Iatrogenic doramectin poisoning in newborn beef calves

Doramectin is a safe antiparasitic drug widely used in animal production. However, two outbreaks of doramectin poisoning in newborn calves (Bos indicus) occurred after an accidental overdose of the drug was injected to prevent umbilical myiasis on a Brazilian farm. Affected calves exhibited clinical signs including apathy, sialorrhea, ataxia, and death. After a period of clinical manifestation lasting 24 to 48 h, a total of 91 calves died. Doramectin was found in brain samples from affected calves at a concentration exceeding the upper detection limit (25 µg/kg) of the quantification technique used. Diagnosis was based on history, clinical signs, the absence of significant gross and histopathological lesions, and the detection of doramectin in the brains of necropsied calves. These results highlight the importance of using doramectin with caution in newborn calves. Implementing practices such as weighing neonatal calves, especially leaner individuals, is critical to ensure accurate dosage calculations and to mitigate the risk of doramectin overdose.