Heart disease in cats most often stems from genetics, with hypertrophic cardiomyopathy (HCM) accounting for the vast majority of cases. But several other factors can damage a cat’s heart, including overactive thyroid glands, high blood pressure, parasites, and nutritional deficiencies. A 2015 study screening 780 apparently healthy cats in rehoming centers found that 14.7% had HCM, making it far more common than most owners realize.
Genetics and Breed Risk
HCM causes the walls of the heart to thicken, making it harder for the chambers to fill with blood and pump efficiently. In many cats, the underlying trigger is genetic. Maine Coons carry one of the best-studied mutations: a change in a gene that encodes a structural protein in heart muscle cells. Roughly one third of Maine Coons worldwide carry at least one copy of this mutation. A cat with one copy is about 1.8 times more likely to develop HCM than a cat without it. A cat with two copies faces an 18-fold increase in risk, though having two copies also appears to cause significant embryonic death, so relatively few of these cats are born.
Ragdolls carry a different mutation in the same gene, and several other breeds, including British Shorthairs, Sphynx, and Persian cats, show higher rates of HCM as well. Mixed-breed cats are not immune. Because HCM can develop silently over years, genetic testing is available for Maine Coons and Ragdolls to help breeders reduce the prevalence of the mutation in future generations. A negative genetic test, however, doesn’t guarantee a cat won’t develop HCM from other causes.
Hyperthyroidism and Secondary Heart Changes
An overactive thyroid gland is one of the most common hormonal disorders in middle-aged and older cats, and it directly stresses the heart. Elevated thyroid hormones force the heart to beat faster and contract more forcefully. Over time, this leads to thickening of the left ventricle, mimicking the structural changes seen in primary HCM. If the condition goes untreated, these changes can progress to heart failure.
The encouraging part: once the hyperthyroidism is controlled, whether through medication, dietary management, or other treatments, the cardiac changes often improve or even resolve completely. This makes thyroid-related heart disease one of the more reversible forms, but only if it’s caught before permanent damage sets in.
High Blood Pressure
Systemic hypertension, or chronically elevated blood pressure, leads to cardiac changes in up to 60% of affected cats. When the heart has to pump against higher resistance in the blood vessels, its walls thicken in response, similar to a muscle that’s being overworked. A veterinarian may detect this as a heart murmur during a routine exam.
Unlike in humans, where heart disease frequently causes high blood pressure, in cats the relationship runs the other direction. High blood pressure is usually the result of another condition, most commonly chronic kidney disease. The two problems reinforce each other: hypertension increases protein loss through the kidneys, which shortens survival time in cats with kidney disease, while the kidney disease itself drives blood pressure higher. This cycle makes blood pressure monitoring especially important in older cats with known kidney issues.
Taurine Deficiency
Cats have a limited ability to produce taurine, an amino acid essential for heart muscle function, on their own. Before the late 1980s, many commercial cat foods didn’t contain enough taurine, and veterinarians saw widespread cases of dilated cardiomyopathy (DCM), a condition where the heart muscle weakens and the chambers stretch out, reducing the heart’s ability to pump blood. Once pet food manufacturers began supplementing with adequate taurine, DCM rates in cats dropped dramatically.
Today, taurine deficiency is rare in cats eating standard commercial diets. The risk still exists, though, for cats fed homemade diets, raw food diets formulated without supplementation, or dog food (which contains far less taurine than cat food requires). DCM caused by taurine deficiency is often reversible with supplementation if caught early enough.
Heartworm Infection
Heartworm disease in cats is caused by a parasitic worm transmitted through mosquito bites. When a mosquito feeds on an infected animal (usually a dog) and then bites a cat, it deposits larvae into the cat’s bloodstream. These larvae can mature into adult worms that lodge in the pulmonary arteries and heart.
In cats, heartworm disease primarily damages the lungs rather than the heart itself, a pattern veterinarians call heartworm-associated respiratory disease (HARD). Even a small number of worms can cause serious inflammation in the blood vessels of the lungs, increasing strain on the heart as it tries to push blood through damaged arteries. What makes this particularly dangerous for cats is that the drug used to kill adult heartworms in dogs can be fatal in cats, meaning there is no direct treatment. Prevention through monthly preventive medication is the only reliable protection.
Restrictive Cardiomyopathy
Restrictive cardiomyopathy is the second most common form of cardiomyopathy in cats, after HCM. In this condition, scar tissue (collagen) builds up in the walls of the left ventricle, making it stiff and unable to relax properly between beats. The heart can still contract, but it can’t fill with enough blood, so the pressure inside the chamber rises and blood backs up. The exact cause remains unknown, though prior inflammation or infection of the heart lining is suspected in some cases.
Congenital Heart Defects
A small percentage of cats are born with structural heart abnormalities. The rehoming center study found congenital heart disease in about 0.5% of screened cats. These defects can include holes between heart chambers, malformed valves, or abnormal connections between blood vessels. While relatively uncommon compared to acquired forms of heart disease, congenital defects can cause symptoms early in life and may require specialized treatment.
How Heart Disease Shows Up
One of the most challenging aspects of feline heart disease is that cats are exceptionally good at hiding illness. Many cats show no obvious signs until the disease is advanced. When symptoms do appear, they tend to include increased breathing rate or effort, sometimes with a cat sitting or standing with legs wide apart to make breathing easier. Cats with fluid buildup in the chest may refuse to lie down. Lethargy and loss of appetite are common but easy to overlook as normal aging.
The most dramatic and frightening presentation is sudden paralysis of the hind legs. Cats with heart disease are prone to forming blood clots that break free and lodge in the blood vessels supplying the rear limbs, a condition called feline aortic thromboembolism. This is intensely painful and, for some cats, the very first sign that anything was wrong with their heart. If your cat suddenly can’t move its back legs, is crying out, or the hind paws feel cold to the touch, this is an emergency that requires immediate veterinary care.