Feline Heartworm Disease

Cats can become infested with a parasite called Dirofilaria immitis. This nasty little worm likes to hang around in the heart and the adjacent blood vessels of the lung, leading to heartworm disease. Though most people think of heartworm as being a doggie problem, cats can get this disease as well.

Both dogs and cats get infected when they are bitten by a mosquito carrying tiny immature worms. These enter the body through the bite then go via the circulation to the chest.

Once disease starts in the host, the similarity between dog and cat heartworm ends. Dogs are the natural preferred host, and cats are not! This means that there is a big difference in the way the parasite goes on to develop in the body. It is considered a more serious threat in cats because treatment is generally unrewarding.

In cats, frequently only a few adults (average 2-4) and few if any microfilariae (the baby immature worms) survive, and adult worms have a shorter lifespan (2-3 years) in cats than in dogs. These few adults can wreak plenty of havoc though! This low worm burden makes it harder for vets to pick up an infected cat on the routine heartworm tests.

Successful testing for heartworm in cats often requires an ultrasound examination —sometimes the worms can be seen. X-rays rarely lead to identification of heartworm disease in cats, but sometimes surveys are done to check in on the health of organs and vessels in the chest. It is not unusual to make the diagnosis in cats after death (post mortem).

Frequently, there are no signs the cat has heartworms. The cat may remain normal until serious symptoms occur suddenly, and without warning. Sometimes sudden death occurs. These types of signs are often the result of worm material getting into the lung vessels (embolism) and leading to a severe allergic-type reaction. A common sign of heartworm disease in cats is vomiting. The cat might also be a little less active and develop a picky appetite. Coughing and difficulty breathing also may occur.

Drugs that kill adult worms (adulticides) when used in dogs are fairly safe, though not risk free. In cats, the cure is about as bad as the disease, with no statistical difference in outcome whether or not adulticide treatments are given, so treatment in cats is focused on supportive care.

The chances your cat will get heartworm disease depend a lot on what part of the country you live in. Around the Gulf States , heartworm disease is very common, while in the Northern States, it is less commonly seen. Note that indoor cats get heartworm too! In one study in North Carolina , fully one third of cats testing positive were indoor cats. All it takes is a few skitters coming in through a hole in a screen or open door!

Because cats are not routinely screened for heartworm disease, there are no hard and fast statistics on how many cats have it, but generally, for every 10 heartworm positive dogs in your neighborhood, about 1 cat will be affected. In the geographic hot spots, or in areas where there is an active outbreak, ask your veterinarian about screening your cat, and about protection with heartworm preventive medication. There are three licensed prevention products available; one topical, and the other two are oral medications.