Body

Heartworm Disease

by Howard A. Mintzer DVM

Heartworm disease is an infection of the lungs, major blood vessels and heart caused by the parasite Dirofilaria immitis.

How Can My Cat Get Heartworm Disease?

The parasite is spread by the bite of an infected mosquito. The mosquito drinks blood from another animal (most commonly a dog, coyote, bear, raccoon or other animal that harbors the parasite) and picks up baby heartworms with its meal. While feeding on your pet the mosquito injects baby heartworms (called microfilaria) under your cat's skin. The microfilaria burrow into the blood vessels and reach the lungs and heart where they mature.

How Do the Heartworms Cause Disease?

Most of the heartworms in cats live in the lungs, not the heart. The lungs are severely irritated by these parasites and so they react to the infestation in a similar manner that the body will react to any irritation. At first the lungs swell. This swelling causes the bronchi (the breathing tubes) to be squeezed. The walls of the bronchi also swell and make excess mucous. This causes the cat to cough and wheeze. For an unknown reason many cats also vomit. As the swelling continues some cats will make blood clots called emboli which clog up blood vessels in the lungs and sometimes the heart. If a small blood vessel is clogged, the tiny piece of lung that the blood vessel supplies with food may die, causing a cough, collapsed lung lobe, lung abscess and other problems. If a large blood vessel becomes clogged with a clot, the cat may die suddenly. The only symptom of heartworm disease exhibited by 50% of the cats who have this disease is sudden death. A cat my be perfectly fine when examined and be dead an hour later from heartworm disease!

How Is Heartworm Disease Diagnosed?

Heartworms have adapted to the dog's heart and lungs where they grow, mature and produce babies. The situation is quite different in the cat. The cat's lungs and heart are not a good place for heartwoms to grow. In a cat, the heartwoms rarely grow past the larval stage, and almost never produce babies. Therefore the tests that we perform on dogs which detect either baby or adult worms don't work with cats. In the past 2 years accurate blood tests that reveal the presence of immature worms in the cat's body have become available. We make a diagnosis of heartworm disease in cats after a thorough physical exam, evaluation of a chest x-ray, and the results of certain blood tests including on for antibodies directed against heartworm larvae.

How Will My Cat Be Treated For Heartworm Disease?

In dogs large numbers of adult heartworms grow in the heart and prevent the heart from working correctly. This causes heart failure if the worms are allowed to stay where they are. Therefore treatment of heartworm disease in dogs is aimed at killing the adult heartworms. In cats as noted above, adult heartworms are very rare. The majority of illness caused by heartworm disease is caused by the lungs' inflammatory reaction to the larval heartworms. Therefore in cats we leave the adults alone, if there are any and direct our therapy at the inflamed lungs. The main therapy consists of anti inflammatory drugs to reduce the swelling of the lungs, anti clotting drugs to prevent emboli (blood clots) and antibiotics to treat any infections which have started in the weakened lungs. Any adult worms which are present are usually not part of the problem and will die on their own in about 18 months. Meanwhile to prevent further infection, your cat will be started on heartworm preventative.

How Come All of a Sudden You are Telling Us Cats Can Get Heartworm?

Veterinarians have been missing the diagnosis of heartworm disease in cats for years. We long believed that the cat disease was rare, and if seen would look like the dog disease, i.e. progressive cough leading to heart disease, wasting away, followed by death from heart failure. Recently we've learned that only the very rare case of cat heartworm disease looks like this. Because of new testing procedures we have found that feline heartworm disease is much more common than we ever thought. The majority of cases were diagnosed as chronic bronchitis or asthma. In fact if you have a cat who was diagnosed with asthma or chronic bronchitis in the past and was well controlled with cortisone, he may have had heartworm disease. Asthma, chronic bronchitis and heartworm infection cause inflammation of the lungs and all are treated with cortisone. Therefore even if your heartworm infected cat was misdiagnosed as having asthma, he was treated with the appropriate drug for heartworm disease and probably recovered.

Why Does My Indoor Cat Need to be on Heartworm Preventative?

Depending on the study, 30-50% of heartworm cases in cats occur in indoor cats. This tells us that either most indoor cats sneak out from time to time or we get more mosquitoes in our houses than we think.

How Can I Prevent Heartworm Infestation in My Cat?

There is one approved drug, Heartguard for Cats, available as a chewable pill that when given once a month will prevent heartworm infection in your cat. Unlike dogs your cat need not be tested for heartworm disease to get this drug. That is because this drug can react very badly with baby heartworms (microfilaria). Since cat heartworms only very rarely produce microfilaria, and when they do produce them, they produce very few microfilaria, it is safe to give your cat heartworm preventative without testing him for the presence of microfilaria.

What is Canine Heartworm Disease?

Heartworm disease is an infection of the lungs, major blood vessels and heart caused by the parasite Dirofilaria immitis.

How Can My Dog Get Heartworm Disease?

The parasite is spread by the bite of an infected mosquito. The mosquito drinks blood from another animal (most commonly a dog, coyote, bear, raccoon or other animal that harbors the parasite) and picks up baby heartworms with its meal. While feeding on your pet the mosquito injects heartworm larva under your dog's skin. The larva burrow into the blood vessels and reach the lungs and heart where they mature.

How Do the Heartworms Cause Disease?

The heartworm larva migrate from the site of the mosquito bite through the small blood vessels until the reach the lungs and heart. Here they mature and within 6 months begin to have babies called microfilaria. The adult heartworm block the blood flow through the heart, lungs and liver. These organs become damaged resulting in liver failure, lung disease and most important heart failure. Untreated most dogs die of heartworm disease within two years of becoming infected.

How Is Heartworm Disease Diagnosed?

Heartworm in dogs is easily diagnosed by any number of excellent blood tests. The severity of the disease is determined by a combination of organ function tests, Ekg and x-rays of the lungs and heart.

How Can I Prevent Heartworm Infestation in My Dog?

Keeping your pet free of heartworm infection couldn't be easier. All you need to do is give a chewable tablet once a month. For your convenience we carry Heartguard plus Chewable heartworm preventative for your dog. Not only will it prevent heartworm infection but it will also help prevent your dog from acquiring roundworm and hookworm infections.