Immuno-histochemical and ultrastructural characteristics of a cyst-forming sporozoon associated with encephalomyelitis and myositis in dogs.

Light-microscopy, including immuno-histochemical staining, and transmission and scanning electron microscopy were applied for the characterization of a Toxoplasma-like parasite associated with encephalomyelitis and myositis in dogs of the boxer breed. Most parasites were tachyzoites, occurring in clusters, up to 60 micron in diameter, in parasitophorous vacuoles of host cells. Cyst stages with a definite cyst wall were few, but could be demonstrated in all cases. Immunostaining with antiserum against Toxoplasma gondii was negative, while both tachyzoites and bradyzoites were stained with serum from affected dogs. Ultrastructurally, the parasite was characterized by a great number of rhoptries, few amylopectin granules, lack of micropores in the tachyzoites, and the occurrence of numerous dense bodies in the bradyzoites. Numerous intravacuolar tubules were present in the space of the parasitophorous vacuole. The cyst wall resembled that of T. gondii. Until a definite host is found, the parasite may be regarded as an "orphan", cyst-forming sporozoon that is ultrastructurally different from other known protozoan parasites. It has a light-microscopical similarity to T. gondii but is antigenically different.