Parelaphostrongylus tenuis

Parelaphostrongylus tenuis (also known as meningeal worm or brainworm) is a neurotropic nematode parasite common to white-tailed deer, Odocoileus virginianus, which causes damage to the central nervous system. Moose (Alces alces), elk (Cervus canadensis), caribou (Rangifer tarandus), mule deer (Odocoileus hemionus), and others are also susceptible to the parasite, but are aberrant hosts and are infected in neurological instead of meningeal tissue. The frequency of infection in these species increases dramatically when their ranges overlap high densities of white-tailed deer. The lifecycle begins in infected meningeal tissues in the central nervous system (CNS) where adult brainworms lay eggs. The eggs are dislodged from the CNS and pass into the lungs, where they hatch. The larvae are then coughed up, swallowed, and proceed through the gastrointestinal tract. Snails and slugs then serve as intermediate hosts, which are later eaten by ungulates, allowing the process to continue. Changes in climate and habitat beginning in the early 1900s have expanded range overlap between white-tailed deer and moose, increasing the frequency of infection within the moose population.