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. Lifecycle The lifecycle of P. tenuis is complex and multistaged. Adults lay eggs on the dura mater (the outer layer of the meninges) of the brain or directly into the bloodstream of an infected host. The eggs hatch into first-stage larvae, which travel in the bloodstream to the lungs, where they travel up the respiratory tract, are swallowed, and then pass out of the body in the mucus coating of fecal pellets. Gastropods such as snails and slugs feed upon the mucus coating of the fecal pellets and ingest the larvae. While in the gastropod, the larvae develop into second- and third-stage larvae, which are capable of infection. Gastropods carrying second- and third-stage larvae may be accidentally ingested with plants, which results in the larvae being transmitted to a new host. The larvae then move into the new host's stomach wall and make their way to the CNS, as in white-tailed deer, or the brain as in other ungulates. Once in these tissues, they develop into their adult third stage of life and lay eggs to begin the cycle again. Transmission Adults of P. tenuis can persist for many years in a single host, which allows for many first-stage larvae to be shed in feces. It is quite common in many populations of white-tailed deer, which have built up a strong resistance. After gastropod ingestion, moose or