Major routes of infection

Humans usually acquire the infection incidentally by ingestion. Consumption of undercooked or raw or minced meat containing tissue cysts represents important sources. Toxoplasma cysts have been recovered from pork (5-30%), mutton (12-25%), and more rarely from beef (0-9%) and chicken (Frenkel and Dubey 1972). Handling of raw or undercooked meat and poor kitchen hygiene have been associated with an increased risk of infection (Kapperud et al. 1996). The tissue cysts are destroyed by freezing (-10°C overnight), thawing, cooking until change of colour (>66°C), desiccation, or exposure to water for more than 30 min (Kotula et al. 1991; Dubey 2000). Cooking in a microwave oven, however, is not sufficient to kill the cysts (Lunden and Uggla 1992).

Another source of infection is contact with the oocyst either by consumption of unwashed fruits, berries, and vegetables, which may have oocysts on their surface, or by direct contact with cat faeces. In children, direct ingestion of oocysts from dirt in sandboxes and yards where cats have defaecated, is supposed to be the major route of infection.

Admittedly, the importance of these two main routes of contamination: both consumption of tissue cysts and contact with cat faeces, has been questioned (Klapper and Morris 1990), since the prevalence of Toxoplasma antibodies appears to be the same among vegetarians and meat-eaters and Toxoplasma seropositivity is rather rare in young children.

Occasionally, infection can occur after self-inoculation in the laboratory, after inhalation of sprouted oocysts, and after blood transfusion or organ transplantation. Parasites have been isolated from sputum, saliva, urine, and semen, but transmission through such body fluids has never been proved. In fact, the only proven route of human-to-human transmission is the transmission of tachyzoites across the placenta to the foetus during the primary stage of infection in pregnant women.

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