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

A serologic survey that tested for antibodies to Toxoplasma gondii was conducted, using the modified direct agglutination test, on 6,965 serum samples collected from swine in 179 herds in Illinois in 1992. In breeding swine, results for 1,057 of 5,080 (20.8%) sera tested were positive. In growing/finishing swine, results for 59 of 1,885 (3.1%) sera tested were positive, which was substantially lower than the seroprevalence rate estimated in a serosurvey of pigs from abattoirs in Illinois in 1983 and 1984. Data in the survey reported here were summarized for herds having at least 28 samples/herd. Among all herds, the median, mean, and maximum seroprevalence rates were 6.7, 16.1, and 96.8%, respectively, for breeding swine in 172 herds, and 0.0, 2.8, and 20.0%, respectively, for growing/finishing pigs in 44 herds. Among the 172 herds with breeding swine, 61 (35.5%) had no seropositive pigs. Among the 44 herds with growing/finishing swine, 28 (63.6%) had no seropositive pigs. A logistic regression model was used to estimate that the cumulative risk of T gondii infection for swine in herds containing seropositive pigs was 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%. Analysis of these data would suggest that overall prevalence of T gondii infection in pigs from Illinois is low; nevertheless, there is a small proportion of farms for which the rate of T gondii infection in swine is moderately high.