What causes myxomatous mitral valve disease?

There are no known causes of myxomatous mitral valve disease, but there are certain factors that make your dog more likely to develop the condition.


This condition is more common in certain breeds of dogs, with some more likely to be affected than others. The Cavalier King Charles Spaniel, poodles and certain terrier breeds are much more prone to MMVD than other breeds. This indicates a strong genetic component to this heart complaint. One study (Mattin et al., 2015) gave the following spread of breeds affected by MMVD:

32.4 % Cavalier King Charles Spaniels

4.44 % King Charles Spaniels

6.2 % Yorkshire Terriers

5.4 % Jack Russell Terriers

2.22 % Poodle (including miniature and standard varieties)

3. 95 % Border Collie

2.95 % Shih Tzu

Remaining % was made up of other breeds

Another study gave an even higher prevalence in miniature poodle of 8% (Meurs et al., 2019).

So, owning a breed in this list makes the chances of them developing MMVD more likely.


Small dogs (weighing less than 20kg) are more likely to develop MMVD than larger dogs (weighing more than 20 kg). Dogs weighing 20 kg or more had approximately half the odds of DMVD compared with smaller dogs (Mattin et al, 2015).


MMVD is most commonly diagnosed in middle to old age adult dogs. The average age that MMVD is diagnosed or the presence of a heart murmur was first recorded was 9.5 years of the 116 cases in one study (Mattin et al, 2015).


Being male gives a higher risk of developing MMVD. In one study 62.2% of cases were male (Mattin et al, 2015)

Age versus Weight

There is an increase in odds of having a heart murmur in young dogs (<4 years) that are heavier (>20kg) when compared with dogs of the same age that are lighter. However, the converse is true in older dogs. The chances of discovering a heart murmur in an older dog (>7 years) is higher in dogs that are lighter in weight (<20kg), compared to those that are heavier (Mattin et al, 2015).


There are a couple of possible causes for a heart murmur in dogs. In myxomatous mitral valve disease, a murmur is caused by a turbulent flow of blood because the valve is not functioning as well as it should. This is because of degenerative changes, causing the valve to become thickened and irregular. Murmurs can vary in loudness and are often assigned a grade by the vet. Murmurs are not diagnostic however and more tests will be required. MMVD is more common in certain breeds than others, and more likely to be detected in older adult dogs.

Key takeaway

Heart murmurs, of varying degrees of loudness, are a common finding in myxomatous mitral valve disease. Some breeds are much more prone to developing this condition than others.