Sheep measles: Economic impacts and epidemiological risks associated with sheep measles

Summary

For some time Australian sheep meat processors have been complaining that sheep measles (Taenia ovis) in slaughtered sheep is causing major financial loss due to condemnation of affected meat and offal (hearts) and extra carcass inspection and trimming time. Sheep measles is caused by the intermediate stage of a tapeworm that infects domestic dogs. Eggs of the sheep measles tapeworm pass into the environment with the faeces of infected dogs. Sheep become infected through accidentally ingesting these eggs whilst grazing. Once in the intestine of the sheep, the eggs hatch, releasing microscopic larvae that exit the intestine, enter blood vessels and pass to the musculature of the sheep. Larvae are mainly attracted to the diaphragm and heart, but any muscle of the body can be infected. The parasites develop into cysts in the muscles, each cyst containing a tapeworm head. If cysts are eaten by dogs they become infected with a tapeworm. However, the cysts in sheep meat are only infective to dogs for a short time (2-3 months), eventually being killed by the immune response of the sheep. Dead cysts develop into pus-filled lesions, which over time become mineralised, transforming into gritty masses and then into calcified nodules. From a consumer’s perspective none of these cystic manifestations is acceptable in meat for human consumption.
The study investigated on-farm risk factors associated with sheep measles transmission through a farmer questionnaire, revisited the lifecycle of the sheep measles parasite through surveys of the tapeworms in farm dogs and wild canids (foxes and wild dogs), using traditional parasitological methods and DNA identification to confirm tapeworm species. In addition, data on the financial losses incurred by abattoirs were collected from 5 abattoirs, one each in Western Australia (WA) and Tasmania and three in New South Wales (NSW).
The questionnaires were completed by farmers that had a sheep measles problem in their sheep and by those that did not. Farmers were identified through the National Sheep Health Monitoring Project (Animal Health Australia) data base. Two hundred and thirty nine farmers were invited to undertake the survey, 56 living in Tasmania, 90 in NSW and 63 in WA. The initial questionnaire mail-out was followed by a follow-up letter to 120 producers who did not respond to the first request. Ninety five farmers returned completed questionnaires 42 NSW, 31 Tasmania and 22 WA (To view the questionnaire, see Appendix 1). The questionnaire asked questions on topics such as working and pet dog care and maintenance, vertebrate pest presence/control, home slaughter/hunting and offal disposal, proximity of national /state parks or forests and supplementary feeding of stock. No identifiable risk factors could be determined. In NSW, there was a weak correlation between farmers who bought in hay compared to those who did not. However, this was not evident in either WA or Tasmania.
The survey of tapeworms in farm dogs was undertaken through identification of eggs in faeces. The eggs of all species of intestinal worm found in the faecal samples were identified and a report sent back to the owners. Of the 245 faecal samples examined only one sample contained eggs of the sheep measles tapeworm, and this dog lived in Tasmania.