This pilot study demonstrated evidence of exposure to T. gondii in a population of children from the Midwest of the United States. Results revealed exposure as early as 2–5 years of age and highlighted a significant differential risk associated with farm vs. non-farm living. Toxoplasmosis is an established zoonotic infection in the United States and large-scale prevalence studies have documented its occurrence and distribution. The observed sero-prevalence of T. gondii-specific antibodies in non-farm children in this study was 4.4% (Table
1), which is in general agreement with the national estimates of 3.7 % to 5.8% for children in various age groups
[1, 2]. However, farm residence was found to be associated with a 5 times higher odds of sero-positivity. The study took advantage of existing banked sera from children; therefore, limited study participant data were available to make inferences about risk factors or sources of infection beyond residence. Nevertheless, results suggest that farm-related routes of exposure may play an important role for T. gondii transmission to children.
Post-natal transmission of T. gondii could have occurred by two main routes, exposure to oocysts shed into the environment by felids, including domestic cats, or the consumption of tissue cysts found in undercooked meat from infected livestock. Risk of exposure from these two sources is dependent on environmental factors and human behaviors and choices. Studies indicate that the risk of infection from meat consumption in the United States varies by type of meat and origin. Evidence of exposure can be high in pigs, chicken, and sheep raised outdoors; however, prevalence is commonly low in livestock raised in conventional high-biosecurity facilities
[15, 16]. Cattle are not an important carrier of T. gondii. Furthermore, inactivation of tissue cysts by meat processing (salt treatments), storage practices (freezing), and proper cooking (in particular pig and poultry) contribute to reduction in infection risk from meat consumption
. On the other hand, the large number of oocysts shed by cats and their long survival (up to 18 months under proper conditions) contribute to the potential for widespread environmental contamination
, as well as the unrecognized ingestion of oocysts by people
[18–20]. Congenital transmission cannot be ruled out as a source of infection in the sero-positive children found in this study; however, based on a reported congenital infection rate of 1 in 10,000 newborns
, the likelihood that congenital infection could explain a significant fraction of the observed prevalence is unlikely. The higher risk of infection in children living on farms is consistent with previous reports
[22, 23]. Although not directly comparable, such reports suggested or hypothesized that environmental sources of infection seem to be significant in the studied age groups
[22, 24]; however, they also highlight the difficulties in attempting to identify specific sources from epidemiological data. The actual sources of infection in these young children remain to be investigated, revealing a gap of our understanding of the ecology of the pathogen in farm environments and of the level of awareness about infection risk by parents.
It is also important to note that although congenital toxoplasmosis is the subject of a significant amount of research, primary infection during childhood has received little attention. In this study, 7.8% of the children were already sero-positive by 10 years of age suggesting a potentially important level of exposure during early childhood. Studies of early exposure to T. gondii in high risk countries have found even higher sero-prevalences at these early ages, including a study in Panama reporting a prevalence of 49% by 10 years of age
. The significance of early infection in a still developing brain
 by a pathogen with tropism for the CNS has not been examined. Furthermore, because of the differentially higher risk of infection on farms, it would be important to investigate the current incidence of infection in women of child-bearing age living on these farms areas and examine the status of public health prevention programs targeting rural populations.
As it is often done in prevalence studies, the reported OP estimate was the result of applying an imperfect laboratory assay (SE and SP are not 100%). Adjusting the OP estimates for the SE and SP of the assay, including the uncertainty about their real values, allowed us to provide a range within which the actual prevalence would likely fall. The AP estimates was slightly lower than the OP; however, these results still suggest that, even in apparently low-risk populations, the true extent of this common zoonotic infection is often under-estimated. It is important to emphasize that the study was done on sera collected several years ago in a specific group of children and results may not reflect current conditions. It would be suitable to investigate if results would be similar today, and in a larger population, considering changes in food-consumption habits (e.g. higher demand for locally-produced meat or from animals raised outdoors in lower biosecurity production systems) and in disease prevention campaigns.