CONICET | Buscador de Institutos y Recursos Humanos

The main goal of the current trial was to study the clinical efficacy and the epidemiological impact of the eggs excreted to the pasture after the subcutaneous (SC) administration of ivermectin (IVM) and ricobendazole (RBZ) given both separately and co-administered to calves parasitized with resistant nematodes on 4 commercial cattle farms. On each farm, sixty male calves naturally infected with gastrointestinal nematodes resistant to IVM were randomly allocated into four groups (n= 15): Control group: animals did not receive anthelmintic treatment; IVM group: animals were treated with IVM by the SC route (0.2 mg/kg); RBZ group: animals were treated with RBZ by the SC route (3.75 mg/kg); IVM+RBZ group: animals were treated by the SC route with IVM and RBZ (0.2 and 3.75 mg/kg, respectively). The clinical efficacy was determined by Faecal Egg Count Reduction Test at 15 days post-treatment. The excretion of eggs to the pasture was reported as the sum of eggs excreted per day by the 15 animals included in each experimental group. The results are presented as the mean across all tests. The efficacies were 68% (IVM), 97% (RBZ) and 99% (IVM+RBZ). No significant differences were found between RBZ alone and combined with IVM in terms of clinical efficacy (P>0.05). Generous involved in IVM resistance were Cooperia spp and Haemonchus spp. The total egg excretion was reduced to 29.9% (IVM group), 2.9% (RBZ group) and 0.8% (IVM+RBZ group) compared with the control group (32,023,821). Animals treated with RBZ excreted 3.7 times more eggs to the pasture than did those treated with the combination, which accounted to higher pasture contamination after the RBZ alone treatment. In conclusion, although in an anthelmintic resistance scenario no therapeutic advantages were observed for the combination, the epidemiological impact after the combined treatment is not negligible and should be taken into account.