Large-scale spatial drivers of avian schistosomes in Northern Michigan inland lakes

Avian schistosomes are snail-borne trematode parasites (Trichobilharzia spp.) that can cause a nasty skin rash in humans when their cercariae mistake us for their normal bird hosts. We sought to investigate drivers of the spatial distribution of Trichobilharzia cercaria abundance throughout Northern Michigan lakes. For 38 sites on 16 lakes, we assessed several dozen potential environmental predictors that we hypothesized might have direct or indirect effects on overall cercaria abundance, based on known relationships between abiotic and biotic factors in wetland ecosystems. We included variables quantifying local densities of intermediate hosts, temperature, periphyton growth rates, human land use and hydrology. We also measured daily abundance of schistosome cercariae in the water over a 5-week period, supported by community scientists who collected and preserved filtered water samples for qPCR. The strongest predictor of cercaria abundance was Lymnaea host snail density. Lymnaea density was higher in deeper lakes and at sites with more deciduous tree cover, consistent with their association with cool temperature habitats. Contrary to past studies of human schistosomes, we also found a significant negative relationship between cercaria abundance and submerged aquatic vegetation, possibly due to vegetation blocking cercaria movement from offshore snail beds. If future work shows that these effects are indeed causal, then these results suggest possible new approaches to managing swimmer's itch risk in northern MI lakes, such as modifying tree cover and shallow-water vegetation at local sites.

Direct onshore wind predicts daily swimmer's itch (avian schistosome) incidence at a Michigan beach

Swimmer's itch (SI) is a painful rash caused by skin penetration by free-swimming infectious cercariae of avian schistosomes, snail-borne helminth parasites related to the causative agents of human schistosomiasis. The goal of this study was to determine if commonly collected environmental data could be used to predict daily fluctuations in SI incidence at an inland beach in northwestern Michigan. Lifeguards collected daily data over four summers, including the number of self-reported SI cases, total swimmers, water temperature, wind speed and wind direction. Mixed-effects binomial regression revealed that wind direction, wind speed and time of day were the best predictors of daily SI risk. Swimmers entering the water in the morning or on days with direct onshore wind perpendicular to the shoreline had the greatest SI risk. However, there was a negative effect of wind speed after accounting for direction, where SI risk was greatest on days with a gentle breeze originating directly offshore. These results suggest that at this beach, direct onshore winds generate a surface-water current that causes SI cercariae to aggregate in the shallow waters used by swimmers. Data are needed from additional sites to confirm whether the onshore wind is a generally important driver of SI incidence.