Sheep measles in Sheep
Sheep measles are the cystic stages of the dog tapeworm Taenia ovis. These cysts represent the intermediate stage of the parasite and are found in the muscle tissue of infected sheep and goats. The adult stage of this parasite is a tapeworm which can grow to several metres in length and lives in the gastro-intestinal tract of dogs, dingoes and more rarely foxes. While living in the dog’s intestine, the tapeworm releases segments which contain thousands of eggs via the faeces onto pasture. Sheep and goats become infected when they graze infected pasture and ingest these eggs. The eggs hatch releasing larvae which migrate through the sheep or goats body and lodge in muscle tissue where they form small firm cysts.
The most common muscles where cysts are found are the heart, the diaphragm and the jaw muscles but any muscle tissue can be affected. Dogs, in turn, become infected when they eat meat from sheep which has sheep measles cysts in it. This completes the life cycle. The time between a dog ingesting cysts from infected offal to when it passes eggs in its faeces can be a short as 5 weeks. This is important when we consider how often dogs need to be treated for tapeworm infection. Recent surveys in Australia indicate that the prevalence of sheep measles is increasing as indicated by detection of affected animals at slaughter (National Sheep Health Monitoring Project (NSHMP) 2014-2015, Animal Health Australia).
Neither sheep with cysts in their tissues nor dogs carrying adult tapeworms generally show clinical signs or other ill effects to their health. Infected sheep show no effect on productivity or mortality and it is not possible to know whether an animal is infected or not by inspection of live animals. The real importance of sheep measles is the economic cost incurred at the time of slaughter and processing.
Sheep measles is a meat quality problem so carcasses with sheep measles are heavily trimmed or if the numbers of cysts are too high, the whole carcass may be condemned. This in turn disrupts the efficient flow of the slaughter line. The upshot is loss of economic value to the producer and increased costs to the processor.
Due to the special aspects of the life cycle of this parasite, management can be highly effective provided a number of steps are taken. It should be noted that there are no effective products to treat infection in sheep once they have cysts in their tissues. Prevention of infection in sheep necessarily focuses on treating and preventing infection in dogs.
The first step is regular de-worming of all farm dogs with an effective worming product to kill the adult tapeworms. The use of either Drontal or Droncit at the correct dose rates will kill the adult tapeworm. Dogs should be weighed, if possible, and dosed according to that weight to ensure that under-dosing does not occur. All dogs should be wormed at the one time and the dosing repeated at least every 6-8 weeks. Any dogs visiting the farm such as contractors’ working dogs or visitors’ pet dogs should be treated before arrival also.
Secondly, dogs should not have access to raw sheep meat and offal. Any meat derived from on-farm stock and marked for feeding of dogs should be adequately cooked prior to feeding or if in doubt, feed only commercial dog food.