Iatrogenic doramectin overdosing causes toxicity in sheep: A case report

Macrocyclic lactones are widely used endectocides in ruminants, with a high margin of safety for labeled indications. No previous report of iatrogenic doramectin overdosing has been published. We report an outbreak in a sheep flock in Northeast Patagonia, Argentina. Toxicity signs were observed in more than 10% and 59% of ewes and lambs, respectively, particularly those with low body condition, treated with doramectin 3.5% long-acting injectable formulation, presumably at the indicated dose of 700 μg/kg. Clinical signs included lethargy, mydriasis and coma. Doramectin concentration in blood samples was 826.8 (±119.3) ng/ml. Cerebrospinal fluid (CSF) and liver doramectin concentration in euthanized lambs were 3.26-4.28 ng/ml and 8506-8772 ng/g, respectively. Epidemiological and clinical information, and high doramectin concentration were sufficient to confirm the neurotoxicity. Scarce fat deposition could have altered doramectin pharmacokinetic which may have accounted for the observed neurotoxicity. Special care should be taken when animals under similar nutritional conditions are treated with macrocyclic lactones.