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Dr Kim Kendall
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 This is a significant issue in the veterinary field. Cats have been getting heartworm for a long time, but until recently it was consigned to the 'too hard basket'. Most pet owners are now aware of the problem of heartworm in dogs, and responsible owners medicate their dogs regularly to prevent the devastating illness that a large heartworm burden causes for the dog, because it is now a very common problem (80% of dogs not on effective medication have heartworm in Sydney).
In summary, heartworms are transmitted by mosquitoes, so anyone can get bitten. The migrating larvae cause low level illness and no one knows how commonly that occurs. The adult heartworm causes trouble wherever it is (heart, lungs, brain, spinal cord etc.). Treatment is unreliable and unpleasant, and one of the symptoms of the disease is sudden death so there is often no opportunity there to treat anyway. It is possible to prevent it, so you need to find out more and make up your own mind.
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How big is the problem in cats?
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Well, if you are a cat with heartworm, it is (a) going to be very difficult to find out about the heartworm (no good reliable tests in cats yet); (b) you are going to have lung problems for a long time, even if the heartworm is cleared; (c) you have a 1 in 5 chance of dying from the heartworm treatment; and (d) you have a 1 in 5 chance of dying if the heartworm is not treated. And literally right there in front of owner and vet, as the reaction to the dying heartworm is so powerful.
So, what to do? Prevention is much better than cure so read on.
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How common is the problem in cats?
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Once again cats are being secretive, and it is hard to tell how common both the early and late stages of the disease are because there are no tests to find out! In Sydney, a survey showed 2% of cats had heartworm in their hearts and lung arteries. Since one of the symptoms of heartworm disease is sudden death, it is a bit hard to find out how many are really affected. The other main symptom is chronic coughing, often put down to Feline Asthma (or ignored).
Dr. Kendall believes that some of the asthmas are triggered by migrating heartworm larvae, so while they may not kill the cat, they do make life a burden. Some of the other symptoms are vomiting intermittently, and sometimes, just weight loss. In fact, the symptoms are a pretty nebulous lot, with no big clues.
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What is heartworm and where does it come from?
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Heartworms are thin, string-like worms that live and reproduce happily in the heart of dogs. When boy meets girl their offspring are microfilariae (little wrigglers) that swim around in the dog's blood. A mosquito feeds on the blood, gets a microfilaria as part of the dinner, and then injects it back into the next animal it feeds on. If this happens to be your cat, the microfilaria changes character to become a larva and then migrates from the skin to any number of places (heart, lungs, brain, spinal cord, kidney) and causes trouble as it grows.
Fortunately, the cat rejects most of the larvae, but the intense allergic reaction that this sometimes takes may cause problems in itself (coughing being the main one, as most of the larvae do make their way to the lungs). If the larva ends up in the heart, then as it matures into a full-size heartworm and lives there a while, it will cause damage in the tissue around it, and when it dies, it may kill the cat from the intense allergic reaction it induces (1 in 5 cats die, even if the vet is there with all the equipment and drugs necessary).
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What is the difference between heartworm in dogs & cats?
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Heartworm is a parasite that has evolved to live in dogs’ hearts, and a dog can quite often have a few heartworms without any problems, and these are picked up on the routine testing carried out on most dogs at various times. Even small dogs can have a few worms in their heart without showing signs of disease.
However, even one worm can cause a problem in a cat (and interestingly also in ferrets), not because of the comparative size of the worm to the heart, but because it causes a strong allergic reaction in the lining of the cat’s heart and arteries, and that is what causes the symptoms, and also the death of the cat (it is not simply that the worm causes an embolism or blockage of the blood vessels in the lungs when it dies, it is the cat's own over-dramatic response which is life threatening, and known as anaphylactic shock).
Also, the low numbers of worms, and the small chance of having a male and female getting it together in a cat's heart, mean that it is rare to have the microfilariae in the blood to pick up on testing. Tests for the worms themselves are also somewhat unreliable as they rely on there being a female in residence to make some of the tests work and others rely on the cat’s reaction to the worm. It is all very difficult, but testing procedures are improving. For the moment the most reliable way to pick up heartworm problems in cats is by X-ray, looking for the allergic reaction in the blood vessels and followed by ultrasound finding the worm outlines in the heart.
Interestingly, humans also get heartworm, but it is usually an incidental finding during open heart surgery. It would be a bit of a surprise for the surgeon though!
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Who is at risk?
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The mosquito is the bearer of infection, so it is one of the few risks that an indoor cat has, however there is a risk of being a “mosquito meal” anywhere. The main risk is living in an area where there are lots of mosquitoes and lots of dogs on ineffective preventative medication. As even one drop of an infected dog's blood can carry a couple of microfilariae, a large dog with a large number of heartworms can act as a source of infection for lots of mosquitoes. So the answer is, any cat, anywhere that heartworm occurs in dogs is at risk.
With your cat, you have a choice about how many things you take a chance on. If you choose not to take a chance on the distress that heartworm causes, then you should speak to your vet about prevention, and make an informed choice, yes or no. The answer to the question “how common is the problem” will be answered mainly by seeing how many fewer coughing cats are seen by vets over the next couple of years as more people put their cats on heartworm prevention and make sure the larvae go nowhere.
Cats are not becoming harder or more expensive to look after, the change is that cat owners are being given more choice in their cat's health stakes and as we expect to live longer, so too we're expecting our cat companion to see us through more “life stages”!
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Author: Dr Kim Kendall BVSc MACVSc (Feline Medicine) graduated from the University of Sydney in 1982 and established the East Chatswood Cat Clinic in 1994, a dedicated cat-only Veterinary Clinic. The practice does everything for cats and is a full service veterinary clinic.
www.catclinic.com.au
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