Challenges in applying W.A.A.V.P. criteria to diagnosing triclabendazole resistance in Fasciola hepatica, an example from the Southern Tablelands of New South Wales, Australia.

Fasciola hepatica (liver fluke) is a zoonotic parasite of global concern. In Australia, it is the 13th most important cause of economic loss in the sheep meat industry alone. Resistance to the frontline drug, triclabendazole (TCBZ), was first recorded in Australia in 1995 and has since emerged globally. In 2023, producers from the New South Wales (NSW) Southern Tablelands raised concerns over a reported 230% increase in liver fluke, which they suspected was due to drug resistance. To confirm or deny these suspicions, we co-designed a diagnostic field investigation aligned with guidelines from the World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) to evaluate the prevalence and susceptibility of F. hepatica on naturally infected sheep, cattle, and goat properties. Nine mobs (seven sheep, one goat, one cattle) across eight farms were divided into three treatment groups (15 animals/group) and treated with either TCBZ, closantel/abamectin (CLOS/AVM, positive control - sheep), albendazole (ABZ, positive control - goats), or water (H₂O; negative control). Prevalence was determined by sedimentation and faecal egg count (FEC), alongside a commercial coproantigen ELISA (cELISA) and in-house qPCR. Drug efficacy was assessed using faecal egg count reduction tests (FECRT) and coproantigen reduction tests (CRT). Four of the eight farms had a within-herd true prevalence >25%. TCBZ resistance was confirmed on one sheep property (86-89% efficacy). The goat property demonstrated susceptibility to TCBZ (97-98% efficacy), but reduced efficacy of ABZ (79%), representing the first potential report of ABZ resistance in F. hepatica infecting goats. Nemabiome sequencing of co-infecting gastrointestinal nematodes confirmed widespread benzimidazole resistance, underscoring the broader challenges faced by producers. Other potential causes of drug failure included climate variability, pseudo-parasites, and low cELISA diagnostic sensitivity. These results highlight the complexity of diagnosing and managing drug resistance in naturally infected populations and reinforce the need for Fasciola-specific W.A.A.V.P. guidelines.