Surveys based on the presence of antibodies in blood sera have reported a worldwide distribution of T. gondii (Table 6.1). Most of these studies were based on convenience samples collected from slaughtered pigs. Prevalence of T. gondii varied dramatically among the classes of pigs surveyed (market pigs versus sows, indoor pigs from biosecure housing systems versus free-range or organic pigs). In the United States, the pigs sold for fresh pork consumption (feeder pigs, market pigs) are mostly raised indoors in well-managed facilities to prevent access to rodents and cats. In these well-managed facilities, prevalence of T. gondii has declined drastically in the last decade (Table 6.1). In a statistically valid population-based nationwide survey conducted in 1983-1984, seroprevalence was 23% in market pigs and 42% in breeder pigs (sows).319 When pigs from these same areas were tested in 1992, prevalence had dropped to 20.8% in breeders and 3.1% in nisher pigs.344 The institution of a National Animal Health Monitoring System (NAHMS) in the United States for swine now allows periodic surveillance of pigs for microbial infections. The prevalence of T. gondii in four NAHMS swine surveys in 1990, 1996, 1998, and 2006 showed a steady decline (Table 6.1). The prevalence of T. gondii in pigs is also inuenced by management systems. In poorly managed nonconnement systems, prevalence in pigs was as high as 68% (Table 6.1). Reports of T. gondii prevalence in pigs in People’s Republic of China were summarized by Zou et al.1386