The ultrastructure of bradyzoites and tissue cysts of Neospora caninum in dogs: absence of distinguishing morphological features between parasites of canine and bovine origin.

The fine structure of bradyzoites and tissue cysts of Neospora caninum in dogs is described. Original findings consisting of branching, septa-like structures of the cyst wall and peculiar, lipid-like inclusions in the cyst ground substance were described. Tubulo-vesicular structures in the cyst ground substance and distinctive, vesicular organelles containing short, membranous segments and smaller vesicles, described in Neospora parasites of bovine origin and previously undescribed in parasites of canine origin, were also observed. The findings in this study indicate that there are no distinct ultrastructural morphological criteria differentiating N. caninum in dogs and the Neospora-like protozoan of cattle. The previously reported morphological distinction between parasites of canine and bovine origin may be attributed to the relatively small numbers of tissue cysts examined, differences in tissue fixation and processing, and variation in developmental stage of the parasites examined.