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.

The frequency of infection in these species increases dramatically when their ranges overlap high densities of white-tailed deer.

[4] The lifecycle begins in infected meningeal tissues in the central nervous system (CNS) where adult brainworms lay eggs.

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 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.

[4][7] 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.

Deer rarely show any external symptoms of P. tenius infection due to their high acquired resistance.

In 1963, a meningeal brainworm, Pneumostrongylus tenuis, was determined as the etiological agent causing neurological disease in moose.

[20][21][22] White-tailed deer populations in the eastern United States are now known to be commonly infected with meningeal worm.

[9] Deer populations began to move into the southern portions of moose range in the early 1900s following changes in climate,[11] logging, mining, forest fires, and increasing human development.

[14] Upon transmission of the pathogen into moose, the worm causes cerebrospinal nematodiasis, a disease of the nervous system that often results in death.

This disruption of host-pathogen dynamics due to habitat alterations has promoted the spread of this parasite to a naïve host and has potentially contributed to declines in moose abundance and productivity in regions with high deer densities.

[25] In this region, wildlife managers are challenged to predict and attempt to mitigate moose declines resulting from deer-related pathogens.

Because no effective methods to prevent the transmission or infection of brainworm in moose have been found, managers have focused their efforts on decreasing deer densities in these regions.