Blood clot formation in cats is a deadly condition that often strikes cats without warning. Understanding what causes blood clots is key to helping reduce the chances of your cat getting one. Our article today will explain what blood clots in cats are, how they affect a cat, how vets can treat them and how you can help prevent them. 

Table of contents

Normal Blood Clotting

We all know that blood clots, or thrombi, are important following injury – how this normally works is as follows:

Clots form when there is damage to blood vessels, such as if you cut yourself. The damaged blood vessel releases chemicals to attract platelets, and narrows blood vessels to prevent excessive bleeding. These platelets stick to the damaged blood vessel and block the hole. 

Over time a substance called fibrin binds to the platelets to help strengthen the clot. Eventually, the clot starts to break down revealing the repaired blood vessel wall underneath. Blood clots are key to prevent blood loss and prevent infection from entering the bloodstream. 

Abnormal Blood Clotting

However, sometimes blood clots form where we don’t want them to. Small clots generally cause few problems but larger clots can get stuck in important blood vessels. This blocks the flow of blood and causes everything downstream to start to run out of oxygen and nutrients, as well as allow toxic products to build up. Restrict a tissue’s blood supply for too long and it will start to die. For comparison in humans, coronary heart disease is where the heart starts to die as fat build-ups block its blood supply. Luckily for fat cats out there, animals don’t tend to get coronary heart disease.

Aortic Thromboembolism (ATE)

However, what is far more common is a disease called aortic thromboembolism (ATE), also known as a saddle thrombus. This is where a blood clot blocks the descending aorta leading from the heart – the descending aorta supplies blood to the entire back end of the cat. If this supply is suddenly blocked, a cat can become paralysed on its back legs. The leg muscles will be very cold and painful, lack a pulse, and the nails and feet may turn blue. The cat generally will be quite distressed, have a fast breathing rate and may be fully collapsed. ATE is an emergency and the cat must be seen by a vet immediately. In some unfortunate cases, there may be no warning and the cat just suddenly dies.

Pulmonary Thromboembolism (PE)

Clots can also rarely block the blood vessels of the lungs, causing pulmonary thromboembolism (PE). Because the lungs are needed to reoxygenate blood, blocking their supply can cause a cat’s tissues to become deoxygenated body-wide. A cat with a PE may have blue gums, eyes and feet, be slow and collapsed and struggle to breathe. The clot may also cause them to cough up blood as well. PE is also an emergency.

Rarer still, blood clots can block other important vessels, such as those to the brain (causing a stroke), the kidneys (causing kidney disease), the front legs (causing paralysis) or other important organs. 

All of the above blood clots can occur in dogs, but are generally much rarer except in dogs with heart disease.

Causes of Blood Clotting

By far the most common cause of blood clots in cats is heart disease. Most cats with heart disease get enlarged hearts – this causes the blood in the heart to swirl around more. We don’t know why but this promotes blood clot formation. As blood is pushed out the heart, clots dislodge and end up stuck in the descending aorta causing ATE, the lungs causing PE, or elsewhere.

Cats with heart disease often show symptoms such as weight loss, slowing down, difficulties breathing, or a potbelly. These symptoms can be used to diagnose heart disease and help prevent blood clots from forming. However, many cats are only diagnosed with heart disease after presenting as an emergency with blood clots – cats are remarkably good at hiding how sick they are.

Less common causes include those that promote a hypercoagulable state (increased likelihood to form clots), such as an overactive thyroid gland, low protein in the blood, heartworm, pancreatitis, sepsis and immune-mediated haemolytic anaemia. With these conditions, the blood is ‘stickier’, which promotes blood clot formation. As above, the clots can dislodge and get stuck in critical blood vessels, often the descending aorta. Many of these diseases will also have underlying symptoms but may also only be diagnosed after a blood clot emergency. 

How We Diagnose and Treat Blood Clots in Cats

Vets often diagnose cats with blood clots brought in for emergency collapse – ATE is usually fairly obvious based on the cold and painful back legs. PE can be trickier to diagnose as it looks similar to other causes of respiratory distress. If a specific diagnosis is required, detecting no pulse in the limbs or taking a blood sample from affected legs can indicate it is ATE. Often, ultrasound will be used to confirm underlying heart disease, if not already known. 

Usually, there is no time for an extensive diagnosis, and treatment must instead be immediately offered. To stabilise the cat, oxygen and fluid therapy are given. These are important to raise the blood pressure and try and help oxygen reach the rest of the tissues. As blood clots are very painful, strong pain relief is given as well. Underlying heart disease can start to be treated with diuretics to shift excess fluid, and medication to improve the heart’s rate and contractions. Manually moving the paralysed legs can sometimes help restore blood flow, in conjunction with other treatments. 

The clot itself is sometimes broken down with anti-clotting medication. However, these are not always effective and can increase the risk of dangerous haemorrhage. PE may be helped with bronchodilators to widen the airways. Treatment of blood clots takes time, strict cage rest and monitoring, so the cat will have to stay at the vets for a while until the clot has broken down and the cat is stable enough to go home.

The likely outcomes

Unfortunately, even with treatment severe blood clots in cats, especially ATE and PE, do not carry a good prognosis. If the paralysis is severe, a vet may instead advise you to put them to sleep to prevent further suffering. This will also be the case if a cat is still painful despite treatment. Avoiding suffering is the most important thing a vet can do for a cat, and sometimes this may mean having to say goodbye. 

Difficulties walking or even long-term paralysis are possible even with successful treatment. Furthermore, even if one clot is successfully treated, many cats will get clots again. 

Preventing Blood Clots

As we mentioned above, many of the underlying causes of blood clot formation in cats can go unnoticed for months or years. If your cat starts to do anything abnormal, lose weight, slow down or struggle to breathe, we urge you to visit your vet for a checkup. On top of this, it is a good idea for your cat to have a regular checkup anyway, as vets can often spot subtle signs that could indicate disease. The earlier a disease is spotted, the better the prognosis and the more likely it is we can prevent blood clot formation. It is important to treat heart disease in particular to prevent clots.

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