Multiple resistance to anthelmintics in sheep nematodes and comparison of methods used for their detection

Intensification of animal production systems has led to an increased reliance on effective anthelmintics to control parasitic worms. However, the excessive and continued use of these “wormers” can lead to high selection pressures and have resulted in increased reports of emerging nematode populations exhibiting resistance to all of the main anthelmintic classes. Faecal egg count reduction tests (FECRT) were conducted on six farms in England and Wales according to standardised guidelines produced by the World Association for the Advancement of Parasitology. Selected farms were identified from a network of 40 study farms participating in a study investigating implementation of Sustained Control of Parasites in Sheep (SCOPS) principles. Resistance to the larval development test (LDT) to either benzimidazole- or imidazothiazole-group anthelmintics had been previously detected on these farms. In this study, resistance was indicated as present by LDT and/or FECRT to one or more groups of anthelmintics on a number of the study farms. Comparisons were made between results obtained by the two tests on the six farms. Benzimidazole resistance was identified by FECRT on five farms; imidazothiazole resistance on four farms and macrocyclic lactones resistance on five farms. The LDT identified the presence of benzimidazole resistance on all six farms, and imidazothiazole resistance on five farms. Generally, there was good agreement between the two tests in identifying both benzimidazole and imidazothiazole resistance. On one farm, the LDT identified the presence of benzimidazole-resistant nematodes, not detected by FECRT and on two farms the presence of imidazothiazole-resistant nematodes not detected by FECRT. On two farms “triple” resistance (i.e. resistance to all three groups of anthelmintics) was identified by FECRT and on one farm moxidectin resistance was suspected based on an early return to egg laying at 28 days post-treatment. Resistance was present in one or more genera, but most commonly in Teladorsagia on all six farms.