Prevalence of antibodies to Toxoplasma gondii in swine in Illinois in 1992

6965 serum samples from swine in 179 herds were tested for antibodies to T. gondii using the modified direct agglutination test. In breeding swine, results for 1057 of 5080 (20.8%) sera tested were positive. In growing/finishing swine results for 59 of 1885 (3.1%) sera tested were positive, which was substantially lower than the seroprevalence rate estimated in a serological survey of pigs from abattoirs in Illinois in 1983 and 1984. Data were summarized for herds having at least 28 samples/herd. For breeding swine in 172 herds, the median, mean, and maximum seroprevalence rates were 6.7, 16.1, and 96.8% respectively, compared with 0.0, 2.8, and 20% for growing/finishing pigs in 44 herds. 61 herds with breeding swine (35.5%) and 28 herds with growing/finishing pigs (63.6%) had no seropositive pigs. The cumulative risk of T. gondii infection for swine in herds containing sero-positive pigs was estimated as 9.0% by 6 months of age for a herd that had the median seroprevalence rate. In contrast, for pigs in herds in the upper quartile of seroprevalence rates, risk of infection by 6 months of age was estimated to be greater than 20%. T. gondii infection in growing/finishing pigs and breeding swine was not a widespread problem, but was concentrated in a small proportion of herds.