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Domestic sheep are susceptible to infection by several species of tapeworms, some as larval forms, and others as the adult stage1. Tapeworm infections in Western Canadian sheep generally have little impact on overall flock health and performance. They may, however, impact economic health of the flock due to carcass trims and condemnations at slaughter.
Larval tapeworms that infect sheep include Cysticercus ovis, Cysticercus tenuicollis, and Echinococcus spp. C. ovis is the larval stage of the dog tapeworm Taenia ovis. C. ovis appear as fluid-filled cysts or degenerate caseous to mineralized, pale yellow, firm foci, about the size of a large rice grain, scattered throughout cardiac and skeletal muscle. The diaphragm and masseter muscles are commonly affected but cysts can be seen in any muscle mass. The common name for this condition is "sheep measles". This parasite has little impact on the health of the host but there is a concern at slaughter. Meat inspectors will trim, or condemn carcasses entirely, based on severity of infections.
C. tenuicollis is the larval stage of the dog tapeworm, Taenia hydatigena. C. tenuicollis appear as fluid-filled cysts attached to serosal surfaces of the liver, diaphragm and peritoneum of sheep, and are commonly known as "sheep bladder worms". This infection generally results in liver condemnation at slaughter because the parasite migrates throughout this organ before emerging to encyst on serosal surfaces of the peritoneal cavity. Presence of hemorrhagic to fibrotic, serpiginous migration tracts, similar to those produced by immature liver flukes, make the liver unsuitable for human consumption for aesthetic reasons.
The life cycle is similar for both C. ovis and C. tenuicollis. The adult tapeworm grows and lays eggs in the intestine of the definitive host, a domestic dog or wild canid (coyote, wolf, fox). Eggs are passed in canid feces that contaminate pasture from which the sheep ingest the eggs. The greatest risk in most sheep operations is the guardian dogs rather than exposure to wild canids.
Breaking the cycle of infection involves controlling the cycle between sheep and the canine definitive host. Treating dogs regularly with antihelminthes that kill tapeworms and preventing dogs from eating raw sheep/goat meat or offal are necessary to control the cycle of infection. Keeping contamination to a minimum by cleaning up dog feces will also help. This may be practical in small spaces but control of dog feces is obviously less practical on large pastures. The parasite may also cycle between wild deer and coyotes. Pasture and hay or feed storage areas should be fenced to keep coyotes and other wild canids away. If the local coyote population is infected with adult taeniids and is allowed to contaminate pasture or feed, control may be very difficult except by eliminating access of lambs to affected pastures or feed. Hay and grain purchased from outside sources is also a potential source for tapeworm eggs.
Sheep measles and bladder worms are not human health hazards. Both are killed by thorough cooking or freezing to –18oC for 10 days. They do, however, represent a significant aesthetic issue when consumers encounter them in food. On very rare occasions, humans can be an aberrant host for T. ovis with the infection being contracted by human ingestion of tapeworm eggs that are passed in dog feces. This emphasizes the need to wear gloves and wash hands thoroughly with soap and water after handling of dog feces.
Echinococcosis or hydatid disease is relatively rare in domestic sheep but does pose a human health risk if hydatid cysts in food or parasite eggs shed by carnivores are ingested2. Echinococcosis is maintained most commonly in a wild cervid-canid cycle but can also cycle between domestic sheep and dogs and between farmed elk and dogs3. There is potential for infection of sheep in areas where sheep rearing overlap with those where the elk-canid cycle exists. Hydatid cysts are found in organs, most typically in the lung and liver, as large single or multiloculated, fluid-filled cysts containing larval tapeworms, visible as “hydatid sand”.
There are also adult stage tapeworms that infect sheep. Moneizia expansa and Thysanosoma actinoides are not a serious problem in Western Canada but can cause losses in sporadic cases. M. expansa is found in the small intestine of sheep as a very long, characteristically flat, segmented tapeworm. The life cycle starts with tapeworm segments containing eggs passed in feces of the sheep. These are ingested by free-living mites, in which they develop to an infective stage. When sheep graze herbage they ingest the mites containing the infective tapeworm which re-infect the sheep and continue the cycle. There is some controversy over how important this tapeworm is to lamb growth performance. It is generally agreed that they do not cause a problem, except in very heavy burdens when masses of worms may block the small intestine.
T. actinoides is found in the bile ducts, pancreatic ducts, and small intestine of sheep. It is a short tapeworm, generally not longer than about a few centimeters. The life cycle is not completely known but it is believed very similar to Moneizia spp. with the intermediate host perhaps being a bark louse. Similar to Moneizia spp., this tapeworm does not appear to have a negative effect on the growth rate of lambs except in very heavy infections. In that instance, the tapeworms may partially obstruct the flow of bile and pancreatic juice and cause digestive disorders and unthriftiness associated with inflammation of the biliary epithelium and enlargement of the bile duct with fibrosis and hyperplasia. The greatest significance of this tapeworm is that it causes condemnation of livers at meat inspection.
Adult tapeworms found in sheep offal are generally not an issue except where specific ethnic markets demand return of offal to the client. If a sheep producer is attempting to capture these niche markets, a parasite control program specifically to control tapeworms along with other common sheep worms should be discussed with the farm veterinarian. Presence of adult tapeworms in offal is not a human health risk but an aesthetic concern.
In summary, tapeworms are not considered to have serious impact on sheep health and performance in Western Canada. However, they do impact the carcass value at slaughter and restrict market access. Producers need to be aware of these impacts and include tapeworm control, as part of their overall parasite control program, in consultation with their veterinarian.
1 Wenger I. Guide to Parasites in Sheep. A joint publication by the Alberta Sheep and Wool Commission and Saskatchewan Sheep Development Board, Paramount Printers, Ltd. 2006.
2 Maxie M. G. (ed). Jubb, Kennedy and Palmer’s Pathology of Domestic Animals, 5th ed., Vol. 2, pp. 254 – 257. Saunders/Elsevier, Toronto. 2007.
3 Agri-Facts: Hydatid (Cystic) Disease in Alberta’s Game-Farmed Elk. Alberta Agriculture and Rural Development, Agdex 481/655-1; July 2004.
Dr. Jagdish Patel, Animal Health and Assurance Division, 780-644-5093. |
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