Monensin toxicity and acute lethal rhabdomyolysis in accidental exposure in cattle

Abstract

Monensin is a monovalent Na+-selective carboxylic polyether ionophore antibiotic, a metabolic product of Streptomyces cinnamonensis. Monensin is effective as growth-promoting agent and is used in beef cattle rations. Toxicity results from exposure to undiluted premixes or from mixing errors. Interference with membrane cation transport allows ionophores to form complexes that traverse cell membranes and cause cell death by perturbing the intracellular ionic homeostasis and destabilizing membranes. Monensin toxicity is particularly evident in cardiac and skeletal muscle cells. Lesions are associated with muscle degeneration, acute cardiac failure or chronic cardiovascular insufficiency. No antidote or specific treatment for ionophore toxicoses exist. Acute onset of illness was reported in a beef cattle herd of 70 animals, lethally affecting 18 bullocks, in which monensin had been incorporated in rations considerably exceeding levels prescribed. Deaths occurred 24 h after feeding and were preceded by scarce premonitory signs or more typically, affected animals developed anorexia, diarrhea, cardiac failure with tachycardia, dyspnea, muscular weakness with restricted limb movement and severe muscular pain due to rhabdomyolysis, recumbency, and most strikingly myoglobinuria leading to death after pulmonary congestion and rhabdomyolysis with renal failure. The pathological findings confirmed the clinical and laboratory myoglobinuria showing a mosaic pattern of striated muscles, acute myodegeneration, necrosis, and intrassarcoplasmic lipid deposits in skeletal muscle. Renal histopathology showed extensive deposition of casts of myoglobin. Marked increase was evident in multiple serum enzymes, creatine kinase reaching 377,640 U/L (normal < 94 U/L). Infectious diseases and poisoning with cardiotoxins such as selenium were excluded. Although the LD50 of monensin for cattle is not firmly established, and deaths occurred at a non-established and different consumption rate, the history of exposure, development of characteristic clinical changes and postmortem findings were consistent with those expected for monensin. To our knowledge, this is the first report of monensin poisoning in Croatia.