Toxoplasma gondii is an obligate intracellular protozoan parasite which is able to infect all warm- blooded vertebrates including mammals and birds.
Felids, e.g. domestic cats, are definitive hosts of T. gondii, and may excrete oocyst stages which are resistant to environmental influences and remain infectious for longer periods of time. Contaminations of food or drinking water with oocysts can cause infections of intermediate hosts (e.g. humans).
Up to one third of the human population is infected with T. gondii. Depending on the age group, up to two thirds of the German population have antibodies against the parasite. Most primary infections of humans are asymptomatic; some of the postnatally infected patients develop lymphadenopathy or ocular toxoplasmosis. Primary infection acquired during pregnancy may cause severe damage to the fetus. In immunosuppressed patients, reactivation of a latent infection can cause life-threatening encephalitis.
Human infections are mainly caused by consumption of raw or insufficiently cooked meat which contains live, encysted stages of T. gondii, or by ingestion of water or food contaminated with oocysts derived from the faeces of infected felids.
Toxoplasma gondii has a clonal population structure. In Northern America and Europe, mainly three clonal lines are found (lines I, II and III) which can be differentiated by means of genotyping procedures.