Toxoplasma gondii

The parasite

Toxoplasma gondii is an obligate intracellular parasite belonging to the order of Eucoccidiorida and phylum Apicomplexa. It was discovered in 1908 by Nicolle and Manceaux at the Pasteur Institute in Tunis. The parasite cycle includes asexual reproduction that occurs in various tissues in warm-blooded mammals and birds (intermediary hosts) and sexual reproduction that occurs in the digestive epithelium of cats and other felines (definitive hosts). In their faeces, cats excrete oocysts that are not directly infective when excreted; they become so after sporulation (1 to 5 days) and are a potential source of infection for other hosts by ingestion. Faecal excretion of oocysts lasts 7 to 15 days after infection, the time for active immunity to be established. The dissemination of oocysts in the environment is not fully understood. In the intermediary host, ingested oocysts are lysed in the stomach and release forms that quickly disseminate into the bloodstream (tachyzoites). After a brief period of infection of a few days, the parasites encyst (in the form of bradyzoites) throughout tissues, particularly striated muscle tissue and the brain. These cysts can be the source of infection in the definitive host or a new intermediary host through ingestion (meat).

Three main genotypes of T. gondii (genotypes I, II and II) have been identified, equivalent to stable clonal lines through time and space. Genetic transfer can lead to atypical isolates, characterized by the presence of unique alleles not found among the classic alleles of the three main genotypes. This has been observed in metropolitan France. Certain highly virulent genotypes are found in South America, particularly in Guyana, that cause severe acquired toxoplasmosis (ocular or pulmonary forms).  (See Strains Pole)

As the parasite is ubiquitous, toxoplasmosis is found on all continents, but with a variable seroprevalence from country to country, and even region to region. Around 40% of the adult population in France is infected, with wide-ranging regional disparities.

The main modes of infection are through food. Humans are usually infected by ingesting cysts present in mammal meats (including venison), where oocysts from faecal matter of infected cats contaminates fruits, vegetables, water and hands. Infection by ingesting circulating tachyzoites (for example, through raw milk) is possible, though exceptional. The risk of inter-human transmission is rarer. It is observed in cases of congenital transmission when a pregnant woman is infected during pregnancy, or when cysts are transmitted during organ transplantation (from a toxoplasmosis seropositive donor to a seronegative recipient). French legislation demands the determination the recipient and donor immunological status since 1997.

The respective share of contamination by cysts through meat consumption or by oocysts through vegetables and water are not precisely known, particularly given the lack of detection methods. Nevertheless, various studies have identified eating meat as a risk factor for infection. Most infections were caused through food, particularly through eating rare meat (especially rare mutton). Infection by oocyte ingestion through drinking water has been recognized in various epidemics in Canada and Brazil (See Epidemiology Pole)