Sarcocystosis of sheep

SARCOCYSTIS species are common, ubiquitous, sporozoan parasites of herbivores. There are four species in sheep; two nonpathogenic macrocyst species which produce grossly visible cysts in muscle and two potentially pathogenic microcyst species. The two macrocyst species (Sal-cocystis medusifowmis and S ovijelis) are often found in oesophageal muscle and are an occasional cause of carcase condemnation in some countries. They only rarely occur in Britain. Microcyst species are found mainly in the myocardium and striated muscle and are present in at least 85 per cent of sheep of nine months of age. Occasionally, cysts may also be found in the brain. The life cycle of the parasite and the identity of pathogenic species of Sai-cocystis are now well established but diagnosis of disease in field cases remains problematic, mainly because of the vague character of the clinical signs and the lack of a serological test to differentiate severe, acute and subacute infection from asymptomatic intercurrent infection. At present, confirmation of disease is dependent upon histopathology and for this it is necessary to demonstrate the presence of parasites in association with lesions. However, parasites are not invariably detected in the tissues of infected sheep. Saicocv:stis species are obligate two-host parasites. The sheep is the intermediate host of the two macrocyst species, S medusXiftwnis and S ovijelis, which have a sheep-cat cycle. The two microcyst species, S arieticanis and S teniella, usually have a sheep-dog cycle but it has also been shown that the fox is an efficient final host for them. As the macrocyst species are thought to be non-pathogenic, the rest of this article will be concerned only with the pathogenic microcyst species.