World wide the prevalence of Toxoplasma immunity varies according to age, and it also differs from one country to another and even within the same country. Seropositivity is highest in areas with warm, moist climate where chances of survival of the oocysts are good, and lower in cold regions and high altitude. In central and southern Europe, many countries in Africa, and in South America, more than half of the population is infected with T. gondii, while in some countries with tropical climates, such as Guatemala, Costa Rica, and Tahiti, between 85 and 95% of the adult population have antibodies (Asburn 1992). In contrast only 11% of adults living in Iceland and 6% living in north Norway are infected (Jennum et al. 1998a; Jonsdottir 1989). Dry climate may also render the oocysts uninfective. Therefore, in environments with low rainfall or in which soil does not retain moisture, there is less risk of infection from oocysts. In Norway, the dry, cold inland has significantly fewer infections than the coastal areas (Stray-Pedersen et al. 1979). Altitude has also been shown to be inversely related to antibody prevalence. In Columbia, South America, 53% of the population living below 2500 m elevation had Toxoplasma antibodies, while only 43% of those living between 2 500 and 4 500 m had the antibodies. This may be due either to the climate in mountaineous areas being unfavourable for oocyst development or to the presence of fewer cats in the area. The differences in prevalence may also be ascribed to variations in hygiene and eating habits. In France, with the 'French cuisine' and a preference for undercooked and raw meat, the prevalence is especially high, with antibody prevalence exceeding 50% before the age of 10 years. The same is observed in different Asian groups that have a habit of eating raw meat. Otherwise, the Japanese and Chinese custom of cooking meat in small pieces minimizes the risk of acquiring the infection from meat (Wallace 1976). In Europe and in the United States, a decrease in Toxoplasma infection has been observed over the last 20 years. In France, a reduction in antibody prevalence from 80 to 69% during the last decades has been reported in the age group of 20-24 years ( Jeanell et al. 1988). In Sweden, the same tendency has been observed with a decrease from 28 to 10% in the same age group (Forsgren et al. 1991). In other countries the impression today is that an actual reduction in infection is taking place (Gilbert 2000). These changes might perhaps be ascribed to the increased consumption of instant or ready cooked food, to the more common use of home freezers for food storage, or to a general decrease in the infection rate of animals.
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