Infective Endocarditis in Horses: Heart Valve Infection Signs and Treatment
- See your vet immediately if your horse has a fever plus a new or loud heart murmur, fast heart rate, weakness, or trouble breathing.
- Infective endocarditis is a bacterial infection of the inner heart lining, usually affecting a valve and causing leaking, inflammation, and sometimes heart failure or emboli.
- Common clues include persistent or recurrent fever, poor performance, weight loss, lethargy, tachycardia, and abnormal bloodwork such as high fibrinogen or inflammatory markers.
- Diagnosis usually involves repeated blood cultures before antibiotics, echocardiography, CBC/chemistry, fibrinogen or serum amyloid A, and sometimes ECG if an arrhythmia is heard.
- Treatment often requires several weeks of targeted antibiotics and close monitoring, but prognosis is guarded to poor overall and depends heavily on which valve is affected and whether heart failure develops.
What Is Infective Endocarditis in Horses?
Infective endocarditis is a rare but life-threatening infection of the endocardium, the inner lining of the heart. In horses, it most often affects one of the heart valves, especially the mitral valve and less often the aortic or tricuspid valve. Bacteria circulating in the bloodstream can attach to damaged or inflamed valve tissue, forming infected clots called vegetations.
As those vegetations grow, the valve may stop closing normally. That can lead to a heart murmur, blood leaking backward through the valve, enlargement of heart chambers, poor circulation, and in some horses, congestive heart failure. Pieces of infected material may also break off and travel through the bloodstream, causing damage in other organs.
Many horses with infective endocarditis first look like they have a vague systemic illness rather than a primary heart problem. A horse may have fever of unknown origin, reduced stamina, weight loss, or a generally unwell attitude before the heart changes become obvious. Because the condition can worsen over days to weeks, early veterinary evaluation matters.
Symptoms of Infective Endocarditis in Horses
- Persistent or recurrent fever
- New, loud, or changing heart murmur
- Fast heart rate at rest
- Fast breathing or increased effort to breathe
- Lethargy or depression
- Exercise intolerance or poor performance
- Weight loss or poor body condition
- Weakness, collapse, or signs of heart failure
- Irregular heartbeat or arrhythmia
When to worry: a fever plus a heart murmur is a red-flag combination in horses. See your vet immediately if your horse has persistent fever, a newly detected murmur, rapid breathing, weakness, or a sudden drop in performance. Some horses show only vague signs at first, so ongoing fever that does not have an obvious cause deserves a thorough workup.
What Causes Infective Endocarditis in Horses?
Infective endocarditis usually starts when bacteria enter the bloodstream and settle on the lining of a heart valve. Reported bacteria in horses include Streptococcus spp., Actinobacillus/Pasteurella spp., and Staphylococcus spp., with less common reports involving organisms such as E. coli, Pseudomonas, and others. The infection may develop on previously abnormal tissue, but it can also occur on a valve that seemed normal beforehand.
Sometimes your vet can identify a likely source of the bloodstream infection, such as pneumonia, a wound, an abscess, an infected catheter site, uterine infection, dental disease, or another systemic bacterial illness. In other horses, no clear starting point is found. Younger horses appear overrepresented in published case series, although horses of any age can be affected.
The condition is not considered contagious in the usual horse-to-horse sense. Instead, it is a complication of bacteremia. That means prevention focuses less on isolation and more on prompt treatment of infections elsewhere in the body, careful catheter and wound care, and early veterinary follow-up when a horse has unexplained fever.
How Is Infective Endocarditis in Horses Diagnosed?
Diagnosis usually combines physical exam findings, bloodwork, imaging, and microbiology. Your vet may suspect infective endocarditis when a horse has fever of unknown origin together with a new or loud murmur, tachycardia, tachypnea, inflammatory bloodwork, or declining performance. Common lab changes include leukocytosis, neutrophilia, anemia, and increased fibrinogen or serum amyloid A.
Echocardiography is one of the most important tests because it can show thickened valves, vegetative lesions, chamber enlargement, and the degree of valve leakage. Blood cultures are also very important, ideally collected before antibiotics are started. Multiple cultures taken over time improve the chance of finding the organism and help your vet choose a more targeted antibiotic plan.
Depending on what your vet hears on exam, they may also recommend an ECG to assess arrhythmias and cardiac troponin I to look for heart muscle injury. In real-world cases, diagnosis can still be challenging. Some horses have already received antibiotics before referral, which can make blood cultures negative even when infective endocarditis is still present.
Treatment Options for Infective Endocarditis in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm or clinic exam with repeat auscultation
- CBC/chemistry and fibrinogen or serum amyloid A monitoring
- Limited initial stabilization and supportive care
- Empiric antibiotics chosen by your vet when referral is not possible
- Restricted exercise and close recheck planning
Recommended Standard Treatment
- Referral or hospital-level internal medicine evaluation
- Multiple blood cultures collected before antibiotics when possible
- Echocardiography to identify affected valve and severity of regurgitation
- CBC/chemistry, fibrinogen or serum amyloid A, and targeted antimicrobial plan
- Several weeks of antimicrobial therapy with serial rechecks
- ECG if arrhythmia is suspected and activity restriction during recovery
Advanced / Critical Care
- Hospitalization with intensive monitoring
- Intravenous broad-spectrum antibiotics followed by culture-directed adjustments
- Serial echocardiography and repeated lab monitoring
- Management of complications such as arrhythmias, laminitis risk, or congestive heart failure
- Oxygen or additional cardiovascular support if needed
- Specialist consultation in equine internal medicine and cardiology
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Infective Endocarditis in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Which heart valve do you think is affected, and how severe is the leakage?
- Should we collect blood cultures before starting or changing antibiotics?
- What did the echocardiogram show, and do we need repeat scans?
- What organisms are most likely in my horse's case, and how will that affect treatment choices?
- What monitoring should we use to track improvement, such as fibrinogen, serum amyloid A, CBC, or troponin?
- Is my horse showing any signs of heart failure, arrhythmia, or risk for embolic complications?
- What level of exercise restriction is safest right now, and for how long?
- What is the realistic prognosis for comfort, long-term survival, and return to work in this specific case?
How to Prevent Infective Endocarditis in Horses
There is no vaccine that specifically prevents infective endocarditis in horses. Prevention is really about reducing the chance of bacteria entering the bloodstream and making sure infections elsewhere in the body are treated promptly. Good wound care, clean catheter handling, dental care, and timely treatment of respiratory, uterine, hoof, skin, and other bacterial infections all matter.
If your horse has an unexplained fever, do not wait for a loud murmur or breathing changes to appear before calling your vet. Early workup of fever of unknown origin may help identify a bacterial problem before the heart becomes involved. Horses that have already had a serious bloodstream infection may need closer follow-up, especially if a murmur develops during recovery.
Routine antibiotic use without a clear reason is not a prevention strategy and can make later culture results harder to interpret. The most practical approach is thoughtful preventive care, fast attention to systemic illness, and rechecks when a horse's energy, appetite, or performance changes in a way that does not fit the usual pattern.