• Bertram Harmon a publié une note il y a 6 jours et 1 heure

    Conflict of interestAcknowledgementsIntroductionThe resistance of nematodes to macrocyclic lactones (MLs) has become a serious problem to the effective control of nematodes in small ruminants and is an emerging problem for horse and cattle parasites, for which resistance was mistakenly expected to develop slowly (Wolstenholme and Kaplan, 2012; Leathwick and Besier, 2014).The economic impact of anthelmintic resistance in cattle has been underestimated by herd owners. Sutherland and Leathwick (2011) observed a reduction in weight gain, approaching 14kg in calves, following their treatment with an ineffective anthelmintic. A reduction in the intake of dry cyclooxygenase pathway of approximately 680g daily in cattle infected with an isolate of Cooperia punctata resistant to ivermectin (IVM) decreased daily weight gains by an average of 110g (7.4% in total) (Stromberg et al., 2012), confirming the economic effect on both appetite and nutrient uptake. Calves in Brazil co-infected with Haemonchus sp. and Cooperia sp. and treated with anthelmintics with an efficacy of 84% increased live weights by 11.85kg compared to untreated calves, but calves treated with avermectins (AVMs) with efficacies of 0 and 48.2% gained only 9.05 and 9.41kg, respectively (Borges et al., 2013).The production of beef cattle in Brazil is based on extensive grazing, with average herd sizes estimated at 2000–4000 in several regions of the country. Handling these large herds presents operational limitations, and the stress caused by the handling leads to losses in productivity. The development of long-acting anthelmintics thus became an important means of parasitic control. The pharmaceutical industry in Brazil increased the concentrations of their formulations of MLs (doses of 630, 700, 800, or 1000μg/kg) and developed long-acting formulations, which at the time of their release provided high therapeutic efficacies and long residual periods (Borges et al., 2008; Lopes et al., 2013; Felipelli et al., 2014; Neves et al., 2014). Increasing the concentration of an AVM in a formulation with the same vehicle will produce a higher peak in plasma concentration (Cmax) but not a longer persistence period (Lifschitz et al., 2007). Two pharmaceutical modifications were thus made to develop long-acting endectocide formulations: increase the AVM concentration and develop slow or long-acting release vehicles that allow slow absorption and a long residual effect (Lifschitz et al., 2007).These measures have been widely discussed and questioned by the scientific community. The administration of anthelmintics with prolonged persistence in plasma fosters the development of resistant larvae, which will mature, reproduce, and shed eggs to the environment, accelerating the selection for resistance (Dobson et al., 1996; Leathwick, 2004). Field experiments have demonstrated that once resistance is established, increasing the dose may not produce higher anthelmintic efficacies (Borges et al., 2008; Lopes et al., 2013, 2014; Felippelli et al., 2014).Material and methodsResultsThe percent reductions in FECs on day 11 were 53.8% (95% CI: 38.3–64.3) for 3.5% DRM, 0.0% (95% CI: 0–0) for 1.0% DRM, and 13.7% (95% CI: 0–31.2) for 3.15% IVM. The pre- and post-treatment FECs did not differ significantly (p>0.05) for these treatments.The necropsies indicated that Cooperia sp. and Haemonchus sp. were the dominant species, representing 98.6% of all helminths recovered from the untreated calves. C. punctata was the most numerous (83.8%) species of Cooperia, followed by Ctenosaura pectinata (10.6%) and C. spatulata (5.6%). Oesophagostomum radiatum was detected at lower intensities (155.33 worms per animal) in all animals in the study, and Trichuris discolor and Trichostrongylus axei represented only 0.08% of the total parasitic burden (Table 1).The variables of species (<0.001) and treatment (p<0.05) influenced the degree of reduction of parasitic load, but the interaction between them was not significant (p=0.378).

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