T. gondii infection is widespread among humans and its prevalence varies widely from place to place. Dubey and Beattie302 summarized T. gondii prevalences before 1988 and Tenter et al.1271 did so for surveys 1989-2000. Selected reports in pregnant women or women of childbearing age are given in reference 889a and in Table 2.1. In these surveys the prevalences of human infection ranged from a low of 4% in Korea to a high of 92% in Brazil. Sample size, age, and serological techniques may account for some of the differences in the reported prevalences. Infections are found on all continents, including Antarctica. In general, seroprevalences are higher in Latin America than in North America and East Asia. It is of interest that the magnitude of antibody titers in some South American countries is higher than that in the United States, perhaps related to the probability of repeat infections.543a The DT antibody titers in the general population in El Salavador,1091 Costa Rica,543a and six other countries in Latin America were higher than a survey in the United States. The serological test and the cutoff titer used are major considerations in any survey, and, as indicated in Chapter 1, DT titer may decline to the extent that antibodies are detectable only in undiluted serum. In the Costa Rican survey titers of 1:8 or lower accounted for 3% and lower than 1:64 accounted for 14% of the samples. Considering the bioassay data in pigs (see Chapter 6) there is no reason why these low DT titers are nonspecic. Infection is, on the whole, more common in warm climates and in low-lying areas than in cold climates and mountainous regions. This is probably related to conditions favoring sporulation and survival of oocysts. During the last decade, prevalences in general have decreased (Figure 2.1). More data on decreasing seroprevalences are available from countries that have established programs for screening of pregnant women.55,87,781 Berger et al.87 summarized data on French women of ve different ages (20, 25, 30, 35, and 40 years old) from 1995 to 2003 and concluded that prevalence decreased in all ve age groups. The greatest decline was in 20-year-olds (17.4% decrease from 37.6% in 1995 to 31.0% in 2003), and lowest in 40-year-olds (13.1% decrease from 65.4% in 1995 to 56.8% in 2003). These studies were based on 13,459 women in 1995 and 15,108 in 2003 tested by ELISA, and data were pooled from different laboratories. Another example of this decreasing prevalence is given in Table 2.2. A population-based National Health and Nutrition Examination Survey (NHANES) found a decrease in the age-adjusted T. gondii prevalence in U.S.- born persons 12-49 years old, from 14.1% in 1988-1994 to 9% in 1999-2004, a seroprevalence of 11% in U.S.-born women 15-44 years old in 1999-2004, and a seroprevalence of 28.1% in foreignborn women in 1999-2004.738 These population-based surveys are expensive and difcult to conduct in countries with a large population. The constant inux of immigrants with different ethnic backgrounds and cultures in a country such as the United States also affects seroprevalence estimates. In North America (most data are from the United States) there is an upsurge of infection during adolescence, whereas in Central and South America there is a steady rise during childhood.426,652,734 There are only a few reports of seroprevalences in young children. In one study of 500 3-10-year-old

Guatemalan children, antibodies were found in 37.8%; seroprevalence increased from 25% in 3-yearolds to 45% in 5-year-olds.736 Souza et al.1217 found marked differences in seropositivity among 608 school children from two localities, Bonsucesso (166 children) and Jacarepaguá (442 children) and two age groups (6-8-year-olds, and 12-14-year-olds) within the city of Rio de Janeiro, Brazil.