Trematode Communities of the Appalachian Stream Snail, Elimia proxima: the Importance of Scale in Parasite Ecology Research

Understanding temporal variation of host-pathogen dynamics can be important for predicting disease risks and anticipating how disease systems may change in response to natural or human disturbances. Seasonal changes in weather, especially those associated with changes in temperature or precipitation, are often a key component of temporal changes in infection risk and can have important impacts on disease systems. However, these patterns can be difficult to track due to interannual variation and the need for longer-term, multi-year surveillance efforts. We assessed seasonal and annual changes in the trematode component community of firstintermediate host stream snails, Elimia (= Oxytrema = Goniobasis) proxima, across five streams in the southern Appalachian Mountains. Over three years, we found no evidence of consistent seasonal peaks of trematode infection in E. proxima. There was some across site consistency in infection prevalence over four years, as high prevalence sites tended to maintain higher prevalence from year to year, relative to lower prevalence sites. Additionally, we examined the relationship between prevalence of first-intermediate host infection, weather variables and sitelevel factors, including snail density and water quality metrics. Trematode prevalence was negatively related to total precipitation, which may have been due to the movement of infectious parasite stages and infected hosts downstream during high flows. We found no strong relationship between trematode prevalence and snail density or any of the water quality metrics examined in this study, indicating that snail infection may be driven primarily by definitive host activity. Introduction Temporal variation in environmental conditions can have large impacts on host-pathogen interactions and affect infection rates in human, livestock, and wildlife disease systems (reviewed in Altizer et al., 2006). Seasonal changes are one key element of temporal variation that can drive disease dynamics. For example, peaks in human malaria are driven by increases in mosquito abundance and parasite development during warm, rainy seasons (Hoshen and Morse, 2004). Identifying temporal patterns in infection dynamics can inform treatments for controlling parasite infections, and is critical for advancing our understanding of how host-parasite systems might change in response to natural or human disturbance. Digenetic trematodes (Phylum: Platyhelminthes, Subclass: Digenea) are parasitic flatworms with complex, multi-host life cycles. The basic digenean life cycle involves a series of three hosts, including a molluscan first-intermediate host and a vertebrate definitive host, while second-intermediate hosts may be either invertebrates or vertebrates, depending on the trematode species (Bush et al., 2001). Because of the range of hosts incorporated in their complex life cycles, trematodes play important roles in community and ecosystem level processes. Trematode infection can impact interspecific competition and community composition (Mouritsen and Poulin, 2005; Wood et al., 2007), as well as trophic dynamics and energy flow (Lafferty et al., 2006; Kuris et al., 2008). Previous research on larval trematode component communities of first-intermediate host snails revealed temporal variation in both prevalence and composition (Esch and Fernandez, 1994; Esch et al., 2001). Seasonal peaks of trematode infection have been described in various