Resistência anti-helmíntica de nematódeos gastrointestinais de ruminantes naturalmente infectados no estado do Rio Grande do Sul, Brasil

Abstract

Among the main diseases that affect ruminants around the world, infections by gastrointestinal nematodes display an important role. These infections, negatively impact the performance of the animals, compromising the economic viability of the systems, if the correct control of these parasites is not made. In this sense, were performed the evaluation of different compounds with anthelminthic action in sheep and cattle in the state of Rio Grande do Sul. The first chapter presents the study whose objective was to evaluate the anthelminthic resistance of gastrointestinal nematodes from naturally infected cattle with different anti-helminthic therapy commercially available in the state of Rio Grande do Sul; and test the effectiveness of combinations of these principles on multiresistant populations. For both, were selected 70-100 animals, with ages between 7 to 9 months, naturally infected and with egg counts per gram of feces (EPG) ≥200. These individuals were from ten different property located in Rio Grande do Sul, which had received no anthelmintic treatment in the 60 days preceding the beginning of the study. The animals were randomly divided in ten groups (7- 10 animals each) and treated with ivermectin, doramectin, eprinomectin, fenbendazole, closantel, disophenol, nitroxynil, albendazole, levamisole or moxidectin. Additional groups of 7 to 10 calves were used to test six combinations of two drugs each in four of studied properties. In general, the fenbendazole was the most effective drug, followed by levamisole, disofenol and moxidectin. However parasites of genera Cooperia, Trichostrongylus and Haemonchus were the most resistant to different treatments employed. The combinations of moxidectin+levamisole, doramectin+fenbendazole and levamisole+closantel were the most effective. The results of this study demonstrated the existence of multidrug-resistant populations to most of the anthelmintic substances commercially available, while combinations of these may represent an alternative for improvement of the efficacy of treatments. The second chapter had as objective to verify the efficacy of monepantel (Zolvix ®) on gastrointestinal nematodes of naturally infected lambs from two properties in the central region of the state of Rio Grande do Sul, Brazil. For it, 64 animals being, 17 animals treated with Zolvix and 17 control animals not treated in property 1, and 20 animals treated with the same principle and 10 control on the property 2, were used. In both properties, the animals did not receive any anthelmintic treatment in the 30 days prior to the study and had EPG ≥200. The efficacy of monepantel was 25.8% and 78.4%, respectively, in the properties 1 and 2, being Haemonchus spp., Trichostrongylus spp. and Cooperia spp. the genera that presented greater resistance to it. It was demonstrated that the status of anthelminthic resistance in sheep is getting worse once the last drug launched in the Brazilian market for the treatment of gastrointestinal infections of the animals proved ineffective, and further studies are needed to delay the appearance of new cases of resistance to this drug and the other compounds commercially available.