Resistance to triclabendazole in Fasciola hepatica on a commercial sheep farm in Taranaki, New Zealand

To investigate potential resistance to anthelmintics containing triclabendazole in naturally acquired Fasciola hepatica (liver fluke) present on a sheep and beef farm in New Zealand. A necropsy study was conducted with three groups of mixed-age Romney ewes naturally infected with F. hepatica. Ewes were either left untreated as controls or treated with a single oral dose of either a combination of 0.2 mg/kg moxidectin and 10 mg/kg triclabendazole or 10 mg/kg triclabendazole alone, both at the manufacturer’s recommended dose. At necropsy, 49 days later, livers were collected for total adult liver fluke counts. All ewes in the trial had live liver fluke (min 1, max 148 flukes/liver). The efficacy was 30% for moxidectin plus triclabendazole and 21% for triclabendazole alone, confirming triclabendazole resistance on this farm. This is the first reported case of triclabendazole-resistant liver fluke in New Zealand. The finding of triclabendazole-resistant liver fluke in New Zealand should alert farmers and veterinarians to consider triclabendazole resistance as a potential cause of treatment failure, and adapt the farm liver fluke management and treatment regimens. In addition, resistant fluke could be widespread around New Zealand due to livestock being sold from this farm.