Rainfall as a driver of seasonality in parasitism

Parasite burdens are known to vary seasonally in wildlife, and rainfall is one key aspect of seasonality that has been linked to parasitism in a range of systems. Rainfall can have immediate effects on parasitism rates by affecting parasite survival and movement in the environment, or it can have delayed effects by affecting host susceptibility to parasites through changes in host body condition or immune function. In this study, we examined how helminth infection in a wild ungulate (Grant's gazelle, Nanger granti) is impacted by seasonal changes in rainfall. We looked at how the burdens of three helminth parasites varied in relation to current (immediate effect) and prior (delayed effect) rainfall by comparing parasite fecal egg and larval counts to rainfall 0, 1, and 2 months prior to parasite sampling. We found burdens of all three parasites to be negatively associated with rainfall, and that delayed effects were stronger than immediate effects. Our findings implicate rainfall as a driver of seasonal variation in infection and suggest one important mechanism may be through delayed effects on host susceptibility.