ADK Forever Wild: Parelaphostrongylus Tenuis!

As mentioned in the Northern Nature Guide, white-tailed deer can carry a brain parasite that’s usually non-debilitating to them, but fatal to other species such as caribou. During class, someone raised the question of how this parasite is transferred from the white-tailed deer to another animal. Because this scientific inquiry left me curious and intrigued, I decided to do some light research on the subject.

I discovered that the brain parasite carried by white-tailed deer goes by the scientific name Parelaphostrongylus tenuis, or P. tenuis for short. Other common names for this parasite include “moose sickness,” meningeal worm, cerebrospinal nematodiasis, and cerebrospinal Parelaphostrongylosis. P. tenuis is a brain worm carried by most adult deer in northern New York, found in the animal’s subdural spaces located between the brain and the walls of the cranium. Even though white-tailed deer are not apparently affected by this parasite, it is commonly fatal to moose, mule deer, black-tailed deer, elk, reindeer, llama, alpaca, goats, sheep, and, of course, caribou.

So what’s the life cycle of this parasite like? I’m glad you asked. The parasitic adult worm lays its eggs in either a deer’s brain or directly into its bloodstream. Once the larvae hatch, they begin their first stage in the bloodstream, traveling to the lungs, up the trachea, and to the mouth where the deer swallows them. After travelling through the deer’s digestive tract, the parasites are excreted in the mucoid covering of fecal pellets. Once outside of the deer, the parasites enter a gastropod (meaning a slug, snail, etc.) by penetrating its foot, maturing into infective second and third stage larvae within the host. When deer feed on plants and consume infected gastropods, the larvae move through the deer’s stomach wall, following its nerves until they reach the spinal cord and enter the brain. In the brain, the parasites mature to adulthood and the cycle repeats itself. However, if other animals such as caribou ingest an infected gastropod, the parasites still travel to the brain, but instead of harmlessly reproducing, they disrupt nervous tissue and harm the animal. Symptoms of infection include ataxia, listlessness, general weakness, fearlessnesss, apparent deafness and/or blindness, circling, unusual head tilt or neck position, inability to feed/forage, emaciation, paralysis, and fatality.

Unfortunately, P. tenuis is common in northern New York, and if someone thinks an animal is infected, it’s usually shot and sent for testing at the DEC’s Wildlife Pathology Unit. One concern was that the parasite would become a limiting factor to the growth of moose population in New York, but as of 2010, moose populations were still on the rise with a population estimated at 800. Let’s hope this trend continues! Below is an image of the P. tenuis parasite.