Moose sickness is a degenerative condition that occurs in moose populations in central and eastern North America that have been infected with the parasitic worm Parelaphostrongylus tenuis

Parelaphostrongylus tenuis

Parelaphostrongylus tenuis is a small parasitic nematode that infects the brain of many ungulates. Its natural host, the white-tailed deer, is unaffected by its presence; other species, however, suffer severe neurologic damage that eventually leads to death...

. The disease is characterized by stumbling, aimlessness and other odd behavior and is often fatal.

The parasite is carried by white-tailed deer who are otherwise unaffected by it. The worm's life cycle is complex. To get the disease, a moose has to eat vegetation inhabited by snails or slugs that have come in contact with infected deer feces.