Risk Factors Associated with Increased Toxoplasma gondii Seroprevalence in South Australian Sheep

Abstract

Toxoplasma gondii (T. gondii) is a protozoan parasite of substantial impact to small ruminants, with reproductive failure a possible outcome of exposure. This observational study assessed T. gondii prevalence within the South Australian sheep population and investigated on-farm risk factors to T. gondii exposure via a cross-sectional survey. 1433 individual animals, consisting of 1282 mixed-age ewes and 151 mixed-age rams were blood sampled and serologically screened for T. gondii-specific antibodies. A risk-analysis questionnaire was conducted for each participating property. Of the 1433 animals sampled, 530 tested positive, with seroprevalence observed to be 39% (95% CI 28.7% to 49.3%) after accounting for clustering within properties. All properties returned at least one positive result, indicating a flock level seroprevalence of 100%. T. gondii seroprevalence was found to be higher in sheep on Kangaroo Island (46.6%; 95% CI 32.1% to 61.1%) compared to the South Australian mainland (31.3%; 95% CI 18.4% to 44.2%), however this difference was not statistically significant (P=0.125). A significant association was observed between T. gondii seroprevalence and age, with seroprevalence increasing from 30.2% (95% CI 17.7% to 42.6%) in one year old sheep, to 69.7% (95% CI 47.0% to 92.5%) in sheep older than six years (P=0.001). T. gondii seroprevalence was significantly higher in animals drinking water from surface water sources (55.9%; 95% CI 35.2% to 76.6%), compared to those exclusively sourcing reticulated mains water (19.1%; 95% CI 0%-39.0%) (P=0.028). An individual animal exposed to a surface water source was found to be more than ten times as likely to be exposed to T. gondii, than an animal sourcing only reticulated mains water (odds ratio:10.68; 95% CI 1.30 to 87.88). Water source is important in the transmission of T. gondii to South Australian sheep. Mitigation strategies should be developed and targeted at reducing contact between oocysts and water sources and reducing interaction between livestock and contaminated water.