The cavalier can cough, struggle for breath, develop a fluid filled stomach. They may have fainting episodes, and be unable to exercise. There are drugs that can help but they can be very expensive.
MVD is a problem that can affect young cavaliers, but often there are no symptoms to warn of the problem until the dogs are mature and have already been used for breeding.
Good Breeders are expected to try and raise the age of onset of MVD by selecting breeding cavaliers with a good heart record.
There are guidelines issued by the UK Cavalier Club which recommend that the mother of the puppies should be at least two and a half years old with a clear heart. The father should be at least two and a half years old with a clear heart.
To use an older stud dog with a clear certificate issued as late in life as possible is highly desirable.
The breeding guidelines for MVD can be found in the health section of www.thecavalierclub.co.uk