Abstract

Recently, the muscle-dwelling protostrongylid nematode, Parelaphostrongylus odocoilei, was discovered in Dall’s sheep (Ovis dalli dalli) in the Mackenzie Mountains, Northwest Territories, Canada. Subsequently, a survey of thinhorn sheep (Ovis dalli) and mountain goats (Oreamnos americanus) in northern North America has revealed that this parasite is widely distributed in the Subarctic.

In thinhorn sheep both naturally and experimentally infected with P. odocoilei, eggs and larvae caused granulomatous interstitial pneumonia and lung hemorrhage, while adult nematodes were associated with localized myositis and muscle hemorrhage. Experimentally infected sheep showed a consistent pattern of weight loss and decreased muscle mass. At two weeks prior to patency, two of five experimentally infected sheep developed hind end ataxia, loss of conscious proprioception, hypermetria, and an eosinophilic pleocytosis in cerebrospinal fluid. Antibody to Parelaphostrongylus spp. was detected in the cerebrospinal fluid and serum of infected, but not control, sheep. Neurologic signs stabilized at the time of patency and subsequently disappeared until recurrence following treatment with ivermectin. Uninfected control sheep showed no weight loss or clinical abnormalities at any time.

In five thinhorn sheep each experimentally infected with 200 third-stage larvae of P. odocoilei, pre-patent periods ranged from 68–74 days. Shedding of first-stage larvae peaked at >10,000 larvae per gram of feces between 90 and 110 days post-infection. The identity of first-stage larvae was confirmed by comparing sequence of the ITS-2 region of nuclear DNA with known sequence for P. odocoilei. Adult P. odocoilei were recovered from three experimentally infected sheep, but not the two sheep that developed neurologic signs which are currently being monitored.

While other researchers have recovered P. odocoilei adults from the epidural space of experimentally infected mule deer (Odocoileus h. hemionus), we did not find adult P. odocoilei in the spinal canals or cords of ten naturally infected or three experimentally infected thinhorn sheep. Therefore, this is the first evidence of a neural migration for P. odocoilei in experimentally infected thinhorn sheep, and the first description of a clinical neurologic syndrome caused by this parasite in any host species. These findings indicate that this host-parasite relationship is more complex than previously believed. Considering the susceptibility of protostrongylid life cycles and northern hosts to climate change, P. odocoilei may constitute a significant emerging disease risk for thinhorn sheep.