Immune responses against Toxoplasma and Sarcocystis infections in ruminants: diagnosis and prospects for vaccination.

Toxoplasma gondii and a variety of Sarcocystis species are common cyst-forming coccidian parasites of domestic ruminants worldwide. T. gondii infection is a major cause of ovine and caprine abortion and perinatal mortality, but does not appear commonly to cause significant disease in other ruminants. Sarcocystis infections are probably most important in growing ruminants in which they can cause subclinical anaemia and reduced weight gain. The life cycles, pathogenesis and current knowledge of immunity to these protozoa, in ruminants, are reviewed. Although clinical signs may suggest a diagnosis, confirmation requires the use of specific immunological methods such as serology and immunohistochemistry. These are discussed and the necessity for using characterised antigens and antisera is emphasised. Whereas live Toxoplasma "vaccines" may be effective in sheep and goats, the eventual preparation of a killed vaccine capable of inducing long-term protection is of the utmost importance. To achieve this goal it will be necessary to identify the specific antigens capable of stimulating protective immunity and to conduct further research on both the pathogenesis of toxoplasmosis and fundamental aspects of the immune response of pregnant ruminants to T. gondii.