The detection of anthelmintic resistance by the faecal egg count reduction test: an examination of some of the factors affecting performance and interpretation.

This study examines and compares the possible effects of several procedural variants with those of a currently recommended faecal egg count reduction test for the detection of anthelmintic resistance. The results suggest that the failure of an anthelmintic to reduce the arithmetic mean egg counts of 10-15 animals by at least 90%, from either their pre-treatment levels or from those of an untreated control group 5-10 days later, is likely to be an appropriate procedure. It is recommended that such evaluations of drench performance be regularly conducted on farms. Some suggestions as to how this might be encouraged are made, and the role of larval cultures both in increasing the sensitivity and in enhancing the value of the faecal egg count reduction test are discussed.