Heart disease affects roughly 10% of all dogs seen by veterinarians, making it one of the most common serious health problems in the canine population. The causes range from inherited valve defects and genetic muscle disease to parasitic infections and nutritional gaps. Understanding what’s behind your dog’s heart condition helps you recognize early warning signs and make sense of what your vet recommends next.
Degenerative Valve Disease: The Most Common Cause
Myxomatous mitral valve disease, often shortened to MMVD, is responsible for roughly 44% of all heart disease cases in dogs. It’s far and away the leading cause. The mitral valve sits between the left atrium and left ventricle, controlling blood flow in one direction. In MMVD, the valve tissue gradually thickens and weakens over time, causing the valve to leak. When blood flows backward with every heartbeat, the heart has to work harder to keep up. Over months or years, that extra workload causes the heart chambers to enlarge.
Small breed dogs are especially vulnerable. Cavalier King Charles Spaniels and Dachshunds are among the most predisposed breeds, though the disease shows up commonly across many small breeds as they age. The genetics behind this susceptibility are still being mapped, but the pattern is clear: smaller dogs develop valve disease more often and earlier in life than larger breeds. Many dogs live for years with a leaky valve before any symptoms appear, which is why a heart murmur detected at a routine checkup is often the first clue.
Dilated Cardiomyopathy: A Disease of Heart Muscle
Where valve disease is a plumbing problem, dilated cardiomyopathy (DCM) is a muscle problem. The heart walls thin and stretch, making the chambers larger but weaker. The heart loses its ability to pump blood effectively, and eventually this leads to heart failure. DCM accounts for about 3% of cardiac cases in dogs, but it’s disproportionately serious because it tends to progress quickly and strikes dogs in the prime of life.
DCM is recognized as a genetic condition, predominantly affecting large and giant breeds. Doberman Pinschers, Great Danes, and Irish Wolfhounds are among the most commonly affected. In Dobermans especially, DCM often develops silently before causing sudden collapse or rapid heart failure. Cocker Spaniels represent an interesting exception to the large-breed pattern. In their case, the disease is linked to a deficiency in taurine, an amino acid critical for heart muscle function.
Golden Retrievers may also be genetically predisposed to taurine deficiency, and many DCM cases in Goldens show low taurine levels. The FDA investigated a potential connection between grain-free diets high in legumes and peas and an uptick in DCM reports across multiple breeds. While the exact mechanism remains unclear, the investigation highlighted that nutrition plays a measurable role in heart muscle health for certain dogs.
Heartworm: A Preventable Cardiovascular Threat
Heartworm disease is caused by a parasitic worm transmitted through mosquito bites. Adult worms take up residence in the pulmonary arteries, the vessels that carry blood from the heart to the lungs. Once established, the worms trigger inflammation and damage the inner lining of those arteries. Within the first 18 months of infection, the artery walls thicken, the vessels narrow, and blood pressure in the lungs rises. This condition, called pulmonary hypertension, forces the right side of the heart to pump against increasing resistance.
Over time, the right heart chambers weaken and enlarge, eventually leading to right-sided heart failure. In severe infections, worms can migrate backward from the pulmonary arteries into the heart itself, creating a life-threatening emergency known as caval syndrome. The damage heartworms cause is largely mechanical and inflammatory: physical obstruction of blood vessels, chronic irritation of vessel walls, and blockages from dead worm fragments that act like clots. All of this is preventable with monthly heartworm prevention, which is why veterinarians emphasize it so consistently.
Congenital Heart Defects
Some dogs are born with structural heart abnormalities. The three most common congenital defects, consistently across decades of veterinary data, are pulmonic stenosis, patent ductus arteriosus (PDA), and subaortic stenosis. Together these three account for the vast majority of congenital cases.
- Pulmonic stenosis is the single most common congenital defect, representing about 34% of cases. The valve controlling blood flow from the heart to the lungs is abnormally narrow, forcing the heart to pump harder.
- Patent ductus arteriosus accounts for roughly 26% of cases. Before birth, a small vessel connects two major arteries, bypassing the lungs. It’s supposed to close shortly after birth. When it doesn’t, blood flows abnormally between the vessels, overloading the heart.
- Subaortic stenosis involves a ridge of tissue below the aortic valve that narrows the outflow path, making the heart work harder to push blood to the body. It represents about 15% of congenital cases.
These defects occur in both purebred and mixed-breed dogs. Some are mild enough that a dog lives a normal life with monitoring alone. Others require intervention early in life to prevent heart failure.
How Aging Weakens the Heart
Even without a specific disease, the aging process itself takes a toll on heart function. As dogs get older, their cells accumulate damage from reactive oxygen species, which are unstable molecules produced as a normal byproduct of energy generation in cells. Over a lifetime, this oxidative damage compromises the DNA inside heart muscle cells and disrupts the tiny energy-producing structures (mitochondria) those cells depend on.
Damaged heart cells stop dividing and begin releasing inflammatory signals that affect surrounding tissue. The heart loses muscle cells at a steady rate, and the remaining cells enlarge to compensate. Scar-like fibrous tissue gradually replaces functional muscle. The net result is a heart that’s stiffer, less efficient at relaxing between beats, and weaker at pumping. This age-related decline explains why older dogs often develop exercise intolerance or breathing changes even when no single disease is identified. It also explains why conditions like MMVD tend to worsen with age: the heart is simultaneously dealing with valve disease and the background deterioration of aging muscle.
Endocrine Disorders and Heart Function
Thyroid hormones play a direct role in how strongly and quickly the heart contracts. In dogs with hypothyroidism, the reduced hormone levels weaken the heart’s pumping force, slow the heart rate, and alter electrical signals on an ECG. On its own, the heart dysfunction from low thyroid function is rarely severe enough to cause obvious symptoms. But in a dog that already has early heart muscle disease, hypothyroidism can accelerate the decline and push the heart closer to failure. This connection has been studied most closely in Doberman Pinschers, a breed prone to both hypothyroidism and DCM. While hypothyroidism doesn’t appear to directly cause DCM, it can worsen an existing case.
Recognizing the Stages of Heart Disease
Veterinary cardiologists classify heart disease progression into stages that help guide when treatment should start. In the earliest stage, a dog belongs to a high-risk breed but has no detectable heart changes. A Cavalier King Charles Spaniel with no murmur, for example, falls into this category. The next stage involves a detectable murmur or mild valve leakage, but no enlargement of the heart chambers and no symptoms. Dogs at this point are typically monitored without medication.
When imaging shows that the heart has enlarged enough to meet specific thresholds, treatment to delay heart failure becomes worthwhile even though the dog still feels fine. This is a critical window because intervening here can meaningfully extend the time before symptoms develop. Once a dog starts showing signs of heart failure, including rapid breathing, restlessness, coughing, or difficulty breathing, the disease has reached a more advanced stage requiring active management. The final stage describes dogs whose heart failure symptoms no longer respond adequately to standard treatment.
The practical takeaway: most heart disease in dogs develops silently. A murmur heard during a routine exam or subtle changes on a chest X-ray are often the earliest evidence. By the time a dog is coughing or breathing hard, the disease has been progressing for months or years. Regular veterinary checkups, particularly for predisposed breeds, give you the best chance of catching it in the stages where intervention makes the biggest difference.