The neuropathology in toxoplasmosis‐like infection caused by a newly recognized cyst‐forming sporozoon in dogs

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Abstract
The pathomorphological changes in the central nervous system associated with an infection with a previously reported, Toxoplasma‐like, cyst‐forming sporozoon were studied by means of light microscopy, including immunohistochemistry, and transmission electron microscopy in seven dogs, aged 4 months to 2 years. Tachyzoites (endozoites) within parasitophorous vacuoles were found in neurons, astrocytes, macrophages, vascular pericytes and probably also in fibroblasts, while cyst stages were only observed in cells showing features of neurons. Proliferating tachyzoites were associated with lesions of a necrotic/granulomatous type. Perivascular accumulations of lymphocytes and plasma cells, together with an activation of astrocytes and perivascular fibroblasts resulting in marked sclerosis, were prominent features in all cases. The grey matter was most seriously affected in the brain, while the white matter was most often the site of inflammation in the spinal cord. The structural similarity to the newly reported Neospora caninum suggests that we are dealing here with the same parasite. However, the present infection and toxoplasmosis have so many features in common, including the light microscopical morphology of the organisms and host/parasite relationship, that a definite etiological diagnosis should be based on ultrastructural and/or immunohisto‐chemical examinations.