Myocarditis in Horses: Inflammation of the Heart Muscle

Myocarditis is inflammation of the heart muscle, also called the myocardium. In horses, that inflammation can be patchy or widespread. When heart muscle cells become inflamed or damaged, the heart may not contract normally, and the electrical system that controls heartbeat can become unstable.

This matters because some horses show only vague signs at first, such as poor performance, fatigue, or a lower willingness to work. Others develop more obvious problems, including an irregular heartbeat, weakness, fluid buildup, or collapse. In severe cases, myocarditis can lead to congestive heart failure or sudden death.

Myocarditis is not a single disease with one cause. It is usually a consequence of another problem, such as infection, toxin exposure, or systemic illness. That is why your vet will usually focus on both confirming heart involvement and identifying the underlying trigger.

If your horse seems off during exercise, tires unusually fast, or has an abnormal heart rhythm, prompt veterinary evaluation is important. Early recognition can help guide safer activity restriction and treatment choices.

Myocarditis in horses has many possible causes. According to the Merck Veterinary Manual, infectious causes include several bacteria and viruses. Reported bacterial causes include Streptococcus species, Salmonella, Clostridium, and Borrelia burgdorferi, the organism associated with Lyme disease. Viral causes can include equine infectious anemia virus and equine influenza. Parasitic disease such as strongylosis has also been linked to myocarditis in horses.

Noninfectious causes are also important. Heart muscle inflammation can occur after exposure to cardiac toxins, including monensin contamination from cattle feed, blister beetles, and certain poisonous plants. Severe vitamin E or selenium deficiency can also damage muscle, including the heart, especially in young or nutritionally compromised horses.

In some horses, myocarditis develops as part of a broader systemic illness rather than as an isolated heart problem. Severe infection, endotoxemia, inflammatory disease, or other major body stressors may contribute to myocardial injury and arrhythmias. That is one reason your vet may recommend a wider workup than a heart exam alone.

Because the list of causes is broad, there is no safe way to assume the trigger at home. Your vet may need to combine history, travel and vaccination status, feed exposure, bloodwork, and cardiac testing to narrow the cause.

Diagnosis starts with a careful history and physical exam. Your vet will listen for murmurs, extra heart sounds, or rhythm disturbances and assess pulse quality, breathing effort, temperature, and signs of poor circulation or fluid retention. Because myocarditis can mimic other causes of poor performance or weakness, the exam usually looks beyond the heart alone.

Cardiac testing is often the next step. The Merck Veterinary Manual notes that blood tests and an echocardiogram may be used to help confirm the diagnosis. An ECG is especially useful for detecting arrhythmias, and some horses may need longer monitoring if an abnormal rhythm comes and goes. Echocardiography helps your vet evaluate heart size, pumping function, valve changes, and evidence of heart failure.

Bloodwork may include a CBC, chemistry panel, inflammatory markers, and testing for infectious diseases based on your horse's risk factors. Your vet may also look for muscle injury patterns, electrolyte problems, or evidence of toxin exposure. In referral settings, additional imaging or specialized cardiology consultation may be recommended.

A diagnosis of myocarditis is often made by putting several pieces together rather than relying on one test. That combination helps your vet judge severity, guide exercise restriction, and choose the most appropriate treatment options for your horse.

Not every case of myocarditis can be prevented, but reducing infectious, toxic, and nutritional risks can help. Keep your horse on a preventive care plan with your vet, including appropriate vaccination based on region and lifestyle, regular parasite control, and prompt attention to fever, respiratory disease, diarrhea, or other systemic illness. Horses that are sick should not be pushed to work.

Feed safety matters too. Prevent access to cattle feeds that may contain monensin or other additives unsafe for horses. Check hay and pasture for toxic plants, and be cautious in areas where blister beetles may contaminate alfalfa hay. Store feed carefully to reduce contamination and mix-ups.

Good nutrition supports muscle health, including the heart. Work with your vet or an equine nutrition professional if there is concern about low vitamin E or selenium intake, especially in foals, growing horses, or horses on restricted forage programs. Supplementation should be guided by testing and regional risk, because excess selenium can also be harmful.

Finally, take reduced performance seriously, especially after illness. A horse returning to exercise too soon after a systemic infection or inflammatory episode may be at greater risk if heart involvement is present. If your horse has had fever, infection, toxin exposure, or unexplained weakness, ask your vet whether a cardiac check is appropriate before resuming work.