Lack of efficacy of macrocyclic lactones, albendazole, and levamisole against a field population of Dictyocaulus viviparus in cattle.

Dictyocaulus viviparus (Strongylida: Dictyocaulidae) is the etiological agent of pulmonary parasitosis in cattle, causing severe respiratory symptoms and, in extreme cases, death. Outbreaks of D. viviparus were recorded on a property in Brazil in the years 2022 and 2023 in an integrated agriculture and livestock system. Treatment failure was observed for levamisole, macrocyclic lactones and benzimidazoles, and only the combination of levamisole (18.8 %) and moxidectin (1 %) was effective according to fecal larvae counting reduction test (FLCRT). The following efficacy rates were observed: albendazole sulfoxide (15 %), 60.10 %; moxidectin (1 %), 60.30 %; ivermectin (0.8 %) + albendazole sulfoxide (10 %), 43.30 %; ivermectin (3.15 %), 9.85; levamisole (18.8 %), 27.40 %; doramectin (3.5 %), 57.30 % in 2022; moxidectin (1 %), 89.20 %; doramectin (1 %), 31.70 %; ivermectin (0.8 %) + albendazole sulfoxide (10 %), 19.10 %; doramectin (1.1 %), 2.22 %; levamisole (18.8 %), 96.30 %; doramectin (3.5 %), 88.40 %; and levamisole (18.8 %) + moxidectin (1 %), 99.70 % in 2023. The present results underscore the importance of monitoring pulmonary parasitosis and the scenario of lack of efficacy found in the studied population of D. viviparus.