The author reviews the relationship of meningeal worm (Parelapostrongylus tenuis) and its usual host, the white-tailed deer (Odocoileus virginianus). Important alterations in the environment in the past 100 years have greatly expanded the northern range of white-tailed deer and brought host and parasite into contact with other native cervids such as moose (Alces americana), mule deer (Odocoileus hemionus), and woodland caribou (Rangfier tarandus) in which meningeal worm is highly pathogenic. There is evidence the parasite is spreading westward with deer in the aspen-parklands of Canada. Meningeal worm can apparently have considerable impact on moose populations in endemic areas. Possibly the existence of clinical disease in moose in an area should be regarded as evidence of a much more widespread disease problem which may have eventually a serious impact on the population.