CONSIDERING WEATHER-ENHANCED TRANSMISSION OF MENINGEAL WORM, PARELAPHOSTRONGYLUS TENUIS, AND MOOSE DECLINES

The risk of meningeal worm (Parelaphostrongylus tenuis) infection in white-tailed deer (Odocoileus virginianus) and neurologic disease in moose (Alces alces) in eastern North America is influenced largely by the effects of weather on deer density and gastropod intermediate hosts. Frequent, easy winters result in high survival and density of deer with a large proportion of young animals that shed up to 3 x more P. tenuis larvae; both greatly increase the production of first-stage larvae. An early spring increases survival of shed larvae by reducing the timing mismatch between the parasite’s “spring rise” and snow melt; larvae deposited into snow experience high mortality. A wetter and longer growing season with moderate temperatures increases the survival of first-stage larvae dispersed in soil, and the density, mobility, and frequency of infected gastropods, including the abundance of infective larvae in them. This weather-enhanced transmission further increases larval output by reducing the proportion of unproductive unisexual infections in deer. High production of larvae and optimal conditions for gastropods increase rates of transmission to co-habiting moose and the occurrence of neurologic disease which is dose-dependent. The density of infected deer at the northern limit of their range is typically limited by winter severity allowing coexistence of deer, moose, and parasite. However, as in Nova Scotia and northwestern Minnesota and adjoining regions, pronounced and prolonged moose declines associated with sustained high deer densities and meningeal worm infection have occurred twice in the past 95 years. These two regions may be prone to extended periods of mild winters and longer, wetter growing seasons that ultimately enhance abundance and transmission of the meningeal worm implicated in moose population declines