Toxoplasmosis infects vast numbers of people, and most never notice
Estimates suggest between a third and a half of all humans carry the parasite behind toxoplasmosis, usually without a single symptom. The trouble comes when the immune system is weak or when infection strikes during pregnancy, and the dormant cysts it leaves in the brain stay put for the rest of a person's life.
Charles Nicolle and Louis Manceaux first described the organism, Toxoplasma gondii, in 1908, and passage from a pregnant woman to her baby was confirmed in 1941. Around 200,000 cases of congenital toxoplasmosis are thought to occur each year. Infection rates vary widely: roughly 11 percent of people in the United States have been infected, compared with more than 60 percent in some regions.
Most people pick it up by mouth. Common routes include meat that is raw or only partly cooked, especially pork, lamb and venison, knives and boards that touched such meat, fruit and vegetables carrying soil tainted with cat faeces, and hand-to-mouth contact after gardening, emptying a litter tray or playing in a sandpit. Organ and stem cell transplants and blood transfusions are rare routes. Some studies have found the parasite in semen, but sexual spread has not been shown to matter much.
In a healthy adult an acute infection is often silent or resembles flu, with fever, fatigue, headaches and swollen glands, usually in the neck or under the chin. Those glands settle within one to two months in 60 percent of cases, though for some people it takes far longer. People with HIV, recent transplants or certain chemotherapy face much worse: confusion, seizures, brain inflammation or damage to the retina. Most babies infected in the womb look healthy at birth but may develop problems later. For healthy people, treatment is usually unnecessary; in pregnancy doctors may use spiramycin or pyrimethamine with sulfadiazine and folinic acid. Diagnosis generally relies on blood antibody tests or on checking amniotic fluid for parasite DNA.
The parasite is adept at survival. It walls itself inside a membrane built from the host cell, blocks signals that would make infected cells self-destruct, and releases proteins that alter immune signalling. Its dormant stage does not respond to antibiotics. Latent infection has been tentatively tied to subtle behavioural changes and even a higher risk of road accidents.
Source: Toxoplasmosis
