Efficacy of Benzimidazoles and Macrocyclic Lactones Against Equine Nematodes: A Case Study from Maramureș, Romania

Equines worldwide are constantly exposed to infections with multiple species of nematodes, especially strongyles. Currently, three classes of anthelmintics are available for treating such infections: benzimidazoles, macrocyclic lactones, and pyrimidines. If an active substance is used continuously, there is an increased likelihood of parasites developing resistance to it, and to related substances within the same chemical group. This study conducted on a group of 30 equines aimed to evaluate anthelmintic resistance in gastrointestinal nematodes. Two types of tests were performed: In vivo, Faecal Egg Count Reduction Test (FECRT), to assess the efficacy of fenbendazole (FBZ) and abamectin (ABA). In vitro, Egg Hatch Assay (EHA) and Larval Development Assay (LDA), to evaluate albendazole (ABZ), fenbendazole (FBZ), and mebendazole (MBZ) efficacy. FECRT showed ABA as more effective, yet statistical analysis indicated low efficacy for both ABA and FBZ. In EHA, all benzimidazoles showed >50% hatching at the reference concentration. LDA revealed the lowest larval development rate for MBZ (0.59%), followed by FBZ (13.3%). All tested anthelmintics exhibited limited ovicidal activity. A discrepancy was observed between EHA and LDA results for benzimidazole efficacy. The correlation between in vivo and in vitro results for FBZ was 100% only in the case of EHA.