Toxoplasma gondii
On this page
Direct answer
Toxoplasma gondii is an obligate intracellular protozoan whose definitive host is the cat, and humans acquire it by eating undercooked meat containing tissue cysts or by ingesting sporulated oocysts. Infection in pregnancy can cross the placenta, and in HIV it reactivates as cerebral disease when CD4 counts fall below about 100 cells per microlitre. The exam-critical fact is that sexual reproduction happens only in the cat intestine.
What you must remember
- Definitive host: The domestic cat, a felid, sheds oocysts in faeces, and sexual reproduction occurs only in its intestine.
- Infective forms: Tachyzoites cause acute infection, bradyzoites persist in tissue cysts, and sporozoites are released from oocysts.
- Congenital triad: Chorioretinitis, hydrocephalus and intracranial calcifications form the classic triad of congenital toxoplasmosis.
- Timing in pregnancy: Transmission to the fetus rises with gestational age, but damage is most severe after first-trimester infection.
- Cerebral toxoplasmosis: Ring-enhancing lesions in a patient with HIV and low CD4 count are typical.
- Treatment: Pyrimethamine with sulfadiazine and folinic acid is the standard regimen for cerebral disease.
- Serology: IgM can persist for months, so IgG avidity testing helps date an acute infection.
- Prevention: Cooking meat thoroughly, washing vegetables and avoiding cat litter handling in pregnancy reduce risk.
A life cycle that moves between cat and meat
Toxoplasma has a sexual cycle confined to cats and an asexual cycle that infects almost any warm-blooded animal. Each route of infection carries a different risk, which is why the same parasite can harm a foetus and also cause disease in an immunocompromised adult.
- Oocyst shedding: A cat that eats infected prey sheds unsporulated oocysts in its faeces for a short period. The oocysts need about one to five days in the environment to sporulate and become infective.
- Ingestion: Humans become infected by ingesting sporulated oocysts from soil or contaminated food, or by eating undercooked meat that contains bradyzoite tissue cysts.
- Tachyzoite spread: Tachyzoites invade macrophages and other nucleated cells, then disseminate through the bloodstream to cause the acute illness.
- Tissue cyst formation: Immune control converts the parasite into bradyzoites inside tissue cysts, which persist for life in brain and muscle.
- Transplacental transmission: Tachyzoites cross the placenta during acute maternal infection, so the fetus can be infected at any stage of pregnancy.
Where the exam tries to catch you
Toxoplasma questions usually mix up the stage, the host and the drug.
- Trap: Assuming congenital infection is most severe in the third trimester Correct: transmission peaks late in pregnancy, but first-trimester infection causes the most serious fetal damage.
- Trap: Choosing sulfadiazine alone for cerebral toxoplasmosis in HIV Correct: pyrimethamine plus sulfadiazine with folinic acid is the standard regimen, since pyrimethamine adds a second antiparasitic action.
- Trap: Reading IgM positivity as proof of recent infection Correct: IgM can persist for many months, so IgG avidity testing is needed to time the infection.
- Trap: Naming the dog as the definitive host Correct: only felids are definitive hosts, and the sexual cycle ends in the cat intestine.
Frequently asked questions
Which host is definitive for Toxoplasma gondii?
The cat is the only definitive host for Toxoplasma gondii, because sexual reproduction and oocyst formation occur only in its intestinal epithelium. Other warm-blooded animals, including humans, act as intermediate hosts.
Which triad is seen in congenital toxoplasmosis?
Congenital toxoplasmosis classically presents with chorioretinitis, hydrocephalus and intracranial calcifications. Neonatal eye examination and cranial imaging help identify this triad, which is the key clinical pattern tested in exams for congenital toxoplasmosis.
What is the treatment for cerebral toxoplasmosis in HIV?
Cerebral toxoplasmosis in HIV is treated with pyrimethamine and sulfadiazine plus folinic acid, usually for several weeks. Maintenance therapy continues until immune recovery is established on antiretroviral therapy.
Which tests diagnose toxoplasmosis in pregnancy?
Toxoplasmosis in pregnancy is diagnosed by IgM and IgG serology, with IgG avidity testing used to date an acute infection. PCR of amniotic fluid detects fetal infection in suitable cases.
Why is cat litter a risk in pregnancy?
Cat litter can carry sporulated oocysts shed by infected cats, so handling it exposes a pregnant woman to infection. Wearing gloves and removing litter daily reduces this risk.