The bias, accuracy and precision of faecal egg count reduction test results in cattle using McMaster, Cornell-Wisconsin and FLOTAC egg counting methods

The faecal egg count reduction test (FECRT) is the recommended method to monitor anthelmintic drug efficacy in cattle. There is a large variation in faecal egg count (FEC) methods applied to determine FECRT. However, it remains unclear whether FEC methods with an equal analytic sensitivity, but with different methodologies, result in equal FECRT results. We therefore, compared the bias, accuracy and precision of FECRT results for Cornell-Wisconsin (analytic sensitivity = 1 egg per gram faeces (EPG)), FLOTAC (analytic sensitivity = 1 EPG) and McMaster method (analytic sensitivity = 10 EPG) across four levels of egg excretion (1-49 EPG; 50-149 EPG; 150-299 EPG; 300-600 EPG). Finally, we assessed the sensitivity of the FEC methods to detect a truly reduced efficacy. To this end, two different criteria were used to define reduced efficacy based on FECR, including those described in the WAAVP guidelines (FECRT <95% and lower limit of 95%CI <90%) (Coles et al., 1992) and those proposed by El-Abdellati et al. (2010) (upper limit of 95%CI <95%). There was no significant difference in bias and accuracy of FECRT results across the three methods. FLOTAC provided the most precise FECRT results. Cornell-Wisconsin and McMaster gave similar imprecise results. FECRT were significantly underestimated when baseline FEC were low and drugs were more efficacious. For all FEC methods, precision and accuracy of the FECRT improved as egg excretion increased, this effect was greatest for McMaster and least for Cornell-Wisconsin. The sensitivity of the three methods to detect a truly reduced efficacy was high (>90%). Yet, the sensitivity of McMaster and Cornell-Wisconsin may drop when drugs only show sub-optimal efficacy. Overall, the study indicates that the precision of FECRT is affected by the methodology of FEC, and that the level of egg excretion should be considered in the final interpretation of the FECRT. However, more comprehensive studies are required to provide more insights into the complex interplay of factors inherent to study design (sample size and FEC method) and host-parasite interactions (level of egg excretion and aggregation across the host population).