The agent of this disease, the sporozoan protozoan Toxoplasma gondii, is a common parasite of many species of birds and mammals. The organism was first seen in 1908 in the tissues of a Tunisian rodent, the gundi, and fully described in 1909. Human disease was first described in 1923 and congenital neonatal disease was reported in 1939, but the complex life cycle of the parasite was not elucidated until 1970. Serologic tests show that humans around the world harbor T. gondii, but because almost all infections are asymptomatic, very few have the disease. The protozoan is an intracellular parasite of a variety of tissues in warm-blooded vertebrates. It multiplies by binary fission in a host cell, eventually rupturing the cell and releasing parasites to attack other cells. Sexual reproduction can take place only in cats and other felines. These definitive hosts release oocysts, the stage infective for herbivores, in their feces. Asexual intracellular replication takes place in the herbivore, and, if the tissues containing T. gondii are eaten by a carnivore, asexual reproduction may also occur in their tissues. Humans can become infected by eating poorly cooked or raw meat or poultry, by ingesting oöcytes from the feces of cats, or congenitally.

Clinical Manifestations

Human infections are usually inapparent, although they sometimes can lie dormant for years and flare up in weakened or immunodeficient hosts. Most cases in otherwise healthy people are mild and cause vague symptoms like fever and weakness. The disease often mimics infectious mononucleosis. Chronic cases can cause diarrhea, headache, and eye damage.