A sero-survey of toxoplasmosis in farm and non-farm children from Wisconsin, United States, 1997-1999

Background: Toxoplasmosis is among the most widespread and prevalent zoonosis in the world. People can become infected through ingestion of oocysts shed by felids or of tissue cysts contained in meat from infected animals. Acute infection can result in a wide spectrum of consequences, including flu-like illness and retinitis, as well as congenital infection in pregnant women. Severe disease can occur, especially if people are immunocompromised. Frequency of human infection varies substantially by region due to ecological, social, and cultural factors. The most recent nationwide prevalence estimates in children from United States were 3.6% in 6-11 year olds and 5.8% in 12-19 year olds. Because of the limited knowledge of the occurrence of common zoonotic pathogens in children in the United States, the objective of this study was to estimate the sero-prevalence of T. gondii-specific antibodies in children from the Marshfield area in Wisconsin and to examine the association between sero-positivity and farm living. Methods. Banked sera from 342 Wisconsin children collected in 1997-1999, aged 2 to 18 years, were tested for Toxoplasma gondii-specific IgG antibodies using ELISA. Recorded information included age, sex, and whether the child resided on a farm. Impact of assay accuracy, sensitivity and specificity, on sero-prevalence was examined using Bayesian methods. Results: Observed prevalence of T. gondii-specific antibodies was 10.8% (37/347). Adjusting for sensitivity and specificity of the assays yielded a prevalence estimate of 8.0% (95% probability interval: 4% - 12.4%). Children living on a farm had a 5 times higher odds of T. gondii-specific antibodies than children not living on a farm (OR=5.08, 95% CI: 2.2 - 11.6). Conclusion: Results suggest that even in apparently low-risk populations, the true extent of the infection in children is significant. In this study population, children living on farms were differentially exposed, with earlier and higher infection risk than children not living on farms. Findings highlight the need to increase awareness about toxoplasmosis acquired early in life and to improve our understanding of the ecology of T. gondii in rural environments from developed and developing countries.