Métodos de controle anti-helmíntico na pecuária de corte – impactos econômicos e consequências

Abstract

Brazilian beef cattle are constantly evolving and has reached levels that have made it one of the main economic sectors of the country. Infections by gastrointestinal nematodes constitute an important obstacle to the development of beef cattle. That is because, parasites such as Haemonchus spp., Ostertagia spp., Trichostrongylus spp., Cooperia spp. and Oesophagostomum spp. they cause great productive losses - like the reduction in weight gain - as a result of the spoliation of their hosts. Chemical control is the most recommended method to reduce these losses and is performed, especially, by the use of anthelminths of the families of macrocyclic lactones, benzimidazoles and imidazothiazoles. However, the efficacy of these molecules is compromised due to the current scenario of parasitic resistance. Hence, the objective of this thesis was to describe the clinical manifestation of helminthosis in cattle, to demonstrate the management adopted for the control of nematodes in a farm, to evaluate the effectiveness of the products, to analyze the difference in productivity and egg count per gram of feces when the suppressive and strategic treatment is carried out with products of low and high efficacy, besides quantifying the economic impact caused by the ineffective anthelmintic treatment used in the farms. For this, we divided the work into two studies. In the first study, we reported a clinical suspicion of helminthosis, the history of antiparasitic treatments from 2006 to 2016 conducted at the farm, we evaluated the efficacy of seven different anthelmintic products and based on the efficacy of the test result, two groups of ten animals were formed and treated during June, September and November with products of low- and high-efficacy, doramectin and levamisole, respectively, and followed up for 252 days (June 2017 to February 2018), with monthly evaluation of the weight of each animal and the count of eggs per gram of feces. In the second study, we performed efficacy tests of seven anthelmintic and based on the results we formed five experimental groups: 1. Strat-Low strategic treatment (February, May, September and November) with low-efficacy product (<20% ), 2. Supp-Low suppressive (Monthly) treatment with low-efficacy product, 3. Strat-High strategic treatment with high-efficacy product (> 95%), 4. Supp-High suppressive treatment with high-efficacy product, and 5. Control untreated animals. In the experimental period, monthly, the animals' live weight, egg counts per gram of feces were evaluated, and the total cost of the treatments and the cost benefit of each treatment were calculated. The results of the first study confirmed the clinical suspicion of helminthosis and the historical treatment revealed that 96.5% of the treatments were performed with macrocyclic lactones, which in the efficacy test presented an unsatisfactory reduction in the egg count per gram of feces with levamisole and albendazole with the active ingredients being effective over 95%. The animals treated with levamisole gained 12.1 kg more weight than those treated with doramectin. In the second study, the results demonstrated that after the parasitic resistance is installed in the farm, the suppressive treatment does not lead to an improvement in the parasitological and productive parameters of the animals. In addition, the most favorable cost-benefit was observed when the strategic treatment with effective product was performed. We can conclude that macrocyclic lactones are the most used anthelmintic and the parasite resistance to these molecules are present in the studied farms, productive losses are generated by the inadequate control of parasites with increased frequency of treatment not being satisfactory to reverse the scenario parasitic resistance. The perform an efficacy test is mandatory to select an effective anthelmintic drug which allows adoption of an effective and profitable anthelmintic regimen. Our study showed that four anthelmintic treatments per year, using a high-effective drug, are efficient to control GIN of growing heifers raised at cultivated pasture in the conditions of southern Brazil. Moreover, this approach was economic feasible and avoid excessive frequencies of treatments.