Status Of Insecticide Resistance, 2La Chromosomal Inversion And Plasmodium Falciparum Infection Rate In Malaria Vectors In Kwale County, Coastal Kenya

Abstract

There has been increased effort globally to reduce malaria related morbidity and mortality. World Health Organization (WHO) recommends the use of long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) for the control of malaria vectors. Despite both methods contributing greatly to the decline in malaria transmission they both rely on insecticides particularly pyrethroids. The success of malaria control is threatened by several challenges among them being insecticide resistance. This study sought to determine the level of pyrethroid resistance in Kwale County, Coastal Kenya and the effects of the resistance on resting behavior and sporozoite infection rates in malaria vectors. Samples collection was done from Kidomaya and Marigiza villages in Kwale County. Adult mosquitoes were collected from both indoors and outdoors using CDC light traps and prokopack aspirator. The mosquitoes were identified using morphological and molecular techniques. They were tested for the presence of Plasmodium falciparum sporozoites by ELISA and karyotyping of 2La chromosomal conformation was detected by PCR. Mosquito larvae were collected, raised to F1 and tested for phenotypic resistance, genotypic resistance and chromosomal inversion. A total of 1101 Anopheles mosquitoes were collected, 694 as adults and 407 as larvae. Of these, 64.40% belonged to the Anopheles funestus complex and 33.97% to Anopheles gambiae complex. For the Anopheles funestus complex Anopheles funestus s.s was the dominant sub-species while for the Anopheles gambiae complex, Anopheles arabiensis was dominant. Anopheles arabiensis showed resistance to deltamethrin (60.44%) and permethrin (70.42%) while Anopheles gambiae s.s showed resistance to deltamethrin (75%) and susceptibility to permethrin. Anopheles funestus s.l showed 100% susceptibility to both deltamethrin and permethrin. Of 659 mosquitoes tested, 30 anophelines (28 Anopheles funestus and 2 Anopheles gambiae), tested positive for Plasmodium vi falciparum antigen. Despite Anopheles funestus being 100% susceptible they showed the highest infection rate. This could be as a result of low sample size or as a result of minimized contact with insecticides due to early biting times. The allele frequency for the 2La inversion showed a deviation from the Hardy-Weinberg expectations indicative of non-random mating. The 2La inversion frequency was significantly higher in Kidomaya (72.22%) compared to Marigiza (5.56%). There was no association between phenotypic resistance and 2La inversion while it was impossible to test for association between phenotypic resistance and sporozoite infection due to the small sample size. The presence of phenotypic resistance to pyrethroids reported in this study poses a major challenge to malaria control. This highlights the need to intensify resistance management and the search for pyrethroids alternatives. There is need to consider new methods to control outdoor malaria transmission which is on the rise due to behavioral resistance. Although there was no association between insecticide resistance and 2La inversion, evidence of non-random mating in Anopheles gambiae suggests some form of selection which favor individuals with the inversion