Review of monensin toxicosis in horses

Results of experimental studiesindicate that horses are much more sensitive to monensin toxicity than cattle. Single dose studies by gavage with mycelial monensin resulted in an estimated LD₅₀ of 1.38 mg/kg of body weight. Subacute feeding studies indicate that horses will tolerate the highest use level for cattle of 33 ppm without any evidence of toxicity. Evidence of intoxication was apparent with the highest use level for chickens of 121 ppm.In pasture supplement studies, horses ate supplement containing high levels of nonensin upon initial exposure which resulted in toxicity and/or death. Following initial exposure to sublethal levels, consumption of monensin containing supplements was negligible.The clinical signs of toxicity includedpartial to complete anorexia, colicky pain, sweating and tachycardia, uneasiness, polyuria, progressive ataxia, recumbency with frequent attempts to rise and thrashing of the limbs followed by death. Examination at necropsy of animals that die soon after initial exposure to monensin may show no significant lesions, otherwise hemorrhage and pale areas in the heart with transudation into body cavities may be seen. Evidence of degenerative cardiomyopathy and congestive heart failure may be observed on histopathologic examination. Increased activities of muscle origin enzymes may also be found.The results of controlled studiesand evidence from field reports indicated that the greatest risk of intoxication was upon initial exposure to feed or supplement containing monensin. Feed mistakes and mixing errors were the two most common causes of monensin toxicity in field cases.