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Sheep and goats are the intermediate hosts for a range of different types of Sarcocystis, a protozoan organism with cats as the final host. Other parasites exist which have a life cycle involving domestic or feral dogs/ foxes as a second host (e.g. bladder worm and sheep measles which are both caused by dog tapeworms).

Sarcocystis forms cysts in the muscles of sheep and goats. Some types of ‘sarco’ cysts are microscopic, but others are large and white. These large cysts are typically seen within the muscles in sheep carcases at slaughter.

Sheep and goats are infected by eating fully developed (sporulated) oocysts (a type of egg) in cat faeces deposited on pasture. These mature stages once swallowed penetrate the intestinal wall and further develop into a cyst-like stage in the walls of blood vessels and muscles. Cats during scavenging eat muscle (meat) containing these cysts. The stage released from within the cyst once in the small intestine of the cat develops to repeat the cycle again.

The parasite is more common in cooler climates.

Disease is rarely apparent in the prey host (sheep or goats) and never in the final host (cat), but the cysts in muscle tissue are a cause of trimming of carcases or, rarely, condemnation.

It is not treated in sheep but rather is controlled by breaking the life cycle – i.e. exclude cats from sheep pastures, prevent scavengers from having access to any carcases, and avoid feeding raw meat to cats.

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