Monensin, Lasalocid, and Salinomycin Toxicity in Horses
- See your vet immediately if your horse may have eaten cattle, poultry, or other medicated feed containing monensin, lasalocid, or salinomycin.
- Horses are highly sensitive to ionophores. Even small mixing errors or feed contamination can cause severe heart and skeletal muscle damage.
- Common signs include weakness, poor appetite, sweating, colic-like discomfort, muscle stiffness, fast or irregular heartbeat, trouble breathing, recumbency, and sudden death.
- There is no antidote. Treatment focuses on rapid feed removal, supportive care, heart monitoring, fluids when appropriate, and careful prognosis discussions with your vet.
- Some horses that survive the first crisis can have delayed heart failure, poor performance, or exercise intolerance weeks to months later.
What Is Monensin, Lasalocid, and Salinomycin Toxicity in Horses?
Monensin, lasalocid, and salinomycin are ionophore drugs used in some livestock and poultry feeds. They can be useful in target species, but horses are especially sensitive to them. When a horse eats feed contaminated with an ionophore, the drug disrupts normal ion movement across cell membranes and can severely injure the heart muscle and skeletal muscles.
This is a true emergency. Affected horses may show sudden weakness, sweating, colic-like signs, abnormal heart rhythms, or collapse. In some cases, the first sign is sudden death. Other horses survive the initial exposure but later develop poor performance, stiffness, or signs of heart failure.
Monensin is the ionophore most often discussed in horses, but lasalocid and salinomycin can also be dangerous. The exact outcome depends on the dose eaten, how quickly exposure is recognized, and how much heart damage has already occurred. Your vet will help guide next steps based on your horse's signs, exam findings, and likely exposure history.
Symptoms of Monensin, Lasalocid, and Salinomycin Toxicity in Horses
- Weakness or sudden exercise intolerance
- Poor appetite or feed refusal
- Colic-like discomfort
- Profuse sweating
- Muscle stiffness, trembling, or reluctance to move
- Fast heart rate or irregular heartbeat
- Rapid breathing or respiratory distress
- Dark urine from muscle damage
- Recumbency or inability to stand
- Sudden death
- Delayed poor performance or signs of heart failure after apparent recovery
Call your vet right away if your horse may have accessed medicated feed, a mixed ration meant for cattle or poultry, or feed from a mill with a contamination concern. Even vague signs like weakness, sweating, stiffness, or a horse that "isn't acting right" matter here.
This toxicity can worsen quickly, and some horses look stable before developing dangerous heart rhythm problems. If your horse is down, breathing hard, has an irregular pulse, or collapses, treat it as an emergency and seek immediate veterinary care.
What Causes Monensin, Lasalocid, and Salinomycin Toxicity in Horses?
Most cases happen when a horse accidentally eats feed intended for another species. Ionophores are commonly included in some cattle, poultry, and other livestock rations, but they are not safe for horses. A horse may be exposed by getting into another animal's feed, being fed the wrong bag, or receiving a ration that was mixed or labeled incorrectly.
Another important cause is feed mill cross-contamination. Small carryover amounts from medicated feed production can be enough to harm horses. Shared augers, bins, trucks, scoops, or storage containers can also create risk, especially on mixed-species farms.
Dose matters, but horses are sensitive enough that even relatively small exposures can be serious. Merck Veterinary Manual notes that horses are particularly susceptible to ionophore toxicity, and monensin has a very low reported lethal dose range in this species. Because there is no antidote, preventing exposure is far safer than trying to treat the damage after it happens.
How Is Monensin, Lasalocid, and Salinomycin Toxicity in Horses Diagnosed?
Your vet usually diagnoses ionophore toxicity by combining the history of possible feed exposure with exam findings and lab work. Important clues include access to cattle or poultry feed, a recent feed change, multiple horses affected, or illness starting soon after a new batch of grain or pellets was opened.
Testing often includes bloodwork to look for muscle injury and organ stress. Muscle enzymes such as CK and AST may be elevated, although the degree can vary. Your vet may also run electrolytes, kidney values, and other chemistry tests, because severe muscle damage and dehydration can affect the whole body.
Because the heart is a major target, many horses need cardiac evaluation. This may include an ECG to look for arrhythmias and, in some cases, ultrasound of the heart if there is concern for ongoing cardiac damage or heart failure. Feed samples can sometimes be submitted for toxicology testing, and in horses that die or are euthanized, necropsy can help confirm the diagnosis by identifying characteristic heart and skeletal muscle injury.
Diagnosis is not always based on one single test. Often, your vet is making the best clinical judgment from the exposure history, signs, bloodwork, and response over time. That is one reason it is so helpful to save the feed bag, lot number, and a sample of the suspect feed.
Treatment Options for Monensin, Lasalocid, and Salinomycin Toxicity in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Immediate removal of suspected feed and strict stall rest
- Urgent farm exam with heart rate and rhythm assessment
- Baseline bloodwork such as CBC/chemistry and muscle enzymes if available
- Oral or limited supportive care directed by your vet
- Clear home-monitoring plan for appetite, manure, urine color, breathing, and ability to stand
Recommended Standard Treatment
- Hospitalization or intensive day care with serial exams
- IV fluids when appropriate and carefully tailored to cardiovascular status
- Repeated bloodwork to track CK, AST, kidney values, and electrolytes
- ECG monitoring for arrhythmias and treatment of complications as your vet recommends
- Pain control, nursing care, oxygen support if needed, and gradual reassessment over 24-72 hours
Advanced / Critical Care
- Referral hospital ICU care with continuous ECG and frequent blood pressure monitoring
- Advanced imaging such as echocardiography when indicated
- Aggressive management of severe arrhythmias, recumbency, respiratory distress, or shock
- Indwelling catheter care, oxygen therapy, sling or recumbent-horse nursing support, and repeated laboratory monitoring
- Extended hospitalization plus discharge planning for prolonged rest and recheck cardiac evaluation
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Monensin, Lasalocid, and Salinomycin Toxicity in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Based on my horse's signs and feed history, how strongly do you suspect ionophore toxicity?
- Does my horse need immediate hospitalization, or is monitored farm care reasonable right now?
- What blood tests and heart monitoring do you recommend today, and what changes would worry you most?
- Should we submit the feed for testing, and how should I store the bag, lot number, and sample?
- What signs at home mean I should call you back or transport my horse immediately?
- If my horse survives the first few days, what is the risk of delayed heart damage or poor performance later?
- How long should my horse rest before any return to exercise is considered?
- Do my other horses need exams or monitoring if they may have eaten the same feed?
How to Prevent Monensin, Lasalocid, and Salinomycin Toxicity in Horses
The safest prevention step is to keep horse feed completely separate from cattle, poultry, goat, and other livestock feeds. Store bags in clearly labeled areas, use dedicated scoops and bins, and make sure everyone feeding your animals knows that medicated livestock feed can be deadly to horses.
If you buy feed from a mill that also makes medicated rations, ask about their sequencing, flushing, cleanout, and cross-contamination controls. Keep feed tags and lot numbers until the bag is finished. If a new batch smells different, looks different, or multiple horses seem off after a feed change, stop feeding it and call your vet right away.
Mixed-species farms need extra caution. Horses should not have access to other animals' troughs, mineral tubs, or feed rooms. Shared equipment such as augers, buckets, and trailers can also create contamination risk if they are not cleaned thoroughly.
Finally, act fast if exposure is possible. Remove the feed, save the bag and a sample, and contact your vet immediately. Early recognition will not create an antidote, but it can improve the chance of supportive care before severe heart damage progresses.