The Meningeal worm of whitetail deer (Parelaphostrongylus tenuis) has an interesting life cycle. The the intermediate host is a snail. Deer consume the snail when grazing and the larva migrate through the tissues to the spinal cord and ultimately the brain. Deer are a well-adapted host and develop little or no problems with worms migrating through their brain. However, if the snail is consumed by a sheep, goat llama, elk or moose the migration causes a severe inflammatory response. This usually occurs when the parasite enters into neurologic tissue such as the spinal canal. The intense inflammation results in paresis (neurologic lamness) or paralysis of the hind legs. The severity of signs varies from knuckling on their fetlock or dragging a toe all the way to complete paralysis and the entire spectrum in between.
Diagnosis of P. tenuis is suspected in areas where both white-tail deer and the intermediate host snail are present. This usually means within 200 miles of the Great Lakes although cases have been suspected in other areas. Confirmation of P. tenuis is difficult as there is no live animal test and necropsy is difficult as the larva are microscopic. Treatment of affected animals is aimed at killing the larva with an avermectin such as Ivomec, ivermectin (generic) or Dectomax at normal label dose. I prefer to use the injectable products as they reach higher blood concentrations more quickly. Some Veterinarians include Safe-guard (fenbendazole) daily for five days in addition to ivermectin.
Decreasing the immune response is another component to treatment, usually with dexamethasone daily for three days or Predef if the ewe is pregnant. Lifting the ewe and repositioning a down ewe on deep bedding is important to minimize muscle damage. Anti-oxidants such as Vitamin E should also be considered. Affected animals should be re-evaluated for improvement daily. If the ewe cannot stand and no improvement is made after five days humane euthanasia should be considered. If improvement in mobility occurs with treatment the sheep may have a full and complete recovery or may have permanent neurologic problems for the rest of its life.
If P. tenius is suspected, treatment of the entire flock with an avermectin is warranted. If one sheep ingested snails with larva it is a good bet that other sheep are also infected with larva but it just has not made it to the spinal canal. In endemic areas treatment every three weeks should be considered in the prime infection season (late summer and fall). Restricting grazing of low lying areas during fall will also help reduce risk.
