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Heartworm in cats

In some parts of the world, such as the United Kingdom, heartworm infection is not something that those with cats have to think about. In other places the risk is severe, and warnings about heartworm are displayed prominently in veterinary clinics. Though it was once thought that heartworm mostly affected dogs, this opinion has been proven very wrong. Heartworm infection is common in cats, and it can be a killer; but the prevalence of the infection depends on where your cat lives.

What is heartworm? As the name suggests, it is a worm. More precisely, it is a roundworm, a thread-like parasite which is spread by mosquitoes. It is called a 'heartworm' because as as the name says, the worms infest the heart. That is, an adult worm makes its home in the right ventricle of an infected animal. There, it can live for many years.

Because mosquitoes spread the parasite, areas where mosquitoes are most common tend also be the areas where the risk of heartworm infection is higher. However it is safe to say that there are very few countries in the world which are risk-free, so if you have any doubts, it is best to consult a vet as soon as possible.

There is a good reason why heartworm infection was originally believed to be a problem which affected dogs. Dogs suffer severely when infected and can have heart failure, lung problems or both. This is because, when infected, dogs end up with large parasite populations. However, when a cat picks up the infection, it normally ends up with just a single worm in the ventricle (though this is bad enough).

Heartworm is a serious condition in cats, which can be life-threatening. However, feline bodies can resist the infection better than dogs are able to, though (as will be seen below), this immune reaction has its own serious issues.

Disease spread and symptoms

The heartworm parasite starts as a larva in mosquitoes. The creature spends about 14 days developing in its host and then is ready to move on to a larger animal - for example a cat. When a heartworm-carrying mosquito bites a cat, the larva is passed into the cat's bloodstream while the mosquito is sucking blood out. Once in the blood, the larva travels into cat's lungs where it will slowly mature. It can take up to 8 months before the larva has matured to its final stage. It is now a full grown worm, and in its case, home is literally where the heart is. The worm is usually about 6 inches (about 15 cm) long, but some worms can be up to three times as long.

Unlike dogs, the feline immune system responds strongly to a heartworm larva in the lung. The cat's body fights hard to destroy the intruder, and in the fight, the battleground - the lungs - suffer collateral and often irreparable damage. Infection in a cat frequently leads to a chronic respiratory condition known as Heartworm Associated Respiratory Disease (H.A.R.D).

The onset of the disease can be severe and rapid. A cat with H.A.R.D shows signs like asthma or chronic bronchitis. The cat has difficulty in breathing, it coughs often and secretes mucus. It becomes lethargic, and will often neglect its grooming.


Diagnosing a heartworm infection is difficult in cats because they fight the parasites so well. Therefore there are not many heartworms for a test to detect. For example, cats can test negative for adult heartworm, but still be carrying a larval infection in their lungs and suffering from H.A.R.D.

The main test for the heartworm infection in cats is something called an antibody/antigen test. The antibody test does not look directly for the heartworm, but to see whether the cat's body has already detected the parasite. If it has, there are already antibodies to the larva in the cats blood, so the test looks for these antibodies. Unfortnately, this test can often be difficult to interpret. Also, the test works best while the parasite is at the larval stage, and is less useful for finding mature worms. Because of this problem, a test might report that a cat is clear of worms while there is still an infection.

Another test for heartworms is to look for them directly using an ultrasound machine. Again this is more useful in dogs because the heart may have a large number of worms which are easier to see. But it is easy to miss a single thin, thread-like worm in a cat's heart. A further cause for gloom is that even a definite diagnosis is not as helpful as it might be. There is no approved treatment for a safe removal of adult heartworm in cats. With severe H.A.R.D., steroids and a low dose of aspirin can be used to reduce the inflammation or allergic reaction, but there is little more that can be done without causing even more damage than the parasite is already doing.

Prevention and treatment

Therefore heartworm is definitely a case where prevention is better than cure. There are two forms of prevention. Oral treatment comes in the form of chewable tablets which contain Ivermectin. Ivermectin is very effective in preventing heartworm infections, and if a cat can be persuaded to chew the tablet - and some cats positively enjoy doing so - then your pet should be well protected.

The second type of prevention is a 'spot on' treatment, which is applied monthly on the cat's skin. One extra benefit of the 'spot on' treatment is that it not only protects a cat from heartworm but also from other infections, for example, fleas, ear mites and intestinal parasites.

Given the problems that heartworm can cause, if you live in an area where there is even a slight risk of infection, not protecting your cat should not be an option. Consult your vet.

Note: This information is for guidance only. It is not intended to replace consultation with a licensed practitioner.


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