Organism The coccidian protozoa Toxo-plasma gondii, found in the cat.

Clinical features The toxoplasma, which develop from the oocysts, disperse to many parts of the body including the CNS where they form small inflammatory foci (pseudo-cysts). They result in surprisingly little pathology to their host occasionally producing lymphadenopathy and a low-grade fever, but congenital infection can result from an acutely infected mother. The infant may have hepatosplenamegaly, corioretini-tis or mental retardation and quite often dies. Toxoplasmosis can also become reactivated in the immunocompromised (e.g. in the HIV-infected individual), causing pneumonia, myocarditis or fatal encephalitis.

Recent evidence has suggested that far from the pseudocysts causing little pathology when they attach to the brain of the non-neonate, they might produce some alteration in human response. Experiments in mice show that by the parasite selectively attacking the brain, it disables the animal's response to the presence of a predator so making it more easily caught and the parasite being transmitted. In the human, the effect is to prolong the reaction time making the victim more likely to have a road-traffic accident.

Diagnosis can be made by a rise in specific IgM during the first 8 weeks, IgG over several years or by lymph node biopsy.

Transmission Oocysts are passed in cat faeces and if accidentally swallowed by humans they become infected. Children are commonly infected when playing with pets, or sand and soil in which cats defecate. Adults are more commonly infected from swallowing pseudocysts in undercooked meat (generally mutton or pork). Congenital infection occurs when the mother becomes infected during the early course of her pregnancy. Oocysts can be inhaled or drunk in contaminated water, and toxoplasma tachy-zoites are passed in cow and goat milk.

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