Large scale patterns of trematode parasitism in a bivalve host: no evidence for a latitudinal gradient in infection levels

Climate is a key driver of parasite transmission and disease dynamics. For trematode parasites, the high temperature sensitivity of transmission between first and second intermediate hosts may lead to higher infection rates with global warming, or spatially with warmer latitudes. However, spatial heterogeneities are common, and local factors are known to play crucial roles in determining infection levels. Using the latitudinal and sea temperature gradient along the New Zealand coastline, we assessed if this temperature sensitivity indeed translates into higher parasite abundance towards lower (i.e. warmer) latitudes in the cockle Austrovenus stutchburyi which serves as second intermediate host for several echinostome trematode species. Seventeen mudflats were sampled, and host densities and infection levels (i.e. metacercariae abundance) were measured for cockles, as well as for whelks and mudsnails (prevalence; first intermediate hosts). No evidence was found for a latitudinal pattern of metacercariae abundance in cockles. Instead, whelk prevalence per site and cockle foot size were found to be the main predictors. This highlights the importance of local factors — in particular, infection levels in first intermediate (i.e. source) hosts. These results indicate that, at least at large spatial scales, the temperature sensitivity of host-parasite systems may be offset by other ecological factors that confer resilience against on-going and predicted climate change.