Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Monensin Toxicity

Synonyms: Cattle Feed Poisoning


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.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

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The prognosis for horses in heart failure is poor.

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.

Horses that do survive may have long term heart problems and be incapable of athletic function. In horses that survive, information on prognosis can be gained using echocardiography and heart stress tests.

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.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
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Questions To Ask Your Vet:
  • Can the feed be tested for monensin?

Never give your horses cattle feed, or allow them access to areas containing cattle feed.

Related References:

Doonan GR, Brown CM, Mullaney TP, et al. Monensin toxicity in horses - an international incident. Can Vet J Feb 1989; 30(2): 165-69.

Hughes KJ, Hoffmann KL, Hodgson DR. Long-term assessment of horses and ponies post exposure to monensin sodium in commercial feed. Equine Vet J. 2009 Jan;41(1):47-52.

Divers, TJ Marc, S. , Kraus et. al. Clinical Findings and Serum Cardiac Troponin I Concentrations in Horses after Intragastric Administration of Sodium Monensin : Journal of Vet Diagn. Investigation: May 2009: Vol 21: no.3: 338-343

Author: Doug Thal DVM Dipl. ABVP