Quality Control for the Molecular Diagnosis of Toxoplasmosis

Abstract

Toxoplasmosis is an endemic parasitic disease due to the protozoon Toxoplasma gondii. The definitive host is the cat, in which the parasite develops in the intestinal epithelium, before being eliminated as oocysts in the faeces. When oocysts, for example in contaminated soil, are ingested by humans, other mammals or birds, they pass through the stomach and excyst in the intestine; the released sporozoites invade the intestinal wall and give rise to generalized infection (Remington & Desmonts, 1995; Wong & Remington, 1993). Two tissue stages are important in pathogenesis: tachyzoites and bradyzoites. Initially, the infection consists mainly of rapidly dividing tachyzoites. Then, with developing immunity, persistent tissue cysts containing hundreds of slowly multiplying bradyzoites develop, especially in the muscle and brain. This ability to live inside cells without destroying them allows evasion of host immunity and the development of a chronic stage of infection, lasting for years. Intermediate hosts become infected either by ingesting soil, water or foodstuff contaminated with cat faeces or by ingesting raw or undercooked meat containing tissue cysts. T. gondii is responsible for generally benign infections except when the disease occurs in pregnant women (congenital toxoplasmosis) or in immunocompromised individuals, such as human immunodeficiency virus-positive or grafted patients, in which cases the vital prognosis may be involved. In certain countries of Europe, including France, toxoplasmosis is regarded as a serious health problem. In France, the prevalence of acquired toxoplasmosis in adults is 44%, and the estimated yearly incidence of contamination in women during pregnancy and of congenital toxoplasmosis are high, respectively, 6-7/1000 and 0.1% of births (Berger et al., 2008, King et al., 2008). Prevention of congenital toxoplasmosis (CT), including prenatal diagnosis (PND), has become a national policy in France since 1978 (Thulliez, 1992). Serological screening and follow-up is established for non-immunized pregnant women, and associated with monthly ultrasound examinations. PND is proposed in case of seroconversion between 6 to 38 weeks of amenorrhea. For this, PCR-based molecular diagnostic tests using amniotic fluid, introduced in the early 90's, have rapidly eliminated the need for cordocentesis, and have competed with more classical biological methods. Wherever it has been implemented, PND of CT has clearly improved the prognosis and outcome of infected children (reviewed in (Bastien, 2002)).