ABSTRACT

Background and Aim: Anthelmintic resistance among gastrointestinal strongyle nematodes threatens goat productivity and sustainable parasite control. Field efficacy assessments combined with molecular surveillance may improve resistance monitoring, particularly where mixed nematode infections complicate interpretation. This study evaluated the in vivo efficacy of five anthelmintics against gastrointestinal strongyles in smallholder meat goats in Nakhon Ratchasima Province, Thailand, and investigated the relationship between farm-level levamisole efficacy and resistance-associated molecular signals.

Materials and Methods: A field study was conducted on 13 smallholder goat farms between November 2024 and March 2025. Of 738 goats screened, 423 met the inclusion criteria for paired fecal egg count reduction testing (FECRT). Five anthelmintic treatments representing three drug classes were evaluated: albendazole, fenbendazole, ivermectin–clorsulon, eprinomectin, and levamisole. Treatment allocation within farms was nonrandomized and constrained by the number of eligible goats available for treatment; consequently, not all five anthelmintics were evaluated on every farm. Paired fecal egg counts from Day 0 and Day 14 were used to calculate farm-level fecal egg count reduction percentages and 90% confidence intervals, and treatment responses were classified according to the 2023 World Association for the Advancement of Veterinary Parasitology guidelines. FECRT reflected the combined treatment response of mixed strongyle populations. Strongyle genera were identified using polymerase chain reaction assays targeting pooled egg samples, and levamisole-associated Hco-acr-8 S168T allele signals were assessed qualitatively in Haemonchus-positive pooled egg DNA.

Results: FECRT classified all evaluated farms as resistant for fenbendazole (10/10), ivermectin–clorsulon (12/12), and eprinomectin (12/12). For albendazole, nine of 10 farms were classified as resistant and one as inconclusive. Levamisole showed comparatively favorable but variable efficacy, with four farms (30.8%) classified as susceptible, two (15.4%) as inconclusive, and seven (53.8%) as resistant. Haemonchus and Trichostrongylus were detected on all farms, whereas Oesophagostomum was detected on two farms. Susceptible-associated Hco-acr-8 signals were detected on all 13 farms, while resistant-associated signals were detected on 11 farms, resulting in co-detection of both signals on 11 farms. The qualitative pooled-sample assay did not determine individual parasite genotypes or allele frequencies. Molecular allele signal patterns did not consistently correspond with phenotypic levamisole responses.

Conclusion: Reduced field efficacy of multiple anthelmintic classes was widespread across the surveyed farms. Levamisole remained effective on some farms but requires farm-specific efficacy confirmation. The incomplete correspondence between FECRT classifications and pooled molecular signals supports their complementary use in resistance surveillance; however, the molecular assay should not be interpreted as an independent predictor of farm-level levamisole resistance. Longitudinal investigations are needed to determine the predictive value of resistance-associated markers.