Significance of Timing on Effect of Metaphylactic Toltrazuril Treatment against Eimeriosis in Calves.

In this multicentric, randomised, blinded and placebo-controlled field study, the effect of treatment with toltrazuril (Baycox(®) Bovis, Bayer) on oocyst excretion, diarrhoea score and weight gain was studied in Danish dairy herds with confirmed history of eimeriosis (coccidiosis) and prevalence of Eimeria bovis and Eimeria zuernii. Three commercial herds and a total of 71 calves, aged 48 - 135 days, were included. Treatment with a single oral dose of toltrazuril (15 mg/kg) was given after relocation to common pens and one week before expected outbreak of eimeriosis. The effect of treatment was followed by weekly faecal sampling and weighing initially and at the end of a study period of 8 weeks. In Herd 2 and 3 toltrazuril treated calves gained on average 7.95 kg more than placebo treated calves (p = 0.007), and both oocyst excretion and prevalence of Eimeria spp. were significantly reduced the first weeks post treatment. In Herd 1, by contrast, the farmer made some unforeseen changes in the management which entailed relocation to large deep-litter pens 3 - 6 weeks post treatment. In addition, many calves were not treated metaphylactically while few calves excreted oocysts when the trial was initiated. Thus, no significant difference in weight gain was found between toltrazuril and placebo treated calves (p = 0.523), and the oocyst excretion of toltrazuril treated calves was significantly higher during week 7 and 8. Significant differences in faecal scores were observed between the herds (p<0.002) but not between treatment groups in any of the herds. In conclusion, timing of treatment is crucial for optimal effect of metaphylactic toltrazuril treatment on weight gain and oocyst excretion.