First report of Dermatobia hominis resistant to doramectin in cattle

The objective of this study was to evaluate the therapeutic efficacy of 1 % injectable doramectin to control Dermatobia hominis in naturally infested beef cattle in a farm with history of clinical parasitism after avermectin use. The study was conducted in a commercial beef cattle farm in Brazil. Twenty crossbred cattle (Nellore x Angus) were selected based on the minimum number of 10 live larvae. Assessment of infestation was performed based on tactile-visual inspection of the entire body of the animals. The animals were allocated to two experimental groups of ten animals each: treated group - dosed with doramectin 200 μg/kg live weight (LW), and control group - dosed with saline solution 1 mL/50 kg LW. Both treatments were administered subcutaneously in a single dose. After treatment, larval counts were performed at 1, 3, 7 and 14 days post-treatment (DPT). For the treatment to be considered efficient, it should reach an efficacy greater than or equal to 90 % by the 10th DPT and the average larval count in the treated animals should be statistically lower than the average larval count in the control group. The geometric means of larval counts at the beginning of the study were 27.2 and 26.3 for the control and treated groups, respectively. After 72 h of treatment, there was a significant reduction (p < 0.01) in the larvae counts of the treated group, extending to the 14th DPT, at which point maximum efficacy for doramectin (86.3 %) was obtained. After treatment, 90 % of the animals remained parasitized with live larvae on the 7th DPT and 70 % on the 14th DPT, while in the control group, all animals were infested on all dates. The administration of injectable doramectin was not effective in controlling D. hominis in naturally infested cattle.