Anthelmintic resistance of gastrointestinal nematodes in dairy calves within a pasture‐based production system of south West Western Australia

The objective of this study was to determine the prevalence of gastrointestinal nematodes among post‐weaned calves aged between 4 and 12 months old within a pasture‐based system of south west Australia and quantify the level of anthelmintic resistance. Pre‐treatment FECs were monitored on 14 dairy farms. Anthelmintic resistance was assessed on 11 of the farms. Control FECs were compared with anthelmintic FECs at 14 days post‐treatment with doramectin (injectable), levamisole (oral), fenbendazole (oral) and a levamisole/abamectin combination (pour‐on). Results demonstrate a strong level of anthelmintic resistance, with at least one class of anthelmintic failing to achieve a 95% reduction in FEC in one or more gastrointestinal nematode species. Doramectin was fully effective against Ostertagia, but C. oncophora displayed resistance in 91% of the farms. Conversely, levamisole was fully effective against C. oncophora, but Ostertagia displayed resistance in 80% of the farms. Fenbendazole resistance was present in both C. onocphora and Ostertagia in 64% and 70% of the farms, respectively. Trichostrongylus showed low resistance, occurring in doramectin (14%) and levamisole/abamectin combination (14%). This study confirms that anthelmintic resistance is common. Regular FEC reduction testing is recommended to monitor and guide decision‐making for appropriate anthelmintic usage.