Degenerative Mitral Valve Disease in Dogs

Degenerative mitral valve disease, also called myxomatous mitral valve disease or chronic degenerative valve disease, happens when the mitral valve between the left atrium and left ventricle becomes thickened and less effective. Over time, the valve does not close tightly, so some blood leaks backward with each heartbeat. That backward flow is called mitral regurgitation. Your vet may first notice the problem as a heart murmur long before your dog seems sick.

This is the most common heart disease in dogs and is seen most often in older, small-breed dogs. Cavalier King Charles Spaniels, Dachshunds, Miniature Poodles, Cocker Spaniels, Miniature Schnauzers, and many small terriers are overrepresented, although any dog can be affected. In many dogs, the disease progresses slowly over months to years. Some remain stable for a long time, while others eventually develop heart enlargement and congestive heart failure.

The disease is usually staged using the ACVIM system. Stage A dogs are at risk but do not yet have disease. Stage B1 dogs have a murmur without heart enlargement. Stage B2 dogs still have no outward signs, but imaging shows enlargement of the heart. Stage C means congestive heart failure has developed, and Stage D refers to heart failure that is harder to control with standard therapy. Staging matters because treatment choices change with each step.

For pet parents, the key point is that a murmur is not the same as heart failure. Many dogs with early disease feel normal and can enjoy a good quality of life with regular monitoring. Early follow-up with your vet helps identify when the disease has moved from watchful monitoring to a stage where medication may help delay worsening.

Diagnosis starts with a history and physical exam. Your vet will listen for a murmur, assess heart rate and rhythm, and look for clues such as increased breathing effort, crackles in the lungs, weak pulses, or abdominal changes. A murmur can suggest mitral valve disease, but it does not tell the whole story. Dogs with the same murmur grade can have very different stages of disease.

Chest X-rays are commonly used to look for heart enlargement and signs of fluid in the lungs. Echocardiography, an ultrasound of the heart, is the best test to confirm mitral valve degeneration, measure chamber enlargement, and assess how severe the regurgitation is. This is especially helpful when deciding whether a dog is still in Stage B1 or has progressed to Stage B2, where medication may be considered.

Your vet may also recommend blood pressure measurement, an electrocardiogram if an arrhythmia is suspected, and bloodwork to check kidney values and electrolytes before or during treatment. NT-proBNP testing can sometimes help when the cause of cough or breathing changes is unclear, or when your vet wants more information about cardiac strain.

Because treatment decisions depend on stage, follow-up testing matters. Dogs with early disease may only need periodic rechecks, while dogs on heart medications often need repeat exams, imaging, and lab monitoring. If your dog has a new cough, faster sleeping respiratory rate, or reduced stamina, tell your vet even if the last visit seemed stable.

The exact cause of degenerative mitral valve disease is not fully understood, but it is considered a chronic degenerative process of the valve tissue. The valve leaflets thicken and become less stable, and the chordae tendineae that support the valve can also weaken. In some dogs, these changes are strongly linked to inherited risk.

Age is one of the biggest risk factors. The disease is uncommon in young dogs but becomes much more common in middle-aged and older dogs. Small-breed dogs are affected far more often than large-breed dogs. Cavalier King Charles Spaniels are a classic high-risk breed and may develop disease earlier than many other dogs.

Breed predisposition has also been reported in Dachshunds, Miniature Poodles, Cocker Spaniels, Miniature Schnauzers, and small terrier breeds. Males may be diagnosed more often in some populations, though either sex can be affected. Body size, genetics, and age together appear to shape risk more than lifestyle factors.

It is important to know what does not cause this disease. Degenerative mitral valve disease is not caused by infection, and dental disease does not cause it, even though older dogs may have both problems at the same time. Pet parents should also avoid assuming every cough in a senior small dog is heart disease. Airway disease, collapsing trachea, and other conditions can look similar, which is why your vet needs to sort out the cause.

There is no proven way to prevent degenerative mitral valve disease from developing in a dog that is genetically predisposed. What pet parents can do is focus on early detection. Routine exams are the best tool because a murmur often appears before any outward signs. For high-risk breeds, regular listening exams become especially important in middle age and beyond.

If your dog already has a murmur, prevention shifts toward slowing complications rather than preventing the disease itself. That means keeping follow-up visits, getting recommended imaging, and starting medication only when your vet believes the stage supports it. In dogs with Stage B2 disease, pimobendan has been shown to delay the onset of congestive heart failure, but it has not been shown to help dogs in Stage B1.

Good general health still matters. Keeping your dog at a healthy body weight, maintaining appropriate exercise for their condition, and avoiding unapproved supplements or medications can help reduce avoidable stress on the heart. Pet parents should always check with your vet before adding cough remedies, anti-inflammatory drugs, or supplements because some products can interfere with heart care.

For breeding decisions, dogs with inherited or strongly suspected familial valve disease should be discussed carefully with your vet and breed-specific health guidance. In predisposed breeds, selective breeding practices may help reduce risk across generations, even though they cannot eliminate the disease completely.

Prognosis depends heavily on stage at diagnosis. Dogs in Stage B1 may live comfortably for years with monitoring alone. Dogs in Stage B2 can also do well for long periods, and appropriate use of pimobendan in that stage may delay the onset of congestive heart failure. Once heart failure develops, the disease becomes a long-term management issue rather than something that can be cured.

Many dogs with Stage C disease can still have meaningful, happy time with a tailored medication plan and close follow-up. Quality of life often improves once fluid in the lungs is controlled, but medications usually need to continue long term. Recheck visits are important because kidney values, hydration status, appetite, and breathing patterns can change as treatment is adjusted.

Recovery after a heart failure episode is variable. Some dogs stabilize quickly and return home within a day or two, while others need repeated medication changes or future hospital visits. Stage D disease is more difficult to control and may require specialty care, more frequent monitoring, and conversations about goals of care.

At home, pet parents play a major role in prognosis. Tracking sleeping respiratory rate, appetite, energy, and coughing can help your vet catch worsening disease earlier. A sudden increase in breathing rate, new nighttime restlessness, or reduced interest in food should prompt a call to your vet. Early response often helps avoid a more serious crisis.