Parasitic flatworms in the family Schistosomatidae infect millions of people worldwide and contribute to thousands of deaths each year. Swimmer’s itch is a nasty rash caused by schistosome species that normally infect snails and birds. Snails release thousands of cercariae that swim through the water and burrow into human skin. This model system was used to conduct a survey of 14 sites on 8 lakes in northern Michigan. The goal of this survey was to fill knowledge gaps in the literature by tracking daily fluctuations of cercaria abundance in the water and to identify environmental predictors of swimmer’s itch transmission.
Temporal and spatial data on parasite and host populations, as well as environmental variables were collected. Variables measured included water temperature, light intensity, water nutrient levels, periphyton growth, and riparian land use. Snail populations were assessed using visual quadrat surveys. Daily cercaria samples were collected from each site from July 6 through August 2, 2015 by filtering 50 liters of surface water through a nitex mesh filter. Cercarial DNA was extracted from these samples and quantified using qPCR. R was used to perform multiple linear regression models for the site-level analyses.
Results/Conclusions
Preliminary analyses of site-level response variables showed several correlations with environmental predictors. When predicting cercaria abundance, snail density (β=0.22,p<0.05) and percent forested area in the watershed (β=0.02,p<0.05) were both significant predictors with an overall model fit of Adj R2=0.64. Host snail density was significantly predicted by urbanized land cover within one mile of the shoreline (Adj R2=0.39, β=0.97, p<0.01). Other positive correlates of aquatic snails included light intensity and periphitic algae. Periphyton growth was explained by light intensity (β=0.13,p<0.01) and percent lawn coverage at the site (β=224,p<0.05), both as significant predictors (Adj R2=0.64). We did not find significant effects of nutrient concentrations on periphyton, snails, or cercariae.
These results may support several mechanistic hypotheses for the variable prevalence of swimmer’s itch in Michigan. Links between urbanization, water clarity, periphyton, snails, and cercariae have been attributed to chemical run-off in previous studies. Experiments have shown that herbicides and insecticides can increase snails via direct and indirect effects on arthropod predators and periphyton. These mechanisms warrant further exploration, especially as observations point to shifting benthic communities in Michigan lakes. This study will help to inform management efforts for swimmer’s itch and may provide baseline data to build predictive models of exposure risk in other snail-trematode systems such as human schistosomiasis.