Monensin Toxicosis in Horses

Key Takeaways

Monensin poisoning occurs when horses ingest a toxic dose of monensin, an antibiotic commonly used as a feed additive to promote cattle and poultry growth. 

Monensin is highly toxic to horses and commonly fatal

• The majority of monensin toxicosis cases occur as a consequence of improper feed preparation

• When ingested, ionophore antibiotics can cause permanent heart damage in horses

• Symptoms of monensin toxicosis include loss of appetite, exercise intolerance, weakness, and excessive sweating

• There is no specific diagnostic test for monensin poisoning, and diagnosis is typically based on physical examination, history of exposure, and feed testing

• There is no antidote for monensin toxicosis

• Prognosis of monensin toxicosis is guarded 

• Horses that are able to recover may not return to previous performance levels due to heart damage

A Closer Look: What is Monensin Toxicosis in Horses?

Monensin poisoning can be characterized as acute or chronic.

Acute monensin toxicosis occurs when large amounts of antibiotic-containing feed are ingested over a short period of time. In these cases, symptoms develop suddenly.

Chronic monensin toxicosis occurs when small amounts of monensin-containing feed are ingested over an extended period of time. Initially, these cases may show no symptoms, and exercise intolerance and weakness develop over time. Chronic toxicosis is rare in horses due to the highly toxic nature of the antibiotic in this species, which makes even small amounts of the toxin more likely to cause severe, sudden symptoms.

Risk Factors

Monensin toxicosis is rare in horses, and usually occurs when there are mixing errors at the feed plant or horses gain access to medicated cattle or poultry feed. Monensin toxicosis is commonly life-threatening and must be treated as an emergency. 

As little as 2 milligrams of monensin per one kg of body weight is toxic to horses. Horses that live on a farm with cattle or poultry are at higher risk of monensin toxicosis.

Possible Causes

Monensin toxicosis is caused by ingestion of a toxic dose of monensin, an ionophore antibiotic commonly found in cattle and poultry feed. In these species, ionophores are used as growth promoters. In horses, monensin damages muscle cells, particularly those found in the heart. 

Cross-contamination between feeds intended for different types of animals or allowing horses access to cattle or poultry feed are the most common causes of monensin toxicosis.

Main Symptoms

Symptoms of monensin toxicosis include:

Exercise intolerance 

• Excessive sweating 

Appetite loss 

Abdominal pain (colic)

Uncoordinated movement that progressively worsens 

• Weakness

• Recumbency 

• Sudden death 

In some cases, multiple horses ingesting the same feed products are affected.

Testing and Diagnosis

Diagnosis of monensin toxicosis is often based on symptoms, physical examination, and history of exposure. Diagnostic tests that can identify the heart damage characteristic of this toxin include:

• Blood work

• Diagnostic imaging, including ultrasound and X-rays

• Electrocardiograms

Definitive diagnosis is obtained by identifying the presence of monensin in feed or the animal's stomach contents.

Steps to Recovery

There is no antidote or definitive treatment for monensin toxicosis. Treatment options include: 


• Gastrointestinal decontamination to remove contaminated feed

• Activated charcoal to bind the toxin

Note: decontamination is only effective if contaminated feed is still in the stomach. Activated charcoal should only be administered by a veterinarian. There is no safe way to decontaminate the GI tract at home. 

Supportive and symptomatic care:

• IV fluid therapy and supplemental oxygen 

• Stall rest to reduce workload of the heart

• Thiamine or vitamin E supplementation

• Antiarrhythmic drugs to stabilize the heart rhythm

Prognosis of monensin toxicosis is guarded and depends on the amount of antibiotic ingested and the resulting degree of heart damage. 

Severe heart damage is often permanent, and the horse may suffer from decreased performance and long-term exercise intolerance. Horses recovering from monensin toxicosis must be withdrawn from riding activities or athletic pursuits for a minimum of 4 months, then evaluated for heart disease. Some horses continue to have heart arrhythmias for the rest of their lives, making them unsafe for riding. Long term, affected horses may develop congestive heart failure.


Monensin toxicosis is not contagious. Strategies to prevent exposure include:

• Selecting feed from companies that use separate machinery for producing horse feed, or have thorough cleaning procedures between batches

• Restricting access to cattle and poultry feed

Is Monensin Toxicosis Common in Horses?

Monensin toxicosis is rare in horses. It is more common in horses on farms with cattle and poultry.

Typical Treatment

• Decontamination 

• IV fluid therapy 

• Supplemental oxygen 

• Stall rest

• Vitamin E and thiamine supplementation

• Antiarrhythmic medications

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