Toxoplasma gondii !

Parasite
Exam priority: high

A parasite caught from undercooked meat, water or soil with cat feces. Usually silent, it can harm the fetus, the eye and the brain in people with AIDS.

Updated 2026-11-01

Also called: toxo

At a glance

Shape: Crescent tachyzoite; tissue cyst with bradyzoites; oocyst in cat feces
Family: Sarcocystidae
Type: Obligate intracellular apicomplexan protozoan (coccidian)
Stain: Giemsa or hematoxylin-eosin in tissue; serology is the usual diagnosis
Size: Tachyzoite about 4 to 8 µm

Morphology

Tachyzoite: the fast-dividing, crescent-shaped form that infects any nucleated cell.
Bradyzoite: the slow form inside tissue cysts in brain, retina, heart and muscle, which persist for life.
Oocyst: shed only by felids in feces; it sporulates in the environment in 1 to 5 days.

Transmission

Eating raw or undercooked meat with tissue cysts (pork, lamb, beef, game).
Swallowing oocysts in untreated water, poorly washed produce, soil or sand with cat feces.
Transplacental, when the mother is first infected during pregnancy.
Organ transplant and transfusion, less often.

Reservoir: Felids (definitive host, especially the cat); Birds, rodents and food animals (intermediate hosts)

Isolation precautions

What the care team adds to standard precautions.

Standard

Life cycle

The cat sheds unsporulated oocysts in its feces for 1 to 3 weeks after infection.
In the environment the oocysts sporulate within days and become infective.
Intermediate hosts (rodents, birds, farm animals, humans) swallow oocysts or tissue cysts.
Parasites released in the gut become tachyzoites and spread through the blood.
Tachyzoites turn into bradyzoites and form tissue cysts in muscle, brain and eye.
The cat is infected by eating prey with cysts; the sexual phase in its gut produces oocysts.

Virulence factors

Active invasion of any nucleated cell via the apical complex.
A parasitophorous vacuole that avoids fusion with lysosomes.
Switch from tachyzoite to bradyzoite: latent cysts that reactivate when cellular (CD4) immunity falls.
Atypical South American strains described as more aggressive to the eye.

Diseases it causes

Acquired toxoplasmosis: nearly always silent, or cervical lymphadenopathy and a mononucleosis-like illness. Toxoplasmosis →
Congenital toxoplasmosis: chorioretinitis, hydrocephalus and intracranial calcifications; many infants look well at birth. Toxoplasmosis →
Ocular toxoplasmosis: necrotizing retinochoroiditis, the most common infectious cause of posterior uveitis. Uveitis →
Toxoplasmic encephalitis in advanced HIV: multiple ring-enhancing brain lesions. HIV infection and AIDS →

Clinical picture

Immunocompetent: painless cervical node, low fever, tiredness; resolves on its own.
Congenital: the risk of transmission rises with gestational age, but fetal damage is worse when infection happens early.
Ocular: blurred vision and floaters; an active white lesion beside a pigmented scar, with vitritis («headlight in the fog»).
HIV with very low CD4: headache, fever, focal deficits, seizures and confusion.

Diagnosis

Serology: IgG means infection at some time; a negative IgM essentially rules out recent infection; IgM can persist for months.
In pregnancy, IgG avidity and confirmatory panels help date the infection.
PCR on amniotic fluid to learn whether the fetus is infected.
Newborn: IgG, IgM and IgA; eye exam, cranial ultrasound or CT.
Encephalitis in HIV: MRI or CT with ring lesions in the basal ganglia; CSF PCR; response to empiric treatment. Primary CNS lymphoma is the key differential.

Treatment

Combined antifolates: pyrimethamine plus sulfadiazine, always with folinic acid (leucovorin) to protect the marrow; clindamycin or other alternatives for sulfa allergy. Early in pregnancy without fetal infection, spiramycin is used. Trimethoprim-sulfamethoxazole serves as primary prophylaxis in people with HIV and low CD4. Asymptomatic immunocompetent people usually need no treatment.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

Cooking meat well, washing produce and drinking boiled or treated water.
Handwashing after handling raw meat, soil or sand.
Pregnant and immunosuppressed people avoid changing cat litter; if they must, they use gloves and change it daily.
Serologic screening of pregnant women in prenatal care.
Trimethoprim-sulfamethoxazole prophylaxis for people with HIV, very low CD4 and positive IgG.

Vaccine: There is no human vaccine.

Test yourself

Think of the answer, then tap the question to see it.

What is the classic triad of congenital toxoplasmosis?

Chorioretinitis, hydrocephalus and diffuse intracranial calcifications.

Which drug is added to pyrimethamine to prevent bone-marrow toxicity?

Folinic acid (leucovorin), which protects the marrow without blocking the effect on the parasite.

Which test confirms fetal Toxoplasma infection?

PCR on amniotic fluid obtained by amniocentesis.

What is the main differential of ring-enhancing lesions in a patient with AIDS?

Primary central nervous system lymphoma; toxoplasmosis usually gives multiple lesions in the basal ganglia.

What does positive IgG with negative IgM mean in a pregnant woman?

Past infection, before the pregnancy; in an immunocompetent mother there is generally no risk of passing it to the fetus.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Toxoplasmosis (CDC, 2024); Clinical Care of Toxoplasmosis | Toxoplasmosis | CDC (CDC, 2024); Toxoplasmosis: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024); High frequency of ocular toxoplasmosis in Quindío, Colombia and risk factors related to the infection (Heliyon (PMC, NCBI), 2021).

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Representative model, for educational purposes only.