Monensin is what is known as an ionophore antibiotic. It is added to beef cattle feed to increase efficiency of weight gain. It does this by changing the populations of microbes in the rumen.
Unfortunately, horses are very sensitive to monensin. In horses, even small amounts of monensin can causes muscle damage, primarily (and most importantly) heart muscle damage. Horses should never eat cattle feed because of the potential presence of monensin. Poisoning most often occurs when feed mills accidentally contaminate horse feeds with monensin. Signs of disease are those related to heart failure.
Diagnosis is through compatible signs with history of monensin exposure. Feed can be analyzed for monensin levels.
Treatment is supportive only. There is no antidote. Horses that ingest a large dose and become very ill will probably not survive.
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Prognosis & Relevant Factors
The prognosis for horses in heart failure is poor. Horses that do survive may have long term heart problems.
Factors involved in severity of disease include the dose of monensin. Horses that ingested a dose of monensin but had a full intestine from other feeds may experience more mild signs of illness.
Absorption of monensin from the intestine and subsequent illness may relate to whether monensin is dissolved in vegetable or other oil when it is ingested. It is oil soluble so it is carried into circulation when dietary oils are absorbed.
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