Sheep and goats are the intermediate hosts for a range of different types of Sarcocystis, a protozoon organism with dogs or cats as the final host.

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

Sheep and goats are infected by eating fully developed (sporulated) oocysts (a type of egg) in dog or 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. Dogs or cats during scavenging eat muscle meat containing these cysts. The stage released from within the cyst once in the small intestine of the dog or 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 hosts (dog or cat), but the cysts in muscle tissue are a cause of trimming of carcases or, rarely, condemnation.