Genetic structure of Schistosoma mansoni in western Kenya: The effects of geography and host sharing

Abstract

We examined the spatial structure of Schistosoma mansoni, a parasite of humans, from natural infections at two levels: across the Lake Victoria basin of Kenya and among snail hosts. Using 20 microsatellite markers we examined geographic patterns of relatedness and population structure of cercariae and found weak, but significant structure detected by some, but not all analyses. We hypothesize structure created by aggregations of clonal individuals or adherence of hosts to local transmission sites is eroded by high amounts of gene flow in the region. This finding also supports previous hypotheses concerning the evolution of drug resistance in the region. Intrasnail dynamics were investigated in the context of aggregation and kin selection theory to determine how relatedness and also sex influence host sharing and host exploitation. Cercarial production did not differ significantly between snails with one or two genotypes suggesting that mixed infections resulted in decreased individual fitness and provides a framework for reproductive competition. Coinfection patterns in snails were independent of parasite relatedness indicating that schistosomes were not aggregated according to their relatedness and that kin selection was not influencing host sharing. Additionally, host exploitation in coinfections (measured by cercarial production) was not negatively correlated with relatedness, as predicted by classical models due to increased competition and thus exploitation when parasites are unrelated. Because of the low levels of relatedness within the population, schistosomes may rarely encounter close relatives and kin selection mechanisms that influence the distribution of individuals within snails or the virulence mode of the parasites may simply have not evolved. KeywordsSchistosoma mansoni; Genetic Structure; Lake Victoria; Kenya; Coinfection; Host Sharing; Competition; Kin Selection Understanding the transmission patterns of pathogens among host populations is an important goal for the control of infectious diseases. Population genetics can be used to reveal patterns of pathogen transmission. For instance, population subdivision into separate transmission cycles, the amount of gene flow among subpopulations, and the role of hosts and geography in structuring subpopulations can be revealed with these methods (Tibayrenc, 1999;Criscione et al., 2005;Archie et al., 2009). This information also can be used to predict evolutionary dynamics of pathogens such as the evolution of drug resistance alleles and local adaptation or divergence of pathogens (Criscione et al., 2005;Huyse et al., 2005 NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptSchistosomiasis is a chronic and debilitating disease that is estimated to infect 207 million people world wide, most of whom are in sub-Saharan Africa (King et al., 2005;Steinmann et al., 2006). Schistosoma mansoni is the primary etiological agent of schistosomiasis in the Lake Victoria basin of Africa, a region where schistosomiasis has a serious impact...

Polymorphism associated with the Schistosoma mansoni tetraspanin-2 gene

“…Diversity, measured as the number of alleles sampled per locus, was lower in the current study than a previous one which sampled schistosomes from snails from the same location (Steinauer et al, 2009), but was not significantly so (paired t-test, P = 0.0632).…”

“… n a , number of alleles for each locus; H o , observed heterozygosityfor each locus; H e , expected heterozygosity for each locus; F is , inbreeding coefficient for each locus. a The number of alleles is compared with rarified allelic richness of the schistosome population (Pop) from a previous study (Steinauer et al, 2009). …”

Inbreeding within human Schistosoma mansoni: do host-specific factors shape the genetic composition of parasite populations?

“…Previous studies hypothesized that the lack of spatial differentiation could be due to the absence of boundaries to water bodies, thereby facilitating host movement among transmission sites (Thiele et al, 2008;Agola et al, 2009;Steinauer et al, 2009). Indeed, the open and flat Sahel region within our study area may impose few restrictions on human host movement.…”

This De-Wormed World?

“…From this will emerge a deeper understanding of the parasites being controlled, with a better ability to predict how control programs will affect their biology. For instance, with schistosomes, Steinauer et al (2009) have noted that in the absence of water body boundaries, gene flow can occur across large geographic distances, potentially providing for rapid dissemination of resistance genes should they become common. Furthermore, recent studies suggest that even in areas where repeated praziquantel treatment occurs, such that prevalence and intensity of infection are reduced, given that there are typically so many people remaining who have not been treated and rates of reinfection are high, there may be little impact of control on the genetic diversity of the schistosomes being targeted (Steinauer, personal communication).…”