Temporal and spatial variation in the transmission of parelaphostrongylid nematodes (brainworm (Parelaphostrongylus tenuis), muscle worm (Parelaphostrongylus andersoni)) is regulated by the environment and their hosts. White-tailed deer (Odocoileus virginianus) are the primary ungulate host, depositing first-stage larvae into the environment. Gastropods (slugs and snails) are required by these parasites for development to the stage that infects ungulates. Gastropod distributions vary temporally and in response to environmental variation, but further characterization is needed. Habitat preferences of infected gastropods remain unknown due to low prevalence observed in naturally infected hosts. The first chapter used transect sampling to investigate the effect of temporal variation and several environmental factors on gastropod presence and abundance in habitat where moose (Alces alces) are recovering after population decline. Site and month influenced gastropod species composition, driven by three host species (Deroceras laeve, Succinea ovalis, Zonitoides arboreus). Statistical modelling explained gastropod presence better than abundance. Presence varied spatially and was positively correlated with soil pH and humidity, being most likely at a site where deer are absent, indicating that transmission risk would increase if deer were to begin occupying it. The second chapter concerned gastropod microhabitat preferences and vertical climbing behaviour in an urban park and forest inhabited by deer infected with parelaphostrongylids. From June-October, gastropod presence increased and was associated with higher humidity. Abundance also increased over time, and was associated with higher humidity, along with lower air temperature and soil moisture. Infected gastropods were found, but none exhibited vertical climbing behaviour. These results bring further insight into parelaphostrongylid transmission by identifying factors that influence the gastropod habitat selection, which play a significant role in the life cycles of these parasites. Population declines in wild North American ungulates of conservation concern may be caused in part by the pathology and mortality resulting from infection with parelaphostrongylid nematodes.