Short answer: about 10% of dogs seen in primary-care vet practices have heart disease, and most of it is one age-related condition — myxomatous mitral valve disease (MMVD), which accounts for roughly 75% of canine heart disease cases, according to the 2019 ACVIM consensus guidelines. It rises steeply with age: up to 85% of small-breed dogs show the valve lesion by 13. Most affected dogs have no symptoms for years, and a murmur heard at a routine exam is usually the first sign.
Heart disease in older dogs is common, mostly silent in its early stages, and far more manageable when it’s caught at a checkup rather than in an emergency room. The numbers are also frequently misquoted, so it’s worth knowing what the research actually says.
What Do the Statistics Actually Say?
| Statistic | What it means | Source |
|---|---|---|
| ~10% of dogs have heart disease | Share of dogs presented to primary-care vets | ACVIM consensus guidelines (Keene et al., 2019) |
| ~75% of heart disease is MMVD | Share of canine heart disease cases caused by mitral valve disease | ACVIM 2019; Merck Veterinary Manual |
| Up to 85% by age 13 | Small-breed dogs showing the valve lesion by 13 years | ACVIM 2019 |
| ~30% develop heart failure | Share of dogs with mitral regurgitation that ever develop congestive heart failure | Merck Veterinary Manual (2025) |
Is it true that 75% of senior dogs have heart disease?
Not as stated. The “75% of senior dogs” line appears on many clinic and pet websites, but the 75% figure in the veterinary guidelines is the proportion of heart disease cases that are mitral valve disease — not the proportion of older dogs who are affected. The best age-based figure, from the ACVIM guidelines, is that valve changes are found in up to 85% of small-breed dogs by 13. That sounds even higher, but “has a valve lesion” is not the same as “has heart disease that will shorten its life”: many of those dogs never progress to heart failure.
What Is Mitral Valve Disease?
MMVD is a slow, degenerative thickening of the mitral valve — the valve between the left atrium and left ventricle. As it thickens, it stops sealing completely, so a little blood leaks backward with every beat (mitral regurgitation). That leak is what your vet hears as a murmur. Over years, the extra workload can enlarge the heart, and in some dogs fluid eventually backs up into the lungs — congestive heart failure (CHF).
The Merck Veterinary Manual notes that there are no clinical signs in the early and middle stages, and that because the heart compensates so well, only about 30% of dogs with mitral regurgitation ever develop CHF. The ACVIM guidelines add that most dogs have a recognizable murmur for years before heart failure begins.
How Do Vets Stage Heart Disease in Dogs?
The ACVIM guidelines sort MMVD into stages, which decide what happens next:
- Stage A — at risk, no murmur yet. Breeds with higher-than-average risk, such as Cavalier King Charles Spaniels, Dachshunds, and Miniature and Toy Poodles.
- Stage B1 — murmur, heart not significantly enlarged. No treatment; the guidelines suggest re-checking with echocardiography (or X-rays) every 6–12 months.
- Stage B2 — murmur plus heart enlargement, still no symptoms. Treatment is recommended before signs appear.
- Stage C — current or past signs of heart failure. Managed with several medications.
- Stage D — heart failure that no longer responds to standard treatment.
The B1/B2 line matters. In the EPIC trial (Boswood et al., 2016), 360 dogs with MMVD and an enlarged heart but no symptoms received either pimobendan or a placebo. Median time to heart failure or cardiac death was 1,228 days on pimobendan versus 766 days on placebo — roughly 15 extra months symptom-free. That’s why a vet who hears a new murmur will usually want imaging rather than just “keeping an eye on it.” Dosing is your vet’s call and depends on the stage.
What Are the Warning Signs at Home?
Heart failure signs to watch for in an older dog:
- Faster breathing at rest or while asleep
- Labored breathing or breathing with an open mouth at rest
- A cough, especially at night or after lying down
- Tiring quickly on walks or reluctance to exercise
- Fainting or collapse
- A swollen belly (advanced cases)
The most useful home check is the sleeping breathing rate. Count chest rises for 30 seconds while your dog is asleep and multiply by two. VCA’s guidance, co-written by a veterinary cardiologist, puts the normal range at 15–30 breaths per minute and says rates consistently above 30 are abnormal — contact your vet as soon as possible. A dog that is struggling to breathe, has blue or grey gums, or collapses needs an emergency vet immediately.
Which Dogs Are Most at Risk?
MMVD is primarily a disease of older small-breed dogs. Cavalier King Charles Spaniels and Dachshunds inherit it, according to Merck. The ACVIM guidelines note that Cavaliers develop MMVD at a relatively young age, although their progression to heart failure isn’t markedly faster than in other small breeds — see our Cavalier vet costs and heart disease guide. Miniature and Toy Poodles are also named as at-risk breeds in the ACVIM guidelines.
Large and giant breeds are more likely to face a different disease: dilated cardiomyopathy (DCM), which weakens the heart muscle rather than a valve. The Merck Veterinary Manual lists Doberman Pinschers, Boxers, Great Danes, German Shepherds and Labrador Retrievers among the breeds at risk, and notes that DCM has a long subclinical phase in dogs, which is why breed-specific screening is often discussed for these dogs — see our Great Dane aging guide.
What Should You Do If Your Vet Finds a Murmur?
A new murmur in an older dog is common and usually not an emergency. Typical next steps:
- Staging — chest X-rays and/or an echocardiogram to see whether the heart is enlarged (B1 vs. B2).
- A plan based on the stage — rechecks every 6–12 months for B1; medication discussion for B2.
- Home monitoring — record your dog’s sleeping breathing rate a few times a week so you know its baseline.
- Weight and exercise — keep your dog lean and active within comfortable limits unless your vet advises otherwise.
Because murmurs are found during routine exams, the twice-yearly senior visit is doing real work even when nothing seems wrong. When a dog counts as senior depends on size — see our senior age by breed size chart — and our breakdown of senior dog vet bills covers what imaging and rechecks typically cost.
The Bottom Line
Heart disease is common in older dogs, but the scary “75% of seniors” figure is a misreading. What the guidelines show is that mitral valve changes become very common in small breeds with age, most affected dogs never develop heart failure, and those who are heading that way can often be identified — and helped — years in advance through a stethoscope, staging, and a simple breathing-rate count at home.
This article is for informational purposes only and does not replace guidance from your veterinarian.
Sources
- Journal of Veterinary Internal Medicine — ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs (Keene et al.), 2019
- Merck Veterinary Manual — Myxomatous Atrioventricular Valve Degeneration in Dogs and Cats, May 2025
- Journal of Veterinary Internal Medicine — Effect of Pimobendan in Dogs with Preclinical Myxomatous Mitral Valve Disease and Cardiomegaly: The EPIC Study (Boswood et al.), 2016
- VCA Animal Hospitals — Home Breathing Rate Evaluation, March 2025
- Merck Veterinary Manual — Dilated Cardiomyopathy in Dogs and Cats, September 2024
