Question
Intermediate and definitive host for toxoplasma gondii.
Answer
Toxoplasma gondii is an obligate intracellular protozoan parasite with a life cycle involving both a definitive and a wide range of intermediate hosts.
Definitive host: members of the cat family (Felidae), particularly the domestic cat — the only hosts in which the parasite undergoes its complete sexual (enteroepithelial) life cycle within intestinal epithelial cells, ultimately producing environmentally resistant oocysts that are shed in cat faeces and become infective after sporulation in the environment (typically 1–5 days).
Intermediate hosts: a very wide range of warm-blooded animals, including humans, sheep, pigs, cattle, and birds — in these hosts, only the asexual cycle occurs; ingested oocysts (or tissue cysts from undercooked meat) release parasites that disseminate and form tissue cysts (bradyzoites), particularly in muscle and neural tissue, where they can persist for the lifetime of the host in a latent state.
Human infection (humans as an accidental/incidental intermediate host) occurs via:
- Ingestion of oocysts from cat faeces-contaminated soil/food/water (e.g., unwashed vegetables, or accidental hand-to-mouth transfer while gardening/handling litter boxes).
- Ingestion of undercooked/raw meat containing tissue cysts (bradyzoites), particularly pork, lamb, or venison.
- Transplacental (congenital) transmission — of major clinical importance, since primary maternal infection during pregnancy can cause severe congenital toxoplasmosis (chorioretinitis, hydrocephalus, intracranial calcification, and other malformations, particularly severe if acquired in early pregnancy).
- Rarely, via blood transfusion or organ transplantation from an infected donor.
Clinical significance: usually asymptomatic or mild/self-limiting in immunocompetent hosts (occasionally causing a mononucleosis-like illness with lymphadenopathy); causes serious disease in the immunocompromised (particularly AIDS patients, in whom reactivation of latent infection can cause toxoplasma encephalitis, an AIDS-defining opportunistic infection) and in congenital infection.