Anthelmintic Combinations: a Sustainable Strategy to Optimize Parasite Control on Commercial Cattle Farms?

CANTON, C; DOMINGUEC, P; CEBALLOS L; MORENO L

Madison

Conferencia; 27º Conference of the World Association for the Advancement of Veterinary Parasitology; 2019

In an attempt to minimize therapeutic failures following anthelmintic treatments and to delay the development of resistance, the combined use of nematodicidal compounds with different mechanisms of action has been proposed. A pharmaco-parasitological assessment of different nematodicidal combinations was performed at different cattle commercial farms in Argentina. The Ivermectin(IVM)-ricobendazole(RBZ), IVM-levamisole(LEV) and RBZ-LEV combinations were assessed in calves naturally infected with susceptible/resistant gastrointestinal nematodes. The observed pharmacokinetic(PK) data demonstrated that the co-administration of two anthelmintics did not modify the plasma PK behaviour of either drug in cattle. In fact, no adverse PK interactions were observed after each combined treatment. Similar PK parameters(P>0.05) were obtained between the single-drug and the combined-based strategies. The IVM-RBZ, IVM-LEV and RBZ-LEV combinations were the only treatments achieving 100% clinical efficacy, even when a highly IVM-resistant Haemonchus spp. isolate was present(45% efficacy for IVM alone). In fact, the combination IVM-RBZ maintained its 100% efficacy against an IVM-resistant Haemonchus spp. population over the last five years. Additionally, the weight gain was significantly higher in calves treated with the IVM-RBZ combination. Overall, after the combined treatments only a therapeutic additive effect is achieved. However, the efficacy of IVM-RBZ against nematodes resistant to IVM and RBZ was greater than an additive effect. In fact, in two farms with multiple resistance to IVM and RBZ, while the efficacies during the first year of study were 54%(IVM), 84%(RBZ) and 98%(IVM-RBZ) in Farm A, the egg reductions were 40%(IVM), 64%(RBZ) and 90%(IVM-RBZ) in Farm B. During the second year of study, the high efficacy of the combination IVM-RBZ could be maintained in Farm A(97%), but not in Farm B(67%). Overall, the potential advantages of anthelmintic combinations should be carefully assessed before being extensively recommended. Their rational use should be strongly supported by pre-treatment diagnosis and considering the epidemiological situation of each individual farm.