Heartworm disease is rare in cats but can be fatal. The disease can be prevented through oral and topical products available by prescription from a veterinarian.

Heartworm disease, also known as dirofilariasis, occurs when a parasitic worm (dirofilaria immitis) makes its way into a cat's system. The disease is serious and causes heart failure, severe lung disease, organ failure, and death. The risk of complications from heartworm disease depends on the number of worms that are in the cat's body, the cat's response to the worms, and the period of time the worms remain inside of the cat.

Because symptoms of heartworm disease are similar to those of other diseases, it's important to visit the veterinarian for a proper diagnosis. The symptoms are most noticeable when the immature heartworms arrive in the arteries of the heart and lungs (approximately three to six months after infection) and when the mature worms die and release toxins into the cat's body. These symptoms include:

Heartworm disease is caused by the bite of a mosquito that has bitten another animal infected with the parasite. Because of this, outdoor cats are twice as likely to contract the parasite as indoor cats. Once the cat's skin has been pierced, the heartworm larvae are able to enter the cat's bloodstream. From here, the larvae make their way into the cat's blood vessels in the heart and lungs where they mature and reproduce. The cat's body reacts to the parasite with a severe inflammatory response known as heartworm associated respiratory disease (HARD).

The veterinarian will need to know the cat's health history, if any mosquito bites have occurred within the last year, when symptoms first began and a detailed list of all of the symptoms. Because there is no specific test to diagnose heartworm disease, the veterinarian will run several tests to rule out other respiratory or heart conditions that could be causing the symptoms. A complete blood count, biochemical profile, urinalysis and a physical examination will be performed to help rule these other conditions.

An antigen and antibody test may also be done on the cat. These tests will look for the presence of foreign bodies and proteins that the immune system creates to fight these foreign bodies. An X-ray will be taken to determine if they veins and arteries that are typically associated with heartworm disease are enlarged. An echocardiogram (ECG), also known as a heart ultrasound, will be done. The echocardiogram can be used to look for the presence of the worms in the lungs or heart and rule out other cardiac conditions that can cause the same symptoms.

Because heartworms in cats have a shorter lifespan than in other animals, the condition can sometimes remedy itself when all of the worms die without reproducing. The veterinarian may opt to monitor the cat to see if the disease cures itself spontaneously. During this time, the cat will need to have frequent X-rays and echocardiograms to monitor the growth of the worms and their effect on the heart and lungs.

Cats who are severely dehydrated as a result of vomiting and diarrhea may need to be hospitalized to receive intravenous fluids to prevent organ failure from occurring.

If the cat isn't able to get enough oxygen on its own as a result of the lung and heart complications, the cat may need to receive oxygen in the hospital through a nasal cannula or face mask.

Though there are no approved medications to kill the adult heartworms that reside in the cat's body, there are medications available to help control the symptoms. Corticosteroids, such as prednisone, may be prescribed in gradually increasing doses to help treat the lung symptoms. Heart medications may also be prescribed to help the heart function at its best. If an infection occurs as a result of the complications, antibiotics will be prescribed in order to clear the associated infection.

Mature heartworms may need to be surgically removed from the heart and lungs. Surgery is risky and not always effective, because if any of the heartworms are missed or not removed completely, they can grow and reproduce. 

The cat will need to follow-up with the veterinarian regularly after treatment in order to ensure that no heartworms are continuing to grow. The cat will be given medication to prevent the disease from occurring again in the future. If the lungs or heart were permanently damaged as a result of the parasite, these conditions will need to be treated and monitored.