OldieDegenerative mitral valve disease is the most common heart disease of dogs, and it is a disease of age and of small breeds. In 112,000 dogs in English practices, 0.36 percent had the diagnosis and over 3 percent had a murmur consistent with it; breed and age were strongly linked1. The valve between the left atrium and ventricle thickens and leaks, the heart enlarges, and, in a proportion of dogs, congestive heart failure follows years later.
Most dogs are found by a murmur at a routine examination, one more reason for twice-yearly senior checks. The ACVIM consensus guidelines stage the disease from a murmur with a normal heart to heart failure, and their 2019 revision changed management before signs appear: dogs whose hearts have enlarged on imaging now start medication that delays failure2.
At home the signs that matter are a cough, especially at night or after lying down, faster breathing at rest, tiring on walks, and a swollen belly. Counting breaths while the dog sleeps, and noting the number in Oldie, gives the vet an early warning: a sustained rise in resting breathing rate is a reason to call2.
This is general information, not a diagnosis. Your vet knows your dog.
Sources
- Prevalence of and risk factors for degenerative mitral valve disease in dogs attending primary-care veterinary practices in England (Mattin MJ et al.). Journal of Veterinary Internal Medicine 29(3):847-854, 2015 (peer reviewed). Accessed 2026-09-22.
- ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs (Keene BW et al.). American College of Veterinary Internal Medicine, Journal of Veterinary Internal Medicine 33(3):1127-1140, 2019. Accessed 2026-09-22.