The efficacy of toltrazuril treatment for reducing the infection intensity of Theileria orientalis Ikeda type in dairy calves

•20 treatment and 20 control calves randomly selected on two dairy farms.•Toltrazuril treatment given 4 weeks post-turnout.•Theileria orientalis infection intensity, haematocrit and weight change measured over 16 weeks.•No effect of toltrazuril on infection intensity, haematocrit and weight gain. The aim of this study was to test the hypothesis that toltrazuril administered at 4 weeks post-turnout reduces the infection intensity of Theileria orientalis Ikeda type in dairy calves and so prevents serious clinical disease in these animals at 2–3 months of age. Two groups of 40 dairy calves on two separate dairy farms in the Waikato were followed for 16 weeks post-turnout onto pasture. On each farm, 20 calves were randomly selected and orally treated with toltrazuril (15 mg/kg) at 4 weeks post-turnout, whilst the remaining 20 calves were left untreated. All 40 calves were blood sampled and weighed at 2, 4, 6, 8, 12, and 16 weeks post-turnout i.e. 6 samplings per calf. A random subset of 10 calves from each treatment group on each farm were faecal sampled at each visit. The blood samples were used to estimate the T. orientalis Ikeda type infection intensity and haematocrit for each calf and the faecal samples were used to estimate the number of coccidia oocysts per gram of faeces. Three linear mixed effects models, to evaluate the effect of toltrazuril treatment on infection intensity, haematocrit (HCT) and weight respectively were fitted to the data. No calves on either farm developed clinical theileriosis or coccidiosis and the three mixed effects linear models, controlling for the effect of farm and days from turnout, showed that there was no effect of treatment on infection intensity (p = 0.81), on HCT (p = 0.99) and on weight gain (p = 0.79). In conclusion, this study showed no evidence supporting the use of toltrazuril to control T. orientalis Ikeda type infection levels and prevent disease.