Monensin (Rumensin): poison for horses


Monensin is a common additive in cattle and poultry food - and is often added to drinking water to prevent coccidiosis on dairy farms. Horses are extremely susceptible to its effects as a recent case has shown.

Monensin (Rumensin—Elanco) is produced as an additive for feeds for ruminants. The problem, however, is that horses are extremely sensitive to monensin poisoning. The LD50 for monensin in horses is 2 to 3 mg/kg body weight (LD50 is the dose required to kill half the members of a tested population after a specified test duration)  whereas cattle and poultry can tolerate more than ten times this dose without issue. Locally available calf feeds have Rumensin in them. 10kg of this feed fed over a short period of time would be enough to kill a horse. With smaller doses the horse may survive but will suffer permanent damage to the heart and skeletal muscles. The photo above shows rapid muscle wasting in a horse fed low levels of calf feed containing Rumensin, this horse also showed signs of heart muscle damage and sadly died from suspected heart failure about 2 weeks after being fed the calf pellets, despite supportive treatment.

The highly energetic tissues of the body such as heart and skeletal muscle are affected first and worst.The clinical signs begin 12-24 hours after consumption of an acutely toxic dose but may be delayed for days or weeks in the case of chronic low level intoxication. The signs of acute intoxication in horses may include some or all of the following depending on dose and the individual: colic, intermittent sweating, incoordination, muscle weakness, elevated heart rate, dark urine, kidney failure, respiratory distress and going down. Again, depending on dose and individual susceptibility, death can occur in less than 24 hours.

Animals surviving the acute intoxication and those with chronic intoxication may exhibit signs of progressive congestive heart failure and muscle wasting. The damage to cardiac muscle is irreparable. Animals surviving intoxication have a guarded prognosis for long term survival and return to fitness.

Sudden deaths in the weeks or even months following intoxication have been reported.

Treatment of Monensin poisoning is primarily supportive - there is no antidote.

Keep your cattle and poultry feed well away from your horses and ensure your horses are not drinking water that has been treated with monensin.


Oedema around the eyes indicating congestive heart failure secondary to chronic monensin toxicity. This horse had been fed a few handfuls of calf feed over several days.