Toxoplasma gondii
Also known as: toxoplasmosis
Toxoplasma gondii is an intracellular protozoan parasite that causes toxoplasmosis. Cats are the definitive host, and humans are infected by ingesting oocysts from cat feces or tissue cysts in undercooked meat, or through transmission across the placenta.
Toxoplasma gondii is an obligate intracellular protozoan with worldwide distribution. Members of the cat family are the only definitive hosts, meaning the sexual stage of the life cycle occurs in feline intestine and infectious oocysts are shed in cat feces. Humans and other mammals are intermediate hosts. Infection follows ingestion of oocysts from contaminated soil, water, or unwashed produce, or of tissue cysts in undercooked meat — most often pork or lamb. Vertical transmission across the placenta and transmission through organ transplantation are the other important routes.
In an immunocompetent person, primary infection is usually asymptomatic or produces a mild mononucleosis-like illness with cervical lymphadenopathy. The parasite is never fully cleared: it persists as latent tissue cysts, particularly in brain and skeletal muscle, held in check by cell-mediated immunity. When that immunity fails — most classically in advanced HIV with a low CD4 count — the cysts reactivate and cause toxoplasmic encephalitis, which appears on contrast imaging as multiple ring-enhancing lesions, often in the basal ganglia. The main radiographic differential is primary CNS lymphoma.
Congenital toxoplasmosis follows primary maternal infection during pregnancy and is one of the TORCH infections. The classic triad is chorioretinitis, hydrocephalus, and diffuse intracranial calcifications, though many affected newborns appear normal at birth and develop chorioretinitis or neurologic deficits later. Prevention advice for pregnant patients centers on avoiding undercooked meat and cat litter handling. Treatment of active disease is pyrimethamine plus sulfadiazine with leucovorin rescue, and prophylaxis in severely immunocompromised patients uses trimethoprim-sulfamethoxazole.
USMLE Step 1 tests Toxoplasma gondii heavily in parasitology and in HIV-related opportunistic infection questions. The two highest-yield presentations are ring-enhancing brain lesions in an immunocompromised patient and the congenital triad in a neonate; know the cat and undercooked meat exposures that appear in the vignette stem.
Key takeaways
- Toxoplasma gondii is an intracellular protozoan whose definitive host is the cat.
- Humans acquire it from oocysts in cat feces, tissue cysts in undercooked meat, or transplacental spread.
- Primary infection in healthy hosts is usually mild, but latent cysts persist for life.
- Reactivation in immunocompromised patients causes encephalitis with multiple ring-enhancing lesions.
- Congenital toxoplasmosis classically causes chorioretinitis, hydrocephalus, and intracranial calcifications.