Toxoplasma gondii

The genus name describes the shape of the infectious stage: from the Greek toxon, a bow or arc, and plasma, a form. The species name records the animal in which it was first found, the gundi, a North African rodent (Ctenodactylus gundi), the name altered slightly to gondii. Charles Nicolle and Louis Manceaux described it at the Pasteur Institute in Tunis in 1908, the same year Alfonso Splendore found it independently in a rabbit in Brazil. Its full lifecycle was not established until about 1970, when it was shown to be a coccidian parasite of cats, with other warm-blooded animals, including humans, as intermediate hosts.

That lifecycle explains the hazard. Cats, the definitive host, shed durable oocysts in their faeces. In the muscle and brain of intermediate hosts the parasite forms dormant tissue cysts containing the slow stage, the bradyzoites. People are infected in three ways: by eating raw or undercooked meat containing tissue cysts, chiefly pork, lamb and game; by ingesting oocysts from cat faeces, soil, water or unwashed produce; or across the placenta, from a newly infected mother to the unborn child. Perhaps a third to a half of all people carry it, usually without symptoms.

For most people the infection is mild or silent. Its seriousness is in two groups. In pregnancy a first infection can cross to the foetus and cause congenital toxoplasmosis, with damage to the eyes and brain, hydrocephalus, or stillbirth. In the immunocompromised, dormant cysts can reactivate and cause encephalitis. This is the basis for the advice to avoid raw cured meats in pregnancy.

**How it endangers cured meat.** Its tissue cysts are present in the raw muscle of infected pork and lamb, and because these meats are often made into products eaten without cooking, cured pork is a recognised vehicle. The position for the curesmith is more favourable than for Trichinella, because the cysts are more sensitive to salt. Cooking above about 66 to 67 degrees C kills them quickly, and freezing to about minus 12 to minus 20 degrees C is generally effective, though domestic freezing is less reliable, being warmer and less constant, and one study recovered viable cysts from ham frozen at minus 20 degrees C. The main control is the cure itself: salt inactivates the cysts readily and early, with sodium chloride at 1.3 per cent and above shown to kill the bradyzoites in the early stages of dry curing, so a properly salted or long-cured product is considered safe. United States regulation requires curing to be validated against Trichinella but sets no such requirement for this protozoan, because ordinary curing controls it reliably.