The most common cause of blood clots in cats is heart disease, specifically a condition called hypertrophic cardiomyopathy (HCM), which accounts for the vast majority of cases. About 25% of cats with HCM will eventually develop a blood clot. The clot typically forms inside an enlarged chamber of the heart, breaks free, and lodges in a blood vessel, most often at the point where the aorta splits to supply the hind legs. This is sometimes called a “saddle thrombus,” and it’s one of the most dramatic emergencies in feline medicine.
How Heart Disease Creates Clots
In HCM, the muscular walls of the heart’s main pumping chamber thicken abnormally. This stiffness makes it harder for the chamber to fill with blood, which causes blood to back up into the left atrium, the smaller chamber just upstream. Over time, the left atrium stretches and enlarges. A healthy cat’s left atrium has a maximum diameter of around 16 mm; when that measurement exceeds 25 mm, the risk of clot formation rises significantly.
The enlarged atrium can’t contract as efficiently, so blood pools and moves sluggishly through it. On an echocardiogram, vets can sometimes see what’s called “smoke,” a swirling haze in the heart chamber caused by blood cells clumping together in the slow-moving flow. That smoke is an early warning sign. If conditions don’t change, those clumps can solidify into a true clot, usually forming in a small pouch called the left atrial appendage. The clot may then eject into the bloodstream and travel until it gets stuck in a vessel too narrow to pass through.
Three Factors That Drive Clot Formation
Clot formation in cats follows the same three-part framework that applies across species. Any combination of sluggish blood flow, damage to the blood vessel lining, and a tendency for blood to clot too easily can trigger the process. In cats with heart disease, all three often occur at once.
Sluggish flow is the most obvious contributor. When the left atrium enlarges and weakens, blood sits in place long enough for clotting proteins and blood cells to aggregate. Studies using cardiac imaging have shown that cats who develop clots have noticeably lower blood flow speeds in the left atrial appendage compared to cats who don’t.
Damage to the inner lining of the heart also plays a role. Autopsies of cats with HCM have revealed injury to the tissue lining the heart chambers. When that lining is damaged, it stops releasing molecules that normally prevent clotting and instead releases compounds that promote it, including a sticky protein called von Willebrand factor that helps platelets latch on and form a plug. Cats with blood clots have elevated levels of this protein in their blood, and it’s been found in abundance within the clots themselves.
The blood itself also changes. Cats with HCM show signs that their platelets are “primed,” meaning more reactive and quicker to clump together than normal. The more severe the heart disease, the more activated the platelets tend to be. On top of that, certain immune cells release web-like structures originally designed to trap bacteria. These structures are potently pro-clotting and have been found woven into the fabric of clots removed from cats.
Non-Cardiac Causes
Heart disease is by far the leading cause, but it isn’t the only one. Hyperthyroidism, which is common in older cats, can alter the clotting system independently of any heart changes. Hyperthyroid cats have higher levels of fibrinogen (a key clotting protein) and von Willebrand factor, suggesting their blood vessel linings may be dysfunctional. While researchers haven’t fully mapped the mechanism, the changes resemble those seen in hyperthyroid humans, who also face elevated clot risk. These clotting abnormalities tend to normalize after the thyroid condition is treated.
Cancer can also shift the balance toward clot formation, as tumors release substances that activate the clotting cascade. Severe infections and inflammatory conditions may do the same. In some cats, a clot is the first visible sign of an underlying disease that hadn’t been detected yet. Only about 15% of cats in one major study had a known history of heart disease before their clot event.
What a Blood Clot Looks Like in a Cat
A clot that lodges at the base of the aorta, cutting off blood supply to the hind legs, produces a sudden and unmistakable set of symptoms. The onset is acute. Cats typically cry out in pain, and one or both hind legs become paralyzed. The affected legs feel cold to the touch because warm blood is no longer reaching them. The paw pads may look pale or bluish instead of their normal pink. You won’t be able to feel a pulse in the affected legs. Some cats also breathe rapidly or with visible effort, especially if the underlying heart disease has caused fluid to build up in or around the lungs.
Not every clot goes to the hind legs. Clots can lodge in vessels supplying a single front leg, the kidneys, the brain, or the intestines, though these locations are less common. The symptoms depend entirely on where blood flow gets cut off.
Which Cats Are Most at Risk
Male cats are disproportionately affected. In one large clinical trial, 76% of cats who experienced a clot were male. The average age at the time of the event was around 8 years. Mixed-breed cats make up the majority of cases simply because they’re the most common, but certain purebred lines carry higher HCM risk, including Maine Coons, Ragdolls, Persians, Bengals, Sphynx, and Abyssinians.
Specific echocardiographic findings help vets gauge risk in cats already diagnosed with heart disease. The most significant red flags include a visibly enlarged left atrium, reduced left atrial contraction, the presence of “smoke” on echocardiogram, and any history of a previous clot event. Cats with one or more of these findings are candidates for preventive medication.
Prevention With Medication
For cats at risk, preventive treatment centers on medications that reduce the blood’s ability to form clots. The most widely studied drug is clopidogrel, an antiplatelet medication. A landmark randomized trial (known as the FAT CAT trial) compared clopidogrel to aspirin in cats who had already survived a clot. Cats on clopidogrel went a median of 443 days before a recurrence, compared to 192 days for cats on aspirin. Clopidogrel also significantly reduced the combined risk of another clot or cardiac death. Both drugs were well tolerated.
More recent research has explored combining clopidogrel with a newer anticoagulant called rivaroxaban. In one study, none of the 14 cats on this combination experienced a clot, and they had a median survival time of 257 days. Another protocol using a different injectable anticoagulant showed only a 4% clot rate. These combination approaches are increasingly used in high-risk cats, though the choice of protocol depends on the individual cat’s condition and the vet’s assessment.
Prevention is only possible when heart disease is detected before a clot occurs. Because HCM often causes no obvious symptoms in its early stages, it can go undiagnosed for years. Routine veterinary exams that include listening for heart murmurs or abnormal rhythms, along with echocardiography when indicated, offer the best chance of catching the disease before a clot forms.