There are three species of tapeworms in Australia which have their larval or cystic stage in sheep:

1. Hydatid tapeworm (Echinococcus granulosus)
2. Sheep measles (Taenia ovis)
3. Bladder worm (Taenia hydatigena)

Hydatids are important from a public health point of view (transmissible to humans). Prevalence figures of up to 20-30% have been reported historically in defined regions of Australia. Recent studies suggest a decreasing occurrence, probably associated with increased awareness of the dangers of feeding uncooked sheep offal to dogs and public health awareness campaigns in Tasmania, NSW and ACT. Recently Tasmania has applied for declaration of freedom from hydatids following a lengthy eradication campaign.

Hydatids and sheep measles have similar lifecycles: in flocks where one occurs, the other may occur also.

Sheep measles are a cause of condemnation of carcases or parts of carcases at abattoirs.

Find out more about tapeworms of sheep:

• Life cycle
• Location in the sheep and diagnosis
• Effects on the sheep
• Treatment and control
• Summary Table - tapeworm cysts

 Life Cycles of larval tapeworms (cestodes) (tapeworm cysts)

1. Hydatid tapeworm (Echinococcus granulosus)
2. Sheep measles (Taenia ovis)
3. Bladder worm (Taenia hydatigena)

Life cycle
These tapeworms are parasites of dogs, and dingoes.

Dogs defaecate on paddocks and further spread may be via streams, effluent or flies. Thick egg shells protect them from environmental extremes. Eggs are eaten by sheep (intermediate host) and cysts develop in internal organs. Dogs become infected by eating meat or offal containing cysts. Hydatid cysts of Echinococcus granulosus can cause life-threatening disease of humans. Infection is by ingestion of eggs passed by dogs; humans are accidental intermediate hosts.

Sheep acquire strong resistance to prevent new cysts developing but this has little effect on existing cysts. In order to perpetuate the cycle carcasses of infected sheep must be eaten by dogs. Small cysts like Cysticercus (sheep measles, bladder worms) are susceptible to desiccation but large cysts like hydatids are very resilient. Cool, moist conditions are most favourable for survival. Freezing is not likely to be severe enough in the field to kill a significant number of cysts.

Because the incidence of these infections has been falling, especially in Tasmania where hydatid control programs have also reduced Cysticercus infection rates, the diseases now occur in local epidemics. Typically, sheep with no previous experience of infection can be contaminated with eggs from the faeces of a single dog. Sheep kept in areas adjacent to bushland containing dingoes and foxes are at risk of infection.

The likelihood of infection is increased by the proximity of wild dogs, access by intermediate hosts to infected dogs' faeces and cool climates which favour survival of eggs and cysts in the environment. Outbreaks can occur anywhere there appropriate hosts and the parasite come together, but are more common in some parts of Australia than others.

If T ovis (sheep measles) is present in a flock, then hydatids could well be present also.

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 Larval tapeworms (cestodes) - Location in the sheep and diagnosis

Taenia
Cysticercus have a single white head enclosed in a membrane.

Echinococcus granulosus (Hydatid cyst)
Hydatid cysts, some up to the size of oranges or grapefruits several cm in diameter, occur mainly in liver and lungs or occasionally brain and other tissues.

Intermediate hosts are sheep, goats, cattle, horses, pigs, wildlife, and humans.

The hydatid cysts of E. granulosus have thick walls, may contain smaller cysts, and contain many small heads, giving a sandy consistency.

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 Effects of tapeworm cysts on the sheep

Heavy infections of bladder worm (C. tenuicollis) in the liver can appear like liver fluke disease (fasciolosis).

Hydatids (Echinococcus granulosus) and sheep measles (Cysticercus ovis) are rarely detected before death.

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 Treatment and control of tapeworm cysts (larval cestodes) in sheep

No treatment for cysts in sheep is available.

Control
Control is based on breaking the life cycle. Specifically this means preventing dogs from eating sheep carcasses, meat or organs.

Freezing or cooking of meat and viscera may not reliably kill all cysts.

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 Summary table - tapeworm cysts

Larval cestodes of sheep
Source: Love and Hutchinson (2003). Pathology and diagnosis of internal parasites in ruminants. Post Graduate Foundation in Veterinary Science, University of Sydney. Adapted from Coles (1986).

Echinococcus granulosus:

Taenia ovis:

Taenia hydatigena:

Note: Taenia pisiformis, T. serialis and Dipylidium caninum are common tapeworms of dogs, foxes and dingoes that need to be differentiated from T. ovis and T. hydatigena. 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.