Confirmation of ivermectin resistance in Ostertagia ostertagi in cattle in New Zealand

•Confirming ivermectin resistance in field strains of Ostertagia ostertagi.•Dual resistant strains of Ostertagia ostertagi likely to be present in New Zealand.•Moxidectin is efficacious against the ivermectin resistant strains examined. Six suspected cases of ivermectin resistance in Ostertagia spp. in cattle were investigated after routine anthelmintic efficacy testing on commercial farms. On four farms a comprehensive faecal egg count reduction test (FECRT) was undertaken using oral formulations of ivermectin (0.2mg/kg), albendazole (10mg/kg) and levamisole (7.5mg/kg) while on two farms only ivermectin was tested. The proportions of Ostertagia spp. in the untreated control and post-treatment larval cultures were used to apportion egg counts to genera and determine efficacy against this genus. Isolates of Ostertagia spp. recovered from three of the farms were each used to infect 18 six month old calves. The efficacy of oral formulations of ivermectin and moxidectin, both at 0.2mg/kg, was determined against each isolate by slaughter and worm count. The efficacy of ivermectin against Ostertagia spp., based on differentiated FECRT for each of the farms varied from 0% to 88%. The efficacy of ivermectin based on worm counts in the slaughter trial varied from 13% to 75% but moxidectin was >99% effective against all isolates. In addition, in the FECRT albendazole, at a dose rate of 10mg/kg, failed to achieve 95% efficacy against Ostertagia spp. on two farms (82% and 85%). Levamisole consistently failed to achieve 95% efficacy against Ostertagia spp. which is consistent with its known lesser efficacy against this parasite. These results confirm the presence of macrocyclic lactone resistant O. ostertagi in cattle in New Zealand and the likely presence of dual resistance, to macrocyclic lactones and albendazole, in some isolates. Resistant populations of this highly pathogenic parasite are probably not uncommon in New Zealand and pose a significant threat to animal production and welfare in the future.