Malaria continues to be a major global public health burden, causing 250 million clinical cases and over 1 million deaths each year. Sub-Saharan Africa accounts for 90% of these cases . To combat malaria, insecticide-treated bed nets (ITNs) have emerged as an efficacious and cost-effective malaria prevention tool. Several previous trials conducted in areas of different malaria transmission patterns have demonstrated that ITNs reduce Plasmodium falciparum malaria transmission by 70-90%. Most importantly, these trials have provided substantial evidence that use of ITNs significantly reduces all-cause mortality and malaria morbidity in children less than five years of age . Additionally, ITNs have been associated with significant reduction in the adverse effects of malaria during pregnancy . The remarkable effectiveness of ITNs has led to an up-scaling of their use in malaria endemic regions in conjunction with other control and prevention measures . Recently, the World Health Organization (WHO) reported that in countries where ITNs have been effectively scaled up, substantial reductions in malaria cases and deaths have occurred .
Insecticide-treated bed nets work by killing mosquitoes on contact and also by repelling or deterring the vectors from entering houses, thereby reducing malaria transmission . Thus, the use of ITNs or increased distribution of ITNs not only affects the mosquito populations but also changes the dynamics of parasite dispersion in both human hosts and mosquito vectors, which could in turn modify vector-parasite-host interactions, ultimately affecting parasite populations. Several studies have shown that significant suppression of mosquito populations, changes in species distribution and vector behaviour, and changes in population genetic structure and susceptibility of mosquitoes to insecticides are associated with community-based ITNs intervention [6–9]. However, there are still information gaps on how the reduction in malaria transmission by ITNs affects parasite population genetic structure although there were a few earlier studies that reported no change in the proportion of multiple infections after transmission reduction by use of ITNs and curtains [10, 11].
Human Plasmodium parasites undergo asexual multiplication in the human host and obligate sexual reproduction in the mosquito vector, each stage shaping the parasite population genetic structure. Although the asexual multiplication by haploid parasites in humans is clonal, polymorphism can arise from insertion/deletion of tandem repeats through slippage in the parasite DNA sequences or natural mutations from various pressures in the host-parasite relationship [12–14]. On the other hand, transmission of malaria parasites from human to mosquito, which is solely accomplished by a small number of infective male and female gametocytes generated in humans, creates an opportunity for the generation of parasite diversity and the emergence of novel genetic traits . The parasite sexual reproduction stage in mosquitoes allows for recombination and re-assortment of genetic material between genomes of gametes to form diploid zygotes during the development of oocysts. The degree of inbreeding or outcrossing in mosquitoes influences the number of clones that are infective to human . Numerous factors including host responses can indirectly influence competitive advantages or suppression of specific parasite clones. However, the level of transmission intensity has a direct effect on the number of infected hosts and number of parasite clones per infected individual, which affects parasite population genetic structure in different endemic settings [17, 18]. Therefore, it is important to evaluate whether, and how, the transmission reduction (by use of ITNs or other methods), particularly in high transmission areas, affects the parasite population including the extent of multiple infections, genetic diversity, genes involved in transmission, drug resistance and polymorphism of vaccine candidate genes.
The study of parasite population structure explores the extent of genetic diversity, allele frequency, genotype distribution and degree of genetic admixture among other measures using statistical methods . Common statistical measurements include expected heterozygosity (He) to test genetic variation, linkage disequilibrium (LD) to assess association of alleles between loci, and fixation index (FST) to evaluate population differentiation [20–22]. Natural Plasmodium parasite populations display extensive genetic variability within species at different geographic locations and different transmission intensity levels, with no predominant overall structure. Some studies of P. falciparum population structure report that areas with intense malaria transmission have higher He and higher rates of outcrossing and recombination which breaks LD, resulting in a more panmictic population structure [17, 23, 24]. Such settings allow faster emergence of novel genotypes reflected as multiple infections. The reverse is expected where transmission is lower with consequent lower He, stronger LD, and higher degree of selfing, resulting in a more clonal parasite population structure [17, 25, 26]. Yet other studies in areas with high malaria transmission have observed strong LD and non-random distribution of specific genotypes, implying inbreeding may be more extensive than expected even in areas with perennial transmission [27, 28]. Although the conflicting results generated from different geographic regions could be partially due to the differences in genetic markers used, methods for estimation of allele frequencies or sampling of parasites at different life cycle stages, they underscore the need to study the relationship between the transmission intensity and the P. falciparum population genetic structure in same locality where changes in transmission intensity can be monitored. Interventions which impose reduction in transmission, such as ITNs at high coverage in malaria-holoendemic areas, provide a field experimental system for research on these questions. The information from such studies is also useful in designing molecular surveillance systems for ITNs and for other adjunct control programmes [29, 30].
This study is part of a two-phase large-scale community-based trial conducted in western Kenya and designed to investigate the impact of ITNs on malaria morbidity and all cause mortality. The overall goal of these parasite population genetics studies is to assess the effects of transmission reduction by ITNs on the population genetic structure of P. falciparum parasites for a sustained period. The current study employed eight single copy multilocus neutral microsatellite markers to study the genetic diversity of P. falciparum using blood stage parasites collected from children less than five years old in the same area prior to and five years after the introduction of ITNs. Genetic diversity of the parasites between the baseline and post-ITNs was assessed by quantifying the extent of multiple infections, allele frequencies, He, LD, and genetic differentiation.