The prevalence and intensity of Sarcocystis spp infections in sheep.

A total of 864 sheep sold for slaughter throughout South Australia were examined for infections with Sarcocystis spp using macroscopic, microscopic and immunoserological methods of detection. Macroscopic cysts were found in 6.7% of the sheep ranging in intensity from 1 to 64 cysts per carcase. Morphometric studies detected 2 types of macroscopic cysts which differed in their size and cyst wall morphology. Large ovoid cysts with thick primary cyst walls were identified as S. gigantea (syn. S. ovifelis) whereas small slender cysts with thin cyst walls were identified as S. medusiformis. The prevalence of each species was 4.5% and 3.1% respectively. In comparison, microscopic cysts were detected in 93.2% of the sheep and stereological calculations of the intensities of infection ranged from 20 to 32,700 cysts per unit volume of muscle. Two types of microscopic cysts were also identified by differences in their cyst wall morphology. Cysts with smooth thin walls were detected in 88.1% of the sheep whereas cysts with radially-striated thick walls were found in 74.7%. Although these 2 types appeared to conform to the original descriptions of S. tenella and S. ovicanis respectively, both were classified as S. tenella (syn. S. ovicanis) pending further taxonomic studies. Specific antibody was detected in 93.7% of the sheep by the complement fixation test and in 96.9% by the indirect fluorescent-antibody test (IFAT). A positive correlation was also found between the prevalence of IFAT antibody and the prevalence of infections by microscopic cysts.