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Brain worm

Other Names: "moose sickness", Parelaphostrongylus tenuis (P. tenuis), meningeal worm, cerebrospinal nematodiasis, cerebrospinal Parelaphostrongylosis.

What Is It?

brain worm adult on moose's spinal cord
An adult brain worm (P. tenuis) on the
spinal cord of a moose submitted
for diagnosis.
~Photo by DEC's Wildlife Pathology Unit~

Brain worm is the term commonly applied to the parasitic nematode (round worm), Parelaphostrongylus tenuis (P. tenuis), which requires a living host to survive. The parasitic worm is frequently found in the subdural spaces (between the brain and the walls of the cranium) in white-tailed deer, which is its definitive (normal) host. The white-tailed deer is usually unaffected by the parasite; in fact, most adult deer in northern New York are parasitized by P. tenuis.

Although brain worm shows no apparent effect on white-tailed deer, it is often fatal to moose, mule deer, black-tailed deer, elk, caribou/reindeer, llama, alpaca, goats, and sheep.

Life Cycle

brain worm life cycle
~Illustration by Natalie Sacco, NYSDEC~

Life Cycle Description in a Deer

  • Parasitic adult worms lay eggs on the dura mater (the outermost of the three layers of the meninges surrounding the brain and spinal cord) or directly into the bloodstream of the deer (primary host).
  • First stage larvae hatch and enter the deer's bloodstream, travel to the lungs, up the trachea (the deer's windpipe) and enter the mouth where they are swallowed.
  • The larvae then pass through the alimentary canal and are excreted with the fecal pellets. The first stage larvae are now within the mucoid coating on the outside of the deer's fecal pellets.
  • The larvae are picked up by a gastropod, such as a slug or snail, by penetrating the gastropod's foot (part of the snail or slug that is used for locomotion). Once in the gastropod, the nematode larvae mature into an infective second and third stage larvae.
  • Deer inadvertently consume the infected gastropod by feeding on plants.
  • The larvae penetrate the deer's stomach wall and travels along the nerves of the deer until it reaches the spinal cord and moves into the brain.
  • In the brain, the third stage larvae mature into an adult, and the cycle continues without affecting the deer.

Life Cycle Description in a Moose

  • The infected gastropod (slug or snail) is inadvertently consumed by a moose.
  • The parasitic larvae travel to the spinal cord and brain of the moose, as it does in the deer.
  • The nematode disrupts the nervous tissue through mechanical destruction, manipulation, and/or inflammation, resulting in neurologic signs and aberrant behavior of the infected animal. Upon infection, there may be periods where the moose seems to recover as the worm or worms move through different portions of the brain or cord; however, the moose typically shows neurologic signs again after several days. An adult P.tenuis within the brain or spinal cord of moose is often fatal. Death can be the result of paralysis, lack of fear/inappropriate behavior (resulting in motor vehicle strike or being shot by police or Environmental Conservation Officer), inability to feed (starvation) or feeding on inappropriate food items (malnutrition).

Signs

Typically there are no signs of infection in the host (white-tailed deer). When P. tenuis infects a moose, mule deer, black-tailed deer, elk, caribou/reindeer, llama, alpaca, goat, or sheep the following signs may be exhibited:

moose showing signs of brain worm
A cow moose (adult female) exhibiting neurologic signs
of brain worm in Rensselaer County, NY
~Photo by NYSDEC Wildlife Pathology Unit~
  • Ataxia (unsteady gait, loss of voluntary muscle control)
  • Listlessness
  • General weakness
  • Fearlessness
  • Apparent deafness and/or blindness
  • Circling
  • Unusual head tilt or neck position
  • Inability to feed/forage
  • Emaciation
  • Paralysis
  • Fatality

Diagnosis

Definitive diagnosis relies on the detection of adult P. tenuis in the spinal cord or brain during necropsy (animal autopsy).
Review a recent moose necropsy Case Report conducted by DEC's Wildlife Pathology Unit, which detects P. tenuis in the spinal cord of the moose.

Presumptive diagnoses are sometimes based on gross or microscopic evidence of worm damage in the spinal cord or brain, or by clinical signs (listed above) when a necropsy is not conducted.

Management Implications

Brain worm is prevalent in New York's white-tailed deer population, yet they apparently suffer little consequence. Brain worm is typically fatal for moose and captive deer/elk, llama, alpaca, goats and sheep. Affected moose cause public concern due to their unusual behavior and Environmental Conservation Officers, DEC Biologists, or local Law Enforcement are frequently called to the scene to evaluate sick moose. They are usually shot and submitted to DEC's Wildlife Pathology Unit for testing of brain worm, Chronic Wasting Disease, rabies, and cause of death determination.

Over the years several biologists have speculated that New York's large white-tailed deer population with its attendant P. tenuis infections would limit the population growth of moose in New York, however, this has yet to be seen. As of 2010, New York moose populations are continuing to rise and are currently estimated at 800 animals. During 2009-2010 the Wildlife Pathology Unit confirmed brain worm infections in six of the 18 moose examined; the infected moose were found in Clinton, Essex, Oneida, Rensselaer, and Saratoga Counties.