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Foxes, the culprit for high levels of sheep measles in Australian sheep?

Sheep measles is a parasitic infection of sheep occurring as small white masses in meat. The importance of this parasite is that it causes financial losses to the Australian sheep meat industry and is a potential barrier to the international trade of Australian sheep meat. Data collected in the National Sheep Health Monitoring Program (co-ordinated by Animal Health Australia and supported by the Sheep Meat Council Australia and Australian Wool Producers) generated data that clearly shows sheep measles to be widespread and often at high prevalence levels in sheep from all sheep producing areas of Australia.

During the last two years, Meat and Livestock Australia (MLA) funded a two-year study charged with determining why sheep measles is so common, revisiting the lifecycle, identifying on-farm risk factors and then determining the financial impact on processors.

The sheep measles parasite, Taenia ovis, has a two-host lifecycle with a different body form in each host. Large tapeworms (up to two metres) occur in the intestine of dogs and small cysts about the size of a small pea grow in the muscles of sheep. Mature tapeworms in dogs produce thousands of eggs that are shed daily into the environment via the faeces of infected dogs. These eggs are accidentally ingested by sheep whilst grazing. They hatch in the sheep releasing microscopic 6-hooked larvae that exit the sheep’s intestine, enter blood vessels and are passed to the muscles. Here, over two months, each develops into a cyst containing a tapeworm head. If these cysts are eaten by a dog (via the infected meat), the dog becomes infected with a sheep measles tapeworm.

Sheep measles does not infect humans, in contrast to its closely related cousin the Hydatid tapeworm (Echinococcus granulosus), and it appears to be of no veterinary importance to either dogs or sheep. Its importance is all down to aesthetics associated with the presence of cysts in meat for human consumption. Cysts in sheep remain infective to dogs for 2–3 months after which time they are killed by the immune system of the sheep, developing into a small pus-filled abscess. These abscesses become mineralized transforming into a gritty masses, evolving to hard calcified nodules that remain in the muscle for the life of the sheep. None of these manifestations is popular with consumers!

Degenerated sheep measles cyst in sheep heart. Source: David Jenkins
Degenerated sheep measles cyst in sheep heart. Source: David Jenkins
Viable sheep measles cysts in sheep heart. Source: David Jenkins
Viable sheep measles cysts in sheep heart. Source: David Jenkins

The MLA sheep measles study was undertaken in WA, NSW and Tasmania. These states were selected since abattoir infection data regarding sheep measles identified high prevalence in WA, medium levels in NSW and low levels in Tasmania. In addition, from a potential for wildlife involvement Tasmania is wild dog and fox-free*. About 100 producers were involved in the study, about two thirds with a sheep measles problem and one third without. We also undertook financial impact studies in 5 abattoirs (3 NSW; 1 WA; 1 Tasmania). Abattoirs killing mainly lambs suffered the lowest losses but mutton abattoirs in NSW were losing about $1,100/day whilst in WA losses averaged $2,200/day with peaks up to $4,000/day. Losses for individual producers are often unexpected and substantial; one producer in this study lost $4,000 on two consignments of lambs sent for slaughter in 2012.

We found no identifiable on-farm risk factors pre-disposing farms to having sheep measles. We examined the faeces of 245 farm dogs and found only one infected with tapeworms, coincidently, these turned out to be T. ovis in a dog from a farm in Tasmania. The low level of tapeworm infection in dogs is the likely result of the common use of dry dog food and wide availability of relatively cheap all-wormers for dogs containing the highly efficient tapeworm-killing drug praziquantel. We concluded that domestic dogs may not be the major source of infection for sheep now, as they once were in the past.

We examined the intestines of 499 foxes and 52 wild dogs. We found T. ovis in two foxes (one from NSW and one from WA). The sample sizes of foxes collected from the various locations were variable. The two infected foxes were recovered from the localities where the largest samples were collected, equating to a prevalence of about one percent in each location. Although not high, given the fact eggs of T.ovis remain infective on pasture for at least 300 days and foxes are highly mobile covering anything from 4 to 16 km/night only a few infected animals need be present in a given geographical area to maintain high exposure pressure on the local sheep population.

Of the 52 wild dogs examined none was found infected with T. ovis, however, this maybe a reflection of the small sample examined. Nevertheless, there was no doubt some wild dogs had been consuming sheep because T. hydatigena tapeworms (bladder worm) were recovered from 4 (8.3%) of the animals. We also identified two foxes infected with T. hydatigena.

Our study showed for the first time that foxes in Australia are able to act as hosts for T. ovis tapeworms, providing an additional (important?) source of infection to sheep. These findings highlight the need to modify current sheep measles control strategies. This does not mean abandoning de-worming of dogs and feeding safe foods; this should continue as it has been highly effective, but we need to consider how we can protect sheep.

In 1989 data were published in the scientific press reporting the development of a highly efficient vaccine for sheep against sheep measles. A few years later, this vaccine was commercialised in New Zealand as a stand-alone vaccine. However, the marketing blunder of producing a stand-alone product ensured the failure of the vaccine because most financial losses were felt by the processors and not the producers. Despite the effectiveness of the vaccine, almost nobody bought it. To be used, the vaccine should be combined with something farmers need to use to ensure the well being of their sheep (such as the current 6-in-1 vaccine). Over time, with wide uptake by producers, this vaccine has the potential to solve the sheep measles problem in Australia, despite the presence of T. ovis-infected wildlife.

(*Foxes have been released illegally in Tasmania, there followed an intense 1080 baiting program. If still present, fox numbers are at such low density as to be irrelevant for transmission of T. ovis in Tasmania)

Further reading

The paper below is in an open access journal site and can be accessed by clicking the reference.

Red foxes (Vulpes vulpes) and wild dogs (dingoes (Canis lupus dingo) and dingo/domestic dog hybrids), as definitive hosts for Australian Taenia hydatigena and Taenia ovis. (2014) Jenkins DJ., Urwin NAR., Williams TM., Mitchell KL., Lievaart JJ., Armua-Fernandes MT. International Journal for Parasitology: Parasites and Wildlife 3; 75-80.