Muscle Worms
Causative Agent
  • Muscle worms are nematodes that parasitize muscle and lung tissue of some cervids.
  • Two species are known to cause disease in BC:
    • Parelaphostrongylus andersoni
      • muscle worm of caribou (Rangifer tarandus) and occasionally white-tailed deer (Odocoileus virginianus).
      • Adult worms, which are delicate and thread-like, are found in association with blood vessels and connective tissue of the loin and thigh muscles.
      • Adult worms are often found in the meat of backstraps, back muscles close to the vertebrae.
      • Adult worms are 19 - 35 mm in length and approximately 0.1 mm in width.
      • The dark, threadlike intestine of adult worms is noticeable against muscle tissue.
    • P. odocoilei
      • muscle worm of primarily of mule and black-tailed deer (O. hemionus); occasionally, mountain goat (Oreamnos americanus), mountain sheep (Ovis canadensis) and caribou are infected.
      • Adult worms are delicate, thread-like, and white, with a central thin, black intestine. Like P. andersoni, they are found in association with blood vessels and connective tissue of the loin and other muscles.
      • Adult worms are about 55 mm in length and approximately 0.2 mm in width.
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Muscle Worms
Adult muscle worm, P. odocoilei, in muscle of mule deer from Alberta.
  • P. andersoni has been documented in northern BC, as well as in the southeastern and south-central areas of the province.
  • P. odocoilei has been documented on Vancouver Island, in the Okanagan Valley, and in coastal mountain goats and some populations of mountain sheep.
  • New infections begin during periods when vegetation is greening up.
  • Larvae are passed in the feces at varying intensities throughout the year.
Hosts, Transmission and Life Cycle
P. anderonsi:
  • Adult male and female worms are often found paired.
  • Females deposit eggs into veins of hosts which are then carried to the lungs where they hatch in small blood vessels.
  • Larvae move up the through the lungs and into the trachea where they are coughed up, swallowed, and passed in the feces.
  • Larvae must then penetrate the body of a land snail to develop into the infective stage.
  • The snail and infective larvae are then inadvertently ingested by caribou and white-tailed deer while feeding.
P. odocoilei:
  • Similar to that of P. andersoni.
Signs and Symptoms
P. anderonsi:
  • Natural infections in caribou and white-tailed deer are generally well tolerated.
  • In general, the signs listed below are observed in heavily infected animals; young are more susceptible than older animals.
  • Notable signs in caribou include:
    • reluctance to stand;
    • weakness;
    • panting;
    • short steps;
    • arched back.
  • Signs in white-tailed deer are more severe and include:
    • hemorrhage on the surface of back and thigh muscles;
    • green abscesses may be observed near eggs in lungs or worms in muscle;
    • small (up to 1 mm in diameter) nodules may be observed in the lung.
P. odocoilei:
  • Outward signs of disease are rarely seen with P. odocoilei.
  • Laboured breathing may be observed in some infected animals.
  • Other signs are similar to those described above for white-tailed deer infected with P. andersoni.
  • Recent research has identified this parasite in mountain sheep and mountain goats. There is some evidence that heavy infections of larvae can damage the lungs of Stone’s sheep, but further studies are required to understand any role the parasite plays in the health of these sheep.
Meat Edible?
  • Carcasses containing P. odocoilei and P. andersoni are safe for human and animal consumption since they do not affect the quality of meat.
Samples for Diagnosis
  • Submission of adult worms from loin or thigh muscles is necessary to confirm infection.
  • Submitting fecal samples may also aid in diagnosis.
Further Reading
  • Lankester M.W. 2001. Extrapulmonary lungworms of Cervids. Pp. 228-278 in W.M. Samuel, M.J. Pybus, A.A. Kocan (eds.), Parasitic Diseases of Wild Mammals. 3rd Ed. Iowa State University Press. Ames, IA.
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