Risk factors for transmission of Toxoplasma gondii on swine farms in Illinois

Two epidemiologic studies of risk factors for transmission of Toxoplasma gondii to swine were conducted for farms in Illinois. The first study was a cross-sectional survey of swine farms from the state of Illinois pseudorabies testing program, in which farm owners or managers were interviewed by telephone regarding presence of risk factors for transmission of T. gondii on the farm. There were 123 farms surveyed that provided blood samples for at least 30 sows. The mean sow seroprevalence was 19.5% (median = 10.0%). Multiple regression analysis of the association of sow seroprevalence with outdoor housing of sows, cat access to sow areas, number of sows, open feed storage and water delivery, delayed removal of carcasses, and presence of rodents on the farm indicated that higher sow seroprevalence was associated with cat access to sows (P = 0.009) and fewer sows in the herd (P = 0.05). The second study was a field investigation of 47 swine farms (37 from the cross-sectional study). Data collection included obtaining blood samples from swine, cats, and rodents, and fecal samples from cats, heart and brain tissue from rodents, and feed, water, and soil samples for T. gondii examination. The risk of T. gondii transmission from cats and rodents to sows and finishing pigs was evaluated, taking into account housing conditions and herd size. Multiple regression analysis indicated that T. gondii seroprevalence in finishing pigs increased with more seropositive juvenile cats on the farm (P < 0.0001) and higher seroprevalence in house mice (P = 0.0023). For sows, the only risk factor associated with increased T. gondii seroprevalence was a higher number of seropositive juvenile cats on the farm (P = 0.0008). Housing swine outdoors was not associated with a higher T. gondii seroprevalence. These results identify T. gondii infection in cats (particularly juveniles) and house mice as indicators of increased risk of transmission to swine.