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.