Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. The parasite infects most general of warm-blooded animals, including humans, but the primary host is the felid (cat) family. Animals are infected by eating infected meat, by ingestion of feces of a cat that has itself recently been infected, and by transmission from mother to fetus. Cats are the primary source of infection to human hosts, although contact with raw meat, especially pork, is a more significant source of human infections in some countries. Fecal contamination of hands is a significant risk factor.
Nicolle and Manceaux first described the organism in 1908, after they observed the parasites in the blood, spleen, and liver of a North African rodent, Ctenodactylus gondii. The parasite was named Toxoplasma (from the Greek words for arc: toxo and creature: plasma), gondii (after the rodent) in 1909. In 1923, Janku reported parasitic cysts in the retina of an infant who had hydrocephalus, seizures, and unilateral microphthalmia. Wolf, Cowan, and Paige (1937–1939) determined these findings represented the syndrome of severe congenital T. gondii infection.
Toxoplasmosis can be difficult to distinguish from primary central nervous system lymphoma, and as a result, the diagnosis is made by a trial of therapy (pyrimethamine, sulfadiazine, and folinic acid (USAN: leucovorin)), followed by a brain biopsy if the drugs produce no effect clinically and no improvement on repeat imaging.
Detection of T. gondii in human blood samples may also be achieved by using the polymerase chain reaction. Inactive cysts may exist in a host which would evade detection.
Toxoplasmosis cannot be detected with immunostaining. Lymph nodes affected by Toxoplasma have characteristic changes, including poorly demarcated reactive germinal centers, clusters of monocytoid B cells, and scattered epithelioid histiocytes.