Toxoplasmic retinochoroiditis: electron-microscopic and immunofluorescence studies of formalin-fixed tissue.

An 82-year-old white man had bilateral, chronic recurrent uveitis caused by Toxoplasma gondii. In spite of extensive therapy for toxoplasmosis, the left eye became painful and blind and was enucleated. Histopathologically, the retina disclosed unusually large numbers of necrotic cysts and a few presumably viable cysts containing crescent-shaped organisms and scanty free forms of T gondii. Electron-microscopic studies confirmed the presence of a true wall of the cysts as well as additional ultrastructural features highly characteristic of the parasite. Immunofluorescent studies of the formalin-fixed tissue demonstrated quite vividly the cysts, which stained variably according to the stage of viability of the organisms. In cases in which toxoplasmic retinochoroiditis has been suspected clinically but routine histologic preparations fail to reveal the organisms, the remaining formalin-fixed tissue should be examined by specific immunofluorescent methods to demonstrate antigenic material of the causative parasite.