Effects of insectcide-treated bednets(ITNs) on genetic polymorphism of the plasmodium falciparum merozoite surface protein-1(MSP-1 19kDa) in a malaria endemic region of Western Kenya

Abstract

The use of insecticide-treated bednets (ITNs) to protect children and pregnant women in endemic areas has been shown to significantly reduce the adverse effects of malaria, a leading cause of morbidity and mortality in developing countries. As a result, large-scale ITN programs have been initiated in many regions. However, immuno-epidemiological studies have demonstrated that acquired immunity to malaria develops gradually with age and requires frequent contact with parasites in order to be maintained. Immune-mediated pressure is considered as an important mechanism by which antigenic polymorphisms are generated in parasite populations. Therefore, this study hypothesized that the significant reduction in parasitaemia associated with the use of ITNs could result in lowered immune pressure, leading to differences in gene polymorphisms in antigens exposed to the host immune system. The current study investigated genetic polymorphisms in a leading P. falciparum vaccine candidate antigen, blood-stage Merozoite Surface Protein-1 (MSP-119_ kDa). Mutations in the highly conserved 19 kilodalton C-terminal domain of the MSP-1 gene were investigated by standard polymerase chain reaction (PCR) and cycle sequencing using an automated ABI 3 100 DNA sequencer. DNA from parasitized blood samples collected in cross-sectional surveys conducted at baseline and at 14, 30, and 46-months post-bednet intervention was analyzed. Samples from intervention and control areas were matched for age and gender. Chromas Pro, Molecular Evolutionary Genetic Analysis (MEGA) and SAS programs were used for data analysis. Point mutations in the MSP-119-kDa were explored using the statistic, the average number of amino acid substitutions between any two sequences and haplotype diversity (Hd), a population genetic index of allelic diversity. Differences in the proportion of haplotypes and were compared by randomization methods. Differences in proportions of haplotypes between ITN and control areas and between surveys were compared by Chi-square test. A total of 24 distinct haplotypes were detected in both ITN and control villages in all the crosssectional surveys. Out of these, eight haplotypes are new and have not been reported in literature. Consistent with earlier studies conducted in the same study area, the E-KESNG-L and Q-KESNG-L haplotypes were the most prevalent (29-40%) in Asembo Bay. The existence of a great number of rare haplotypes (>10) reveals a greater degree of diversity at this locus than previously reported. There was no significant difference in parasite diversity between ITN and control areas and overtime, suggesting that the use of ITNs does not affect parasite diversity. This molecular data will provide useful information of public health relevance in the current and future design and deployment of malaria control strategies including MSP-119_kDa-based vaccines