The structure and identity of macroscopically visible Sarcocystis cysts in cattle.

Macroscopically visible Sarcocystis spp. cysts isolated from the skeletal muscle of slaughtered cattle were examined by light- and electronmicroscopy. Transmission experiments involving cats, dogs and a human volunteer were also carried out. The cysts could only be transmitted to cats which establishes them with a high degree of certainty as Sarcocystis hirsuta. The cyst wall (including protrusions) ranged from 3.3 to 7.0 micron in thickness and the individual cyst wall protrusions from 1.2 to 2.6 micron in width. Transmission and scanning electronmicroscopy revealed previously undescribed features of the cyst wall. It appears that, with increasing age, the cyst wall protrusions become larger and develop a highly irregular surface. Their attachments to the cyst wall are slender and widely spaced indicating that growth of the cyst continues without the formation of new protrusions. Within the protrusions the fibrils become disorganised and numerous osmiophilic granules appear. It is evident that major changes in the structure of sarcocysts can occur with age.