Monensin poisoning in Brazilian horses

Three outbreaks of monensin poisoning caused 12 deaths in 16 horses. The il lnesses were associated with the Ingestion of the same batch of a commercia l ration labeled for feeder calves which contained 180 +/- 20 ppm sodium mo nensin. The morbidity rate was 100% and lethality was 60%, 75%, and 100%. C linical signs were tachycardia and cardiac arrythmia, groaning, incoordinat ion, sudoresis, recumbency, and paddling movements with the limbs before de ath. Two horses had dark discolored urine (myoglobinuria). Serum levels of creatine phosphokinase activity were increased. Main necropsy findings were in the skeletal muscles and myocardium.