Anthelmintic resistance on meat goat properties in Queensland and New South Wales, Australia, with a comparison of statistical methods

Gastrointestinal nematode (GIN) infection has serious deleterious effects on the health of goats, and costs Australian producers $2.5 million annually (Lane et al., 2015). Anthelmintic drugs are heavily relied upon for control of these parasites; however, anthelmintic resistance (AR) has become a major constraint for producers. This study assessed resistance to a range of registered and off-label anthelmintic treatments using faecal egg count reduction tests (FECRT) and compared four methods of FECRT analysis. Twelve meat goat properties in southern Queensland (n = 6) and New South Wales (n = 6) were recruited for resistance testing. Goats shedding a minimum of 150 eggs per gram (epg) were randomly allocated to treatment groups of 15 animals. Faecal egg count (FEC) samples were taken on day 0 and day 14 post-treatment. Four methods were employed to analyse FECRT results: a simple mean percentage reduction of FEC, a Bayesian Hierarchial Model ('eggCounts' package in R Studio), a computational method recommended by the World Association for the Advancement of Veterinary Parasitology (WAAVP) (Kaplan et al. 2023), and a Beta Negative Binomial (BNB) Distribution analysis (FECRT.com). This study identified the first field cases of resistance, classifications to several anthelmintics: Moxidectin (MOX) on 70% of properties, Monepantel (MPL) + Abamectin (ABA) on 50%, Derquantel (DER) + ABA on 33%, ABA+Oxfendazole (OXF) + Levamisole (LEV) on 50% and ABA+Closantel (CLOS) + LEV+Albendazole (ABZ) on 50%. All properties (100%) had resistance to Fenbendazole (FBZ), ABA, Eprinomectin (EPM), Doramectin (DRM) and LEV in combined species tests. Haemonchus contortus was found to be the most commonly resistant GIN species, followed by Trichostrongylus spp. A comparison of the four methods used to evaluate FECRT results revealed that out of a total 104 individual FECRTs performed, 87 tests returned a consistent categorical outcome. The WAAVP method and BNB distribution yielded the most resistant classifications. These findings illustrate widespread AR to commonly used registered and off-label anthelmintic treatments in meat goats across both Queensland and New South Wales. This has significant implications for the longevity of these treatments and poses a serious risk to successful control of GIN in goats using chemicals. The continued reliance on anthelmintics is not sustainable, the incorporation of non-chemical control management practices should be encouraged for all goat producers.