Cysticercus ovis, the infective larval stage of  Taenia ovis, a cestode parasite of dogs (and perhaps free-ranging carnivores), occurs around the world, including in Canada.  The parasite transmits from dogs to sheep by ingestion of infective eggs in dog faeces, and from sheep to dogs by ingestion of infective cysticerci in skeletal muscle, heart and occasionally other tissues.  Although the parasite is usually of minimal clinical significance in dogs and sheep, and is not known to be zoonotic, the presence of the macroscopically visible larval stages in meat from infected sheep is unattractive to consumers and reduces carcass value.  For reasons that are not yet clear, since approximately 2003 C. ovis seems to be an increasing problem in sheep in several parts of Canada, and in 2008 was the most common cause of sheep carcass condemnation in this country. The increase seems to be due to  more than greater awareness.

A recent two-year survey at an abattoir in Ontario found evidence of T. ovis infection in sheep from the four western  provinces, as well as Ontario and Quebec. Only two farms in British Columbia, and only a single farm in the maritime provinces were included, and it was not possible, therefore, to draw conclusions about these regions. Of the sheep surveyed, carcasses from 237 were condemned because of T. ovis cysticerci, and these originated from 133 farms. These data likely underestimate the prevalence of T. ovis infection in sheep in Canada because they were derived only from condemned animals, and excluded animals with light infections, the carcasses of which were trimmed but not condemned. Additionally, a case-control study identified farm dogs scavenging on deadstock and failure to dispose of deadstock as risk factors for carcass condemnation resulting from T. ovis infection.

One of the unresolved issues related to T. ovis is the role of free-ranging carnivores in the life cycle of the parasite. As far as we know, there are no published records of the parasite in coyotes, and only a single record in wolves in Estonia. Sheep producers often implicate coyotes as the source of T. ovis, but the Ontario study did not support this view. It appears that dogs are the sole source of the parasite in Canada, but that view may change in the future.

As a result of the increasing significance of T. ovis to the sheep industry in Canada, considerable efforts have been made to inform sheep producers of the occurrence, significance, and biology of the parasite, and of control measures. The last are centred on the effective anthelmintic treatment of dogs that have contact with sheep and/or sheep pastures, and on prevention of access by dogs to deadstock on sheep farms.

DeWolf BD et al. (2012) Distribution of, and risk factors associated with, sheep carcass condemnations due to Cysticercus ovis infection on Canadian sheep farms. Veterinary Parasitology 190: 434-441.