Thermal preferences of bird schistosome snail hosts increase the risk of swimmer's itch

Abstract

Ambient temperature strongly affects host parasite interactions, especially when both are ectothermic. Bird schistosomes, which cercariae are known as agents of swimmer's itch and their snail hosts can be a good example of this phenomenon. The snails of these parasites play the key role, as the source of harmful larvae. Cercarial dermatitis is noted even in areas when prevalence of parasites in snail populations is very low. The main question is what adaptation in snail-fluke association can lead to a sufficient number of cercariae causing swimmer's itch in lake water? The influence of ambient temperature on snail survival and cercarial production as well as the thermal preferences of two host species naturally infected with bird schistosomes were studied. The 24-h preferences of Lymnaea stagnalis infected with Trichobilharzia szidati, and Planorbarius corneus infected with Bilharziella polonica were recorded using an oblong thermal gradient set (OTGS). Both cercariae releasing hosts of bird schistosomes preferred a significantly lower temperature than non-infected snails. Additionally, at a higher temperature, the survival of snail hosts was shortened as a result of the increase in daily cercarial expulsion. An especially interesting result concerns the release of a significantly larger total cercariae number by L. stagnalis at lower than at higher temperatures. These data indicate that preferences of infected snails to low temperature microhabitats can increase host survival and parasite success, as well as affecting the increase in the number of invasive larvae in the environment increasing the risk of swimmer's itch.

Keywords: Bird schistosomes; Cercarial dermatitis; Snail host; Thermal preferences.