Potential limitations of the undifferentiated faecal egg count reduction test for the detection of anthelmintic resistance in sheep.

Larval cultures were used to determine the identities and occurrences of those parasites (excluding Nematodirus) represented by strongylid eggs at the time of anthehnintic administration in ovine faecal egg count reduction tests submitted to the Batchelar Animal Health Laboratory between 1992 and 1994. The numbers of individual nematode genera recorded in pre-treatment samples from these cases ranged from one to five and included infections of Haemonchus, Ostertagia, Ttichostrongylus, Cooperia and Oesophagostomum/Chabertia. Adequate egg representation for testing purposes by all five genera simultaneously was found to occur in only 17 (10%) of the 163 cases examined, with the majority (71%) of them containing between one and three nematode genera. The greatest representation occurred in those tests conducted during the months of February to May. However, even during this period, worm eggs of all five genera were concurrently present on only 16% of occasions. The importance of knowing what nematode genera are adequately represented at the time of routine faecal egg count reduction testing and the relevance of this information to reducing the likelihood of being misled when under- taking assessments of farm resistance status are discussed.