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P. tenuis (Parelaphostrongylus tenuis) – What You Should Know

P. tenuis (aka meningeal worm, M-worm, or brain worm) is a parasitic nematode (roundworm) that, when accidentally ingested by an aberrant host (alpaca, llama, sheep, and goat), can cause serious neurologic disease and/or death. The infective parasite travels to the spinal cord (meninges) and sometimes the brain, causing inflammation and damage to the nerves. It can take 30-60 days from ingestion of infective stage larvae to the appearance of symptoms. Some neurological signs may include a wobbly hind end, inability to stand, stumbling, circling or head tilt, etc.

(Aberrant host is an accidental or dead-end host, which means that they do not spread infective larvae, and cannot give M-worm directly to another pasture mate)


Non-infective, L1 larvae are carried and shed by a definitive host, the White-Tailed Deer (WTD). Nearly 80% of deer in our region shed this larva through their feces but are asymptomatic. Gastropods (snails & slugs) subsequently pick up the L1 larva from WTD feces. The L1 larvae develop into the infective, L3 stage within the gastropod.

Aberrant hosts become infected by ingesting the gastropod themselves, or food contaminated by the gastropod’s mucus trail. The gastropod must have the infective, L3 stage larva to cause disease in your herd.

EquidDoc veterinarians Treatment

It is possible for your animal to recover from P. tenuis if it is caught early and treated aggressively. Treatment consists of deworming and strong anti-inflammatory medications, and some extra TLC, especially if your animal is unable to stand.  Unfortunately, as prey animals, our alpacas, llamas, sheep, and goats don’t often show signs of disease to their caregivers until it is too late. Be observant! Prevention truly is best practice!


The best way to prevent P. tenuis is to use the lifecycle against itself! This means making your property unwelcoming to deer and snails & slugs.

  1. Remove or reduce food sources for deer
  2. Install deer fencing
  3. Remove leaf litter & brush that snails & slugs take cover under
  4. Keep your animals on a well-drained pasture, especially during times of heavy rain
  5. Remove standing water
  6. Place a gravel path along the outside of pasture fence lines, to reduce gastropod traffic
  7. Do not plant gardens close to pastures

Lastly, EquidDoc veterinarians are happy to help you create a farm plan and prophylactic deworming protocol if P. tenuis exposure is a concern. Call EquidDoc @ 508-885-4205 or email if you have questions or concerns regarding P. tenuis and your farm!


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