Sheep measles – caused by the larval cystic stage of the sheep tapeworm Taenia ovis – is surprisingly common in the Crookwell district. Surprising because while it’s not considered important to the health of sheep or humans, the parasite frequently causes condemnation of carcasses with significant loss of income.
Taenia ovis (formerly known as Cysticercus ovis) occurs wherever there are sheep, goats and domestic dogs. Sheep and goats are the intermediate hosts that support the cystic stage of the worm in muscle tissues (the “measles”), while dogs (and sometimes foxes) are the final hosts and carry the adult tapeworms in their intestines.
The Taenia life cycle is similar to that of the hydatid tapeworm, Echinococcus granulosus, but there are two major differences: taenia grow to a huge 2 metres, while the adult hydatid tapeworm is only 4 to 6mm in length, and taenia are not a public health risk, unlike the hydatid tapeworm (although the calcified cysts are unpleasant and considered unacceptable to eat).
The Taenia ovis life cycle starts when eggs are passed out in a dog’s droppings. Sheep or goats then eat the eggs when grazing pasture. The swallowed eggs hatch into larvae and these develop into cysts in muscle tissue. Dogs get re-infected by eating these infective tapeworm cysts in sheep or goat meat.
The term “sheep measles” comes from the appearance of the multiple small white calcified cysts about 3–10 mm long seen in body tissues at slaughter. It’s important to note that T ovis cysts in infected sheep will not disappear over their lifetime. This means that sheep measles may continue to be detected at slaughter for some time after a control program has been instituted.
Like hydatids, the key to controlling sheep measles is breaking the sheep to dog to sheep life cycle.
The good news is that this program controls both hydatid and taenia tapeworms:
- controlling dog movements
- preventing your canine pets or working dogs from eating sheep or goat meat or offal.
- treating all your dogs every 4-6 weeks with a wormer containing praziquantal.
- always cooking or freezing any sheep or goat meat before feeding to dogs (this inactivates the cysts).
- ensuring contractor’s dogs coming onto your property have been wormed with praziquantal within the previous 4 weeks.
Acknowledgements: NSW Department of Primary Industries and the WA Department of Agriculture and Food.