The prevalence of Toxoplasma antibodies increases with age and the acquisition is directly dependent on the prevalence of Toxoplasma in the environment. All babies born to infected mothers are seropositive. However, after 6-12 months the passively transferred maternal antibodies have disappeared, and the baby is seronegative, except in a very few cases with congenital infection (Stray-Pedersen 1993). Once the child starts to walk and become mobile, contact with the parasite in the form of oocysts in soil, sandboxes, and playgrounds, are possible. The local food habits may also influence the infection during childhood.
In the later years, however, most seroconversions occur in the age group of 15-35 years. In Norway, a steady increase in seroprevalence was observed in the higher age groups (Stray-Pedersen and Jennum 1992). Only 23% of patients of the age of 45, admitted to a district hospital, had antibodies, while 46%
of those aged 84 years or higher had evidence of past Toxoplasma infection, indicating an annual incidence of approximately 0.6%.
Generally, there seems to be no sex differences in the incidence of infection (Beverly et al. 1976). In Norway, the same prevalence of Toxoplasma antibodies was found in military recruits and pregnant women of the same age (Stray-Pedersen et al. 1979). In one study,
however, boys up to the age of 15 years had a higher incidence of infections than girls (Beverly et al. 1976). It is possible that boys, having more outdoor activities than girls, have a higher risk of infection by oocysts. On the other hand, more women than men above 25 years are infected, and this has been ascribed to women having more contact with cats, with raw meat during cooking, and doing more gardening than men. The increased prevalence is thus probably due to higher level of exposure to the parasites.
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