Sheep measles (Cysticercus ovis) cause condemnation of carcases or parts of carcases at abattoirs, but they rarely cause ill-effects in the sheep or goats. Small white cysts appear in certain muscles, and while these cannot infect humans, they are considered unacceptable, leading to carcase condemnation.
The life cycle has two hosts: the definitive host—foxes, domestic dogs and occasionally dingoes and wild dogs; and the intermediate host—sheep or goats.
The tapeworm stage (Taenia ovis) occurs in the small intestine of the dog, with the tapeworm itself being up to two metres long. Tapeworms produce eggs that pass out in tapeworm segments in the dog’s faeces. If these are on pasture, they can be eaten by sheep or goats in which larvae develop and migrate to the heart and diaphragm and some other muscles.
In the muscles they form cysts up to 10 mm long. Over time, the cysts calcify, becoming hard. The cycle continues when dogs eat carcasses that contain viable cysts that develop into tapeworms in the dog’s small intestine.
No treatment for cysts in sheep or goats is available; control is based on breaking the life cycle. Specifically, this means preventing dogs from eating carcasses, meat or organs of infected sheep or goats. Freezing or cooking meat and viscera may not reliably kill all cysts.
Dogs in sheep areas should continue to be regularly treated for tapeworm. Research by Dr David Jenkins, Charles Sturt University, has shown that control of tapeworms in farm dogs and widespread use of dry dog food has largely removed dogs as a definitive host. Foxes, however, have been shown to be active in maintaining the population of sheep measles.
This parasite has two names because the cyst stage was described and named before it was known to be a stage of the life cycle of Taenia ovis.
Other tapeworms in dogs: Taenia pisiformis, T. serialis and Dipylidium caninum are common tapeworms of dogs, foxes and dingoes and should be differentiated from T. ovis and T. hydatigena (bladder worm). The intermediate hosts of T. pisiformis and T. serialis are the rabbit and hare. The flea and possibly the biting louse are the intermediate hosts for D. caninum.
The hydatid tapeworm, Echinococcus granulosus, also occurs in dogs, with the intermediate stage affecting sheep, goats and sometimes humans.