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'Tis the Heartworm Season -- for Your Cat

by Terri Hobbs

Although we normally associate heartworm disease with dogs, cats can and do get heartworms. If you live in an area where there are mosquitoes and a population of dogs with heartworm disease -- like the Southeastern United States, along the Mississippi River and its tributaries, and certain areas of Australia and Japan, for example -- your cat is at risk for contracting heartworm disease.

Heartworm disease is the situation where adult worms (of the species Dirofilaria immitis) have lodged themselves inside the right chambers of the cat's (or dog's) heart. Sometimes the heartworms will be in the large vein that feeds blood into the heart, or occasionally in other parts of the body. A dog can have tens, or even hundreds, of heartworms. Cats normally only have a few, but even one heartworm can be deadly to a cat.

Heartworms can grow to be 4 to 6 inches long, and are skinny and white. They start life, however, as larvae -- tiny creatures that can swim in the bloodstream. The larvae go through several stages before they mature into adult worms. One of these stages must be passed inside of a mosquito in order for the maturation process to complete.

Dogs are normally the host population for heartworm infection. They are the animals in which heartworms live the longest and produce the most larvae. This is because there are more heartworms -- male and female -- inside of dogs, so the worms can reproduce. Because there are so few heartworms inside of cats, the chances of them reproducing and generating larvae is extremely unlikely.

The spread of heartworm disease occurs like this. A dog that has heartworms that have reproduced will have larvae in its bloodstream for a period of time. If a mosquito bites the dog during this time, the mosquito will pick up some of the larvae. The larvae will pass through the next stage of their development in the mosquito. At this point the mosquito can infect another animal. If the mosquito bites a dog or a cat, the larvae get into the animal's skin, continue to develop, and migrate to the bloodstream, where they go to the heart and live as adult worms. This animal now has heartworm disease.

The heartworms can cause allergic reactions, inflammation, and fluid buildup in the lungs. Often, the most common symptoms in cats are coughing and vomiting. Sometimes an infected cat shows no symptoms at all. Unfortunately, the only sign may be sudden death of a cat that appeared normal and healthy just a couple of hours earlier. This severe reaction occurs when a worm dies. But sometimes, the worm or worms will die inside the cat and the cat will recover with no problems. It is impossible to tell in advance how a given cat will react to a heartworm infection.

Because one of the risks of heartworm disease is sudden death, your veterinarian may recommend that your cat be placed on preventative medicine. One such product is Heartgard for Cats ®. This is a pill that is given to the cat once a month beginning one month before mosquito season and continuing until one month after mosquito season. In mild areas, the cat may need to be on preventative medication year-round.

If you have a dog that needs to go on heartworm prevention, you will have noticed that the dog will first be tested for heartworm infection. Cats are not tested beforehand because there are no risks of starting preventative medication if heartworms are already present. In addition, it is much more difficult to detect heartworm infection in cats compared to dogs.

Note that the preventative medication is not a vaccine. It must be given regularly during the time of the year that your veterinarian indicates. When the cat takes the pill, the medication kills any heartworm larvae in the cat's system, preventing the larvae from developing into adult worms. Heartworm disease cannot develop because there are no adult worms. However, if the cat gets bitten again by a mosquito, any new larvae must be killed; therefore, the medication must be given as long as there is a danger of the cat being bitten by a mosquito. If your cat is an indoor cat, don't think your pet is immune. About one third of cats that contract heartworm disease are considered indoor cats.

If you live in an area with a low rate of heartworm disease in dogs, then your cat may not need to go on preventative medication. But the best thing for you to do is discuss heartworm disease and its prevention with your veterinarian. If you do live in a high-risk area, you may save your cat's life with a simple monthly regimen.

About the author:

Terri Hobbs is the webmaster for Crazy for Kitties at, where you can find facts, articles, games, postcards, photos, and a Kitty of the Week. Terri wishes to thank Dr. Susan Oltman, DVM, of the Countryside Veterinary Clinic (410-461-0517) for her helpful advice and information about heartworm disease.

Heartgard for Cats ® is a registered trademark of Merial Limited, Iselin, NJ.

This article may be reproduced if it is reproduced in its entirety, including the About the author section and the registered trademark statement. You can receive this article via email by sending a message with a blank subject line to