Heart disease in dogs has several distinct causes, from inherited valve problems to parasitic infections, and it affects a significant portion of the pet population. Roughly 1 in 10 dogs examined by a veterinarian has some form of cardiovascular disease. The most common culprits are degenerative valve disease, dilated cardiomyopathy, heartworm infection, and congenital defects present from birth.
Degenerative Valve Disease
The single most common heart condition in dogs is degenerative mitral valve disease, sometimes called myxomatous mitral valve disease. The mitral valve sits between the two left chambers of the heart and acts as a one-way gate, keeping blood flowing in the right direction. Over time, the valve’s internal structure breaks down. The supportive collagen layer weakens and thins, while the spongy middle layer expands with excess proteins and sugars. The valve leaflets become thickened, floppy, and unable to close tightly.
When the valve can’t seal properly, blood leaks backward with each heartbeat. The heart has to work harder to push enough blood forward, and the left side gradually enlarges to compensate. Eventually this leads to congestive heart failure, where fluid backs up into the lungs.
Small and medium-breed dogs are most prone to this condition, including Cavalier King Charles Spaniels, Dachshunds, and Miniature Poodles. It typically develops in middle-aged to older dogs and progresses slowly over years. Research has linked the degeneration to abnormal serotonin activity within the valve tissue itself. Affected valves show higher serotonin levels, increased serotonin receptors, and reduced ability to clear serotonin, all of which activate cells that remodel the valve structure in harmful ways.
Dilated Cardiomyopathy
Dilated cardiomyopathy, or DCM, is a disease of the heart muscle itself. The walls of the heart thin out and weaken, causing the chambers to stretch and enlarge. The heart loses its ability to contract forcefully enough to pump blood effectively. This condition primarily strikes large and giant breeds: Doberman Pinschers, Great Danes, Irish Wolfhounds, and Boxers (which develop their own rhythm-specific variant called arrhythmic cardiomyopathy).
In Dobermans, researchers at North Carolina State University identified two specific genetic mutations tied to DCM. One involves a gene called PDK4, where a small deletion in the DNA creates a faulty protein suspected of damaging heart cells. Dogs carrying even one copy of this mutation face a sevenfold increase in their risk of developing the disease. A second mutation, in a gene responsible for building a structural protein in muscle fibers, compounds the risk further. Dogs carrying both mutations are at the highest risk of all. These findings underscore that DCM in many breeds is fundamentally a genetic disease, passed from parents to offspring.
The Grain-Free Diet Question
Starting in 2018, the FDA investigated reports of DCM in breeds not traditionally prone to it, with many affected dogs eating grain-free diets high in peas, lentils, and potatoes. The concern was that these ingredients might interfere with taurine, an amino acid important for heart muscle function. Dogs can normally make their own taurine (unlike cats, who must get it from food), so the question was whether something in these diets disrupted that process.
As of the FDA’s last update in late 2022, no causal link has been established. The agency described it as “a complex scientific issue that may involve multiple factors” and noted that the vast majority of dogs eating these diets have not developed DCM. The investigation remains open but without new public findings. If you’re concerned about your dog’s diet, this is a reasonable topic to bring up with your vet, but the science hasn’t settled on a clear answer.
Heartworm Disease
Heartworm is one of the few forms of heart disease in dogs that is entirely preventable. Dogs become infected when a mosquito carrying heartworm larvae bites them. The larvae migrate through the body over several months, eventually maturing into foot-long worms that take up residence in the pulmonary arteries and, in severe cases, the heart chambers themselves.
The damage isn’t just from physical obstruction. The worms trigger chronic inflammation in the blood vessel walls, causing scarring, thickening, and progressive dysfunction of the vessels that carry blood from the heart to the lungs. This drives up blood pressure in the pulmonary arteries, forcing the right side of the heart to pump against increasing resistance. Exercise makes everything worse by increasing blood flow velocity through already-damaged vessels, creating a cycle of further injury.
Over time, the right ventricle can no longer keep up with the pressure demands, and right-sided congestive heart failure develops. In the most severe scenario, called caval syndrome, worms physically migrate backward into the right atrium and the large veins feeding the heart, obstructing blood flow and creating a life-threatening emergency. Monthly preventive medications break this entire chain by killing larvae before they can mature.
Congenital Heart Defects
Some puppies are born with structural heart problems that were present during fetal development. The most common congenital defect in dogs is patent ductus arteriosus, or PDA. Before birth, a small blood vessel called the ductus arteriosus connects the aorta to the pulmonary artery, allowing blood to bypass the lungs (which aren’t yet in use). This vessel is supposed to close within the first few days of life. When it doesn’t, blood continuously shunts from the aorta into the pulmonary artery, flooding the lungs with excess blood flow and overloading the left side of the heart.
PDA is most common in small-breed dogs and occurs more often in females. Without treatment, the chronic volume overload typically leads to left-sided heart failure within the first one to two years of life. The good news is that when caught early, PDA can be corrected through a minimally invasive catheter procedure or surgery, and dogs treated before symptoms develop generally have an excellent long-term outcome. A characteristic loud, continuous “machinery-like” murmur heard during a routine puppy exam is often the first clue.
Age-Related Heart Changes
Even without a specific disease, the aging process itself takes a toll on a dog’s cardiovascular system. Older dogs experience increased stiffness in both the heart muscle and blood vessel walls, making the heart less flexible and less efficient at filling and pumping. The heart’s electrical system slows, with longer intervals between signal and contraction. Perhaps most importantly, the aging heart loses its ability to ramp up output during exercise, which is why older dogs tire more easily and may seem reluctant to play or walk as far as they used to.
These changes don’t cause heart failure on their own, but they reduce the heart’s reserve capacity. A heart that’s already stiff and slow to respond has far less room to compensate when valve disease, obesity, or another condition adds additional stress.
Obesity and Heart Strain
Carrying excess weight places measurable strain on a dog’s heart. Research comparing obese dogs to normal-weight dogs found significant reductions in how effectively the heart pumps. Obese dogs showed lower ejection fractions (roughly 71% versus 80% in lean dogs) and lower fractional shortening, both key measures of how forcefully the heart contracts. The heart walls were actually thinner in obese dogs despite increased overall heart mass, a pattern consistent with the chambers stretching under chronic excess workload.
Obesity also disrupts the balance of the nervous system’s control over heart rate, tilting it toward a stress-dominant state. On top of that, excess fat tissue generates roughly double the level of oxidative stress molecules compared to lean dogs, creating cellular damage throughout the cardiovascular system. None of this means an overweight dog will inevitably develop heart failure, but it does mean obesity accelerates the progression of any existing heart condition and reduces the heart’s ability to cope with additional challenges.
Dental Disease and the Heart
Severe periodontal disease can become a cardiac risk factor through a surprisingly direct route. When gum disease is advanced, bacteria from dental plaque enter the bloodstream through inflamed, bleeding gum tissue. These bacteria can then attach to heart valves, particularly valves that are already abnormal or damaged, and establish an infection called bacterial endocarditis.
Researchers have confirmed this connection by matching the exact bacterial strains found in dogs’ mouths to those found on their heart valves at necropsy. The infected valve tissue can form clots that break off and travel to other organs, a complication known as thromboembolic disease. Regular dental care isn’t typically thought of as heart disease prevention, but in dogs with existing valve abnormalities, keeping the mouth healthy removes a meaningful source of risk.
How Heart Disease Is Detected
Many forms of canine heart disease are silent in their early stages, which is why veterinary exams matter. Heart murmurs, detected with a stethoscope, are often the first sign of valve disease or congenital defects. Echocardiography (an ultrasound of the heart) is the standard tool for confirming a diagnosis, measuring chamber sizes, and assessing how well the heart is pumping.
A blood test measuring a protein called NT-proBNP can help distinguish heart disease from other conditions that cause similar symptoms like coughing or labored breathing. Dogs with levels below 500 pmol/L are very unlikely to have significant heart disease. Levels between 900 and 1,725 pmol/L suggest moderate disease warranting further investigation, and levels above 1,725 pmol/L are strongly associated with congestive heart failure. In dogs already diagnosed with valve disease, an NT-proBNP above 1,500 pmol/L carries nearly a sixfold increase in the odds of progressing to heart failure.