Pathological changes in the brains of mice infected with Toxoplasma gondii: a histological, immunocytochemical and ultrastructural study.

The pathological changes in the brains of mice infected with T. gondii were studied at various intervals between 7 days and 22 months post-infection using histology, immunocytochemistry and electron microscopy. Initially, a few single parasites were observed (day 7) but necrotic lesions and microglial and inflammatory nodules rapidly appeared (9-I4 days). The majority of the lesions between days 9 and I4 contained proliferating toxoplasma and early cyst formation but from 2I days onwards the vast majority of nodules contained neither parasites nor Toxoplasma antigen. Intact intracellular cysts persisted throughout the period of study eliciting no host response. A generalized meningoencephalitis developed by day II and persisted with varying degrees of severity throughout the 22 months studied. At first, the inflammatory cells consisted of lymphocytes and monocyte/macrophages but during the chronic phase plasma cells predominated. In chronic infections, the number of microglial/inflammatory nodules was relatively constant with only a few containing toxoplasmic material resulting from recent cyst rupture. A few brains contained small nodules of dystrophic calcification. This study shows that in these asymptomatic animals, the major feature is perivascular cuffing by mononuclear cells and localized microglial/inflammatory nodules. After the development of the chronic state, there is no obvious increase or decrease in the severity of the pathological changes with time.