Contaminated Cattle Feed and Monensin Toxicity in Horses

What are Monensin and Rumensin?

• Rumensin is the brand name for the antibiotic monensin. It’s derived from the bacterium Streptomyces cinnamonensis and used to treat ruminants like cattle and sheep. Some literature states the fungus Streptomyces cinnamonensis is the source, but it’s a bacteria.

• Monensin is a type of ionophore antibiotic. Ionophores are sneaky substances that pick up and carry ions through cell walls. Inside a cell, monensin will interfere with sodium, calcium, and potassium, among other things.

Treating coccidiosis in ruminants

• In cows, it treats coccidiosis. Tiny organisms called coccidia damage the intestinal lining, creating dehydration, diarrhea, weight loss, and other digestive upset for the cow. This can impact milk production in the dairy industries and proper body weight gain for cattle destined for slaughter.

• Unfortunately, this powerful antibiotic also affects mammalian cells, but the dangerous doses vary by species. Not all drugs act the same way on all animals. A large amount of one drug in one animal species can be fine but devastating to another species.

• Different amounts of monensin affect different species of animals, like feedlot cattle, swine, poultry, and horses differently. Monensin acts similarly in sheep as in cattle, although sheep are more sensitive to it and can be toxic in smaller doses than cattle. In horses, a toxic dose is 1/100th that of a cow, and for most horses, 1 gram of monensin is a lethal dose.

• Monensin is also a common ingredient in chicken feed.

Ionophores and horses

• When horses eat monensin, it acts on their cell membranes by disrupting the ions that help control the nervous system and muscles, including the heart muscle.

• As monensin interferes with skeletal muscle, horses may shake, wobble, and lose muscle control. They may become paralyzed and have seizures.

• The heart muscle can weaken, making the blood more challenging to pump, creating an unsteady rhythm, and triggering heart failure.

• Some horses experience sudden death.

Can horses recover after eating cattle feed with monensin?

• In many situations, such as the 70+ horses affected in 2024 in one location, the result was devastating, with all horses succumbing. Read more about this here. Weeks after this incident, the mill took responsibility for the contamination. Read more about the outcome of this case here.

• Six horses passed in 2018 from toxicity. Read more about the 2018 deaths here.

• If horses recover, which is unlikely, they will suffer long-term heart trouble. This article discusses a study done about how horses recover from monensin and notes that many do not recover fitness or heart health.

Where you buy your horse’s feed matters. 

How Monensin Contamination Occurs in Horse Feed

• Horses can eat monensin in a few ways. One, the feed mill or manufacturing plant contaminated horse feed with monensin. This could happen during production or storage at the manufacturing level. Contamination can also occur if feeds are improperly stored at farms.

• There’s also the possibility that someone accidentally fed cow or chicken feed containing monensin.

• When monensin toxicity is suspected, there is always an investigation to try and determine the cause. A recall of possibly contaminated complete feeds or other feed products may be issued.

Signs of Monensin Toxicity

• Horses with monensin poisoning often show different signs. Some horses may go “off their feed.” For some horses, you may start seeing strange behaviors within a few hours of eating contaminated feed. Some horses become lethargic and unwilling to move. As the toxicity continues, you may see these clinical signs:

• • Trouble breathing
  • Uncoordinated movement
  • Neurological signs, like ataxia (muscle weakness), tripping, and falling
  • Paralysis
  • Colic-like symptoms like diarrhea and pain

• These signs are all linked to ionophore toxicity. As monensin interferes with ion transport in cells, especially sodium ions and potassium ions, it may create cell death.

Monensin is a key ingredient of feeds for food-producing animals. 

Call your vet right away

• Call your vet immediately at the first sign of trouble, even if you think it’s something mild that will resolve itself. Also, notice if other horses at the barn are acting a bit “off” or showing poor performance, as this is valuable data that your vet needs.

• Many diseases, injuries, and poisonings look the same! Your vet needs as much information as possible, including:

• • Your horse’s vitals signs and if they are changing
  • How signs of poisoning continue, increase, or decrease
  • What your horse eats: pasture, hay, supplements, and any commercial feeds
  • How much they are drinking, eating, urinating, and passing manure and if it’s typical or not

• While there is no cure for this poisoning, horses need supportive care. In some cases, horses deserve kind euthanasia to end suffering. Please call your vet.

Diagnosis and Treatment of Monensin Poisoning

• While there is no definitive test for ionophore toxicity, your vet will gather information about your horse’s vitals, diet, behaviors, and anything suspicious.

• Immediate care means your vet needs to stop more of the toxin from being absorbed. This may include flushing the digestive tract with lavage (like stomach pumping) or giving activated charcoal to absorb the toxin.

• Your horse will need help with electrolyte balance and hydration, so IV fluids and electrolytes are needed. Horses may benefit from pain management, as well. Stall rest is imperative.

• Your vet can advise you on how to proceed if the situation is dire. Euthanasia is a kind alternative and ends needless suffering.

• This article summarizes the difficulties of determining feed toxicities and the steps you and your vet can take to report suspected cases.