mechanisms of
resistance. Despite being susceptible to pyrethroids An. funestus s.l. could be driving
malaria infections in the area.