Mitral valve disease in dogs: symptoms, tests and treatment

Overview

The mitral valve lies between the left atrium and left ventricle. It should close when the heart contracts so that blood moves in the right direction. A degenerated valve does not close tightly, and some blood flows back into the atrium. This is called valve regurgitation. The disease is abbreviated to MMVD.

Heart disease and heart failure are different conditions. A valve can be structurally diseased without visible signs. In heart failure, the heart disease is already causing functional problems, such as fluid accumulating in the lungs. Some dogs never progress to this stage.

Degenerative diseases of the atrioventricular valves account for about 75% of heart disease in dogs. About 60% of affected dogs have changes confined to the mitral valve. In about 30%, the tricuspid valve on the right side of the heart is also affected.

Causes

Degeneration changes the valve's supporting structure and thickens its leaflets. The tendinous cords supporting the valve can also weaken and sometimes rupture. The exact initiating cause is unknown. An inherited component is recognised particularly in certain breeds.

When blood leaks back into the atrium, the heart has to handle a larger volume of blood than usual. This extra load can enlarge the atrium and ventricle. The body can compensate for the leak for a long time, so your dog may seem normal despite significant structural disease.

Compensation also involves the kidneys retaining more water and sodium. Blood volume increases, helping the heart maintain forward blood flow despite the leaking valve. However, do not try to regulate this mechanism by restricting drinking water yourself.

If pressure in the left atrium rises sufficiently, pressure in the lung blood vessels also increases, and fluid may move into lung tissue. This is called cardiogenic pulmonary oedema. It explains why the most dangerous visible change in valve disease may be breathing difficulty rather than swelling of the abdomen or limbs.

Risk factors

The disease becomes more common with age and is particularly frequent in small and medium-sized dogs. Larger and crossbreed dogs can also be affected.

Inherited susceptibility is recognised in Cavalier King Charles Spaniels and Dachshunds. The disease also occurs in breeds such as small Poodles, Chihuahuas and Pomeranians.

The disease is about 1.5 times more common in males than females. It is diagnosed particularly often in dogs weighing less than 20 kg. In large dogs, it often progresses faster, and impaired heart muscle function may be more pronounced.

Cavalier King Charles Spaniels can develop valve disease at a younger age than many other small dog breeds.

Typical symptoms

Early on, dogs often have no signs that you can notice. A murmur may be heard during an examination long before obvious illness develops. Later, you may notice that your dog cannot exercise as well as before.

• Reduced tolerance of walks or play.

• Coughing.

• Faster breathing or difficulty breathing.

• Weakness or collapse.

• Restlessness alongside breathing signs.

A cough alone does not prove heart failure. These dogs can also have tracheal and bronchial diseases that explain the coughing.

Advanced heart failure can involve muscle wasting. Weight loss is not always obvious, even when muscle mass has already been lost.

Complications and consequences

The main complication is heart failure with associated pulmonary oedema. Abnormal heart rhythms can also develop as the heart enlarges. Rarely, a cord supporting the valve ruptures or the atrial wall tears, which can cause a sudden, serious emergency.

In many older dogs, the disease progresses slowly and ultimately does not limit lifespan.

Heart failure develops in about 30% of dogs with mitral regurgitation. Estimates of the proportion developing clinical heart failure signs also range from 25–50%, so not every dog with valve disease reaches a symptomatic stage.

After heart failure begins, some dogs live for more than 1 year with appropriate care. A typical survival time with treatment is about 12–18 months from the onset of heart failure, but individual variation is considerable. These times relate to symptomatic heart failure, rather than the date symptom-free valve disease is diagnosed.

Muscle loss associated with heart disease worsens the outlook and is easier to prevent than to treat later.

Diagnosis at the vet

A murmur heard with a stethoscope is a common first finding. It results from turbulent blood flow caused by the leak. The history of signs and your dog's general condition are assessed alongside the murmur. Ultrasound shows the structure of the valves and heart, as well as blood flow. The aim is to confirm the type of disease and assess the burden it places on the heart.

Chest X-rays provide information about heart size and fluid in the lungs. In a dog with signs, this helps distinguish heart failure from other respiratory diseases. A blood test reflecting cardiac strain can add to the assessment, but cannot replace the full investigation. An ECG is used to investigate heart rhythm if necessary.

Chest X-rays taken before signs appear may be useful later. They provide a comparison if your dog starts coughing or breathing abnormally. Keep a record of where and when previous imaging was performed.

In a dog with signs, a normal or nearly normal result on a blood test measuring cardiac strain argues against heart failure as their cause.

Disease staging guides treatment. ACVIM stage B1 means a symptom-free stage in which heart changes do not meet the criteria for preventive medication. In ACVIM stage B2, the dog is still symptom-free, but heart enlargement has progressed enough to meet treatment criteria.

ACVIM stage C means current or previously diagnosed heart failure caused by the disease. A dog remains in this stage even when medicines bring signs under control. In ACVIM stage D, standard treatment no longer controls the signs adequately.

Blood pressure is also measured to detect concurrent hypertension. Heart ultrasound can also identify raised pressure in the pulmonary circulation. The severity of valve regurgitation is assessed using measures including atrial size; the size of the regurgitant jet on colour-flow imaging is not a reliable measure for this purpose.

A blood count, blood biochemistry and urine tests can be normal in a dog with valve degeneration. If a tendinous cord supporting the valve ruptures, acute heart failure can develop without marked atrial enlargement.

Treatment

The vet prescribes medicines according to the stage of disease. Mild, symptom-free disease can be monitored without medicines. Once the heart is sufficiently enlarged, pimobendan may help delay the onset of heart failure.

Treatment for heart failure includes diuretics and medicines that support heart function. Severe pulmonary oedema may require hospital care, oxygen and prompt diuretic treatment. Do not change the amount of medicine given at home unless your vet has provided written instructions specifically for your dog.

Medicines control signs and cardiac workload, but do not repair the degenerated valve. In selected cases, valve repair surgery or other specialist procedures can be discussed with a vet experienced in heart disease. These techniques are not widely available, and suitability is assessed individually.

Monitoring includes general condition, blood pressure and, when needed, blood tests and imaging. Kidney function and electrolyte balance matter particularly when monitoring heart failure and medication.

Avoid unnecessarily salty treats while ensuring that your dog eats enough and receives adequate energy and protein. Do not restrict the diet so severely that your dog loses weight or refuses food.

In mild, symptom-free disease, another ultrasound examination is usually performed after 6–12 months, depending on the findings. After heart failure develops, blood pressure and blood tests are typically monitored every 3–6 months and after changes in medication.

Prevention

The exact cause of valve disease is unknown, and there is no reliable home measure to prevent it. Regular health checks can, however, detect a murmur before signs appear. Early recognition allows planned monitoring and assessment of when medicines may start to help. Listening to the heart of an apparently healthy dog at risk is therefore a useful part of a routine examination.

If heart enlargement is minimal, your dog can be cared for normally alongside agreed check-ups. Agree limits for exercise and daily activities with your own vet according to any signs or other illnesses.

Some breed-club programmes require a heart ultrasound before breeding if a murmur is heard during examination.

Important for dog owners

Practise monitoring breathing while your dog is asleep or completely at rest. Record results in one place so that a change from the usual baseline is visible. Ask your vet for an individual threshold for making contact and instructions on how to count. If your dog has difficulty breathing, contact the veterinary team rather than stopping to fill in a diary.

In a dog with a previous episode of heart failure, the breathing rate during sleep or complete rest is usually below 30 breaths per minute. This is not a threshold below which every sign is harmless. Always consider your dog's baseline and the effort involved in breathing, and agree with the vet treating your dog what to do about an abnormal change.

Keep an up-to-date medicine list and record treatments if several people care for your dog. At rechecks, ask about the disease stage, the aim of the next investigation and what to do if there is a sudden change. Mention travel plans or changes in routine in advance, so an adequate medicine supply and any monitoring can be arranged.

Frequently asked questions

Does a heart murmur always mean heart failure?

No. A murmur from valve regurgitation can be heard before clinical signs develop, and some dogs remain symptom-free for a long time. Investigations assess heart structure, enlargement and any fluid accumulation. Treatment depends on these overall findings, rather than simply whether a murmur is present.

Does a dog without symptoms need heart medicine?

Some do and some do not. The key issues are how much the heart has changed and whether treatment criteria are met. Mild disease is monitored without medicines, while sufficient heart enlargement may justify medication even before signs appear.

Does coughing in a dog with heart disease always mean pulmonary oedema?

No. The same dog may also have tracheal or bronchial disease. The cause of coughing therefore needs investigation, and a need for diuretics cannot be inferred from coughing alone. Breathing difficulty or a clearly increased resting breathing rate always requires prompt contact with a vet.

Why monitor breathing while my dog sleeps?

The breathing rate during sleep or complete rest helps monitor a dog, particularly after a previous episode of heart failure. A change from their own baseline is useful information for the vet. Ask for individual instructions on counting and interpreting results, with a clear plan for when to get in touch.

Can medicines repair a leaking valve?

Medicines control the effects of the disease and signs of heart failure, but do not repair the valve's structure. Valve repair may be considered separately in selected cases. Discuss with the vet treating your dog what treatment is aiming to achieve and which options are available in practice.