[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Apicomplexans](https://durgical.com/en/microbiology/groups/apicomplexans/) › Toxoplasma gondii

[Microbiology](https://durgical.com/en/microbiology/) · [Apicomplexans](https://durgical.com/en/microbiology/groups/apicomplexans/)

# 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

## Key points

- Definitive host: the cat; humans are intermediate hosts.
- Classic congenital triad: chorioretinitis, hydrocephalus and diffuse intracranial calcifications.
- HIV with low CD4: multiple ring-enhancing lesions; differential is CNS lymphoma; prophylaxis with trimethoprim-sulfamethoxazole.
- Treatment: pyrimethamine plus sulfadiazine with leucovorin; spiramycin early in pregnancy.
- Retinochoroiditis: the most common infectious cause of posterior uveitis.

## 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 →](https://durgical.com/en/conditions/toxoplasmosis/)
- Congenital toxoplasmosis: chorioretinitis, hydrocephalus and intracranial calcifications; many infants look well at birth. [Toxoplasmosis →](https://durgical.com/en/conditions/toxoplasmosis/)
- Ocular toxoplasmosis: necrotizing retinochoroiditis, the most common infectious cause of posterior uveitis. [Uveitis →](https://durgical.com/en/conditions/uveitis/)
- Toxoplasmic encephalitis in advanced HIV: multiple ring-enhancing brain lesions. [HIV infection and AIDS →](https://durgical.com/en/conditions/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:

- [Sulfadiazine](https://durgical.com/en/pharmacology/sulfadiazine/)
- [Calcium folinate](https://durgical.com/en/pharmacology/calcium-folinate/)
- [Clindamycin](https://durgical.com/en/pharmacology/clindamycin/)

## 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.

## In Colombia

Infection is very common in Colombia: in studies from Quindío most adults are IgG-positive, and ocular toxoplasmosis occurs at about 3 cases per 100,000 people a year. Untreated drinking water appears to be an important source in the country. Ocular toxoplasmosis is a significant cause of visual loss, and screening in pregnancy is part of prenatal care.

## Memory aid

«Cat, meat, water» are the doors in; in the baby it leaves «eye, water and chalk»: chorioretinitis, hydrocephalus and calcifications.

## 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.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/microbiology/toxoplasma-gondii/)

## Related

- [Cryptosporidium spp.](https://durgical.com/en/microbiology/cryptosporidium-spp/)
- [Plasmodium falciparum](https://durgical.com/en/microbiology/plasmodium-falciparum/)

## Related conditions

- [Toxoplasmosis](https://durgical.com/en/conditions/toxoplasmosis/)
- [Uveitis](https://durgical.com/en/conditions/uveitis/)
- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Apicomplexans](https://durgical.com/en/microbiology/groups/apicomplexans/)

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](https://www.cdc.gov/dpdx/toxoplasmosis/index.html) (CDC, 2024); [Clinical Care of Toxoplasmosis | Toxoplasmosis | CDC](https://www.cdc.gov/toxoplasmosis/hcp/clinical-care/index.html) (CDC, 2024); [Toxoplasmosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000637.htm) (MedlinePlus (NLM), 2024); [High frequency of ocular toxoplasmosis in Quindío, Colombia and risk factors related to the infection](https://pmc.ncbi.nlm.nih.gov/articles/PMC8056222/) (Heliyon (PMC, NCBI), 2021).

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