What Is Mitral Valve Disease?
Myxomatous mitral valve disease — abbreviated to MMVD and sometimes called chronic valvular disease or endocardiosis — is by far the most frequently diagnosed heart condition in dogs. It accounts for the vast majority of canine heart disease cases seen in general veterinary practice, and in certain breeds it is so prevalent that screening programmes exist specifically to monitor its progression.
The mitral valve sits between the left atrium and the left ventricle, acting as a one-way gate that ensures blood flows in the correct direction as the heart contracts. In MMVD, the valve leaflets gradually thicken, deform, and lose their precise fit. Over months and years, this causes regurgitation — a backward leakage of blood with each heartbeat — which forces the heart to work harder to compensate.
Which Dogs Are Most Affected?
MMVD shows a strong predisposition toward small and toy breeds. The Cavalier King Charles Spaniel is the breed most severely affected, with studies suggesting that virtually all Cavaliers will develop the condition by the time they reach ten years of age. Other commonly affected breeds include:
- Dachshund
- Chihuahua
- Miniature and Toy Poodle
- Maltese
- Pomeranian
- Shih Tzu
- Yorkshire Terrier
- King Charles Spaniel
Large breed dogs can develop MMVD, but it is far less common in them. Males appear to develop the condition slightly earlier and with slightly more severity than females, though both sexes are affected.
How Does MMVD Progress?
MMVD is a degenerative disease, meaning it worsens over time. The rate of progression varies considerably between individuals. Some dogs remain in the early stages for many years with no symptoms whatsoever, while others progress more rapidly toward heart failure.
In the early stages, the leaking valve causes a characteristic heart murmur that a vet can detect through a stethoscope. The body initially compensates for the reduced efficiency through a number of physiological mechanisms — the heart beats faster, blood vessels adjust their resistance, and the heart chambers begin to enlarge to handle increased volumes of blood. This compensation period can last for years.
As the disease advances and compensatory mechanisms become overwhelmed, the dog begins to develop congestive heart failure. Fluid accumulates in or around the lungs, making breathing more difficult and causing the familiar cough associated with cardiac disease.
Recognising the Symptoms
In the early, preclinical stages, there are often no visible symptoms at all. This is why routine veterinary check-ups are so important — a murmur may be present long before any outward signs appear.
As MMVD progresses, owners typically begin to notice:
- A soft, persistent cough, often worse at night or early morning
- Breathlessness or faster breathing at rest
- Reduced willingness to exercise or tiring quickly
- Restlessness at night, particularly difficulty settling
- Reduced appetite
- Weight loss despite normal or reduced food intake
- A distended abdomen
- Occasional fainting or weakness
One early warning sign that is easy to miss is a subtle increase in resting respiratory rate. A healthy dog at rest should breathe fewer than 30 times per minute. Many cardiologists now recommend that owners of at-risk breeds count their dog's resting respiratory rate regularly, as a consistent rise above 30 breaths per minute at rest can signal early fluid accumulation before coughing begins.
Diagnosing Mitral Valve Disease
The journey to diagnosis usually begins when a vet detects a heart murmur during a routine examination. Murmurs caused by MMVD are characteristically heard best at the left apex of the chest and are graded on a scale of one to six based on intensity.
To fully assess the extent of disease, additional tests are usually recommended:
- Echocardiography: This cardiac ultrasound is the gold standard for evaluating MMVD. It allows visualisation of the valve itself, measurement of chamber dimensions, and assessment of how significantly blood is leaking backwards.
- Chest radiography: X-rays can reveal whether the heart has enlarged and whether fluid is present in the lungs.
- Blood pressure monitoring: Elevated blood pressure can worsen valvular disease and needs to be addressed if present.
- Blood tests: These help evaluate kidney and liver function before starting medication, and are important for ongoing monitoring.
Treatment Options
The approach to treatment depends entirely on the stage of disease. Landmark research — particularly the EPIC trial published in 2019 — demonstrated that dogs with certain echocardiographic criteria benefit from starting pimobendan before clinical signs of heart failure appear. Pimobendan works by making the heart muscle contract more effectively and by reducing the resistance the heart must pump against.
Once heart failure develops, treatment typically expands to include:
- Furosemide or other diuretics to remove excess fluid from the lungs
- ACE inhibitors such as enalapril or benazepril to reduce cardiac workload
- Spironolactone to provide additional diuretic effect and some cardiac protection
- Dietary sodium restriction
The QUEEN study and other research have helped refine understanding of when and how to adjust medications as the disease progresses, and most veterinary cardiologists follow established guidelines to optimise outcomes for each individual patient.
Breed Screening and Prevention
While MMVD cannot be prevented, responsible breeding practices can reduce its prevalence in future generations. The Cavalier Health organisation and the Kennel Club support heart testing schemes that assess breeding dogs before mating. Dogs with early-onset severe murmurs are excluded from breeding programmes.
If you own a Cavalier or another predisposed breed, annual cardiac auscultation from age two onwards is a sensible baseline. Earlier or more frequent screening may be recommended depending on your individual dog's findings and family history.
Quality of Life and Prognosis
With modern medication protocols, many dogs with MMVD — even those in congestive heart failure — can enjoy good quality of life for a year or more after diagnosis. The preclinical stage can last for years with no impact on daily life whatsoever. Prognosis is influenced by the rate of disease progression, how well the dog responds to medication, and the presence of other health conditions. Consistent monitoring and open communication with a veterinary cardiologist give your dog the best possible chance.