Anthelmintic resistance in sheep nematodes in Australia: a compilation of recent test results

OBJECTIVE: The results of recent faecal egg count reduction tests (FECRTs) were collated to provide an indication of the prevalence and severity of anthelmintic resistance in sheep nematodes across Australia and of changes in comparison to earlier testing reports. METHODS: FECRTs conducted in New South Wales, South Australia, Western Australia and Victoria from 2021 to 2024 were analysed to indicate relative efficacy against the nematodes Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus species. The prevalence and percentage reductions were compared to previous reports of anthelmintic resistance in Australia. RESULTS: The efficacy of a variety of anthelmintics was assessed in FECRTs in several Australian states, with up to 80 tests for the broad-spectrum anthelmintics and at least 20 tests for closantel and naphthalophos. Resistance (a reduction in worm egg counts of less than 95%) in any one of the major nematodes Haemonchus contortus, T. circumcincta or Trichostrongylus species was seen for albendazole, levamisole, abamectin and moxidectin in 96%, 88%, 91% and 84% of tests, respectively, and in 66% of tests for a benzimidazole-levamisole-abamectin combination. For the newer anthelmintic groups, the comparable prevalence was 23% for both monepantel and a derquantel-abamectin combination, but the comparative efficacy (reduction in worm egg count) was considerably greater than for the older anthelmintics. For individual worm species, resistance in T. circumcincta was common and often severe to the benzimidazoles, levamisole and macrocyclic lactones; in H. contortus to the benzimidazoles and macrocyclic lactones; and in Trichostrongylus mostly to the benzimidazoles and levamisole. For H. contortus, closantel resistance was seen in 60% of tests, but resistance in this species was less prevalent for levamisole (21% of tests) and naphthalophos (31% of tests). For all species and anthelmintics, resistance had increased in comparison to previously reported figures. CONCLUSION: The FECRT figures suggest that there is little role for the benzimidazole, levamisole and macrocyclic lactone groups as single actives and that all anthelmintics will be used most effectively and sustainably as combinations of actives, but their efficacy on individual sheep properties should be confirmed with an FECRT.