Are multiple pre-treatment groups necessary or unwarranted in faecal egg count reduction tests in sheep?

Previously conducted faecal egg count reduction tests (FECRTs) in sheep involving a number of different anthelmintic treatments, were used to examine the effects of comparing post-treatment faecal egg counts (FECs) with pre-treatment counts from either the same treatment groups (matched FECRs) or with those from other treatment groups (unmatched FECRs). Each of these unmatched FECRs were considered to be analogous to those that might otherwise have been obtained by the use of a randomly selected group of animals to provide a single pre-treatment baseline for comparing all post-treatment results. An examination of these comparisons showed that the use of either procedure was likely to result in similar estimates of anthelmintic efficacy and the detection of a comparable number of cases of anthelmintic-resistance. Only on 1.1% of occasions did the FECRs from any of the unmatched groups fall outside the 95% confidence limits of the FECRs of their corresponding matched counterparts and in just 9.8% (54/553) of instances were there any disagreements between the number of cases categorised as either resistant or susceptible on the basis of a < or ≥ 95% FECR. These findings suggest that any improvements in accuracy and reliability that might supposedly be achieved by the use of multiple pre- and post-treatment FECs from the same treatment groups as opposed to those likely to be provided by the use of a single randomly selected representative pre-treatment group, may be largely illusory.