Heartworm Disease in Dogs and Cats


Fido is eating less, he’s tired all the time, and he doesn’t have enough energy for exercise. He’s been having difficulty breathing. You hear abnormal sounds from his lungs, and he has a deep, soft cough. He may be losing weight and vomiting. Fluffy may also be having these problems — or you may see no signs of the condition that could kill her.

You know something is wrong, but what? And what can you do about it?

Introduction to the heartworm

Heartworms are believed to infect over 30 species of animals, including dogs and (less often) cats and ferrets. Animals infected with heartworm disease have been found in all 50 US states and on every continent except Antarctica. In the United States, heartworms are particularly prevalent near and east of the Mississippi River.

They start out as larvae from mosquitoes. When an infected mosquito bites an animal, the larvae enter the bloodstream and work their way to the heart. Although the larvae don’t become mature for several months, they can reproduce before they reach maturity. Female heartworms can produce thousands of offspring every day.

Heartworms and their larvae live in the heart, lungs, bloodstream, and veins around the liver and heart. Male heartworms can reach a length of up to six inches, and females may grow to be twice as long. They can live for about two years in cats and about five years in dogs.

These spaghetti-thin parasites remove nutrients from the body, fill the heart, and block blood flow to the lungs. By the time you see signs of heartworm disease, the heartworms have already done some damage to the heart. Eventually they can kill.

Cats may have only a few heartworms compared to the dozens or more that may live in dogs, but even one heartworm can be lethal. Some cats exhibit no signs of heartworm disease at all until they suddenly die.

Heartworm disease diagnosis

Heartworm cannot be diagnosed by clinical signs alone; the signs are similar to those of other conditions.

Diagnosing heartworm disease in dogs

Two blood tests are commonly used to identify heartworm disease in dogs:

  • The microfilaria test detects microfilaria (heartworms in the prelarval stage) in the blood.
  • The heartworm antigen test looks for the presence of antigens that female worms producing microfilaria give off.

An animal may have just microfilaria or just adult heartworms, so both tests are useful. The antigen test may produce false negative results if the animal has only a few female heartworms. On rare occasions it produces false positive results, especially for dogs that have been treated for heartworm disease.

The antigen test is more expensive than the microfilaria test, so often only the microfilaria test is done at first. Adult heartworms in the heart and lungs may also appear on X-rays.

Annual blood tests help diagnose heartworm conditions in the early stages.

Diagnosing heartworm disease in cats

Cats generally have a lot fewer heartworms than dogs do, which makes it more difficult to diagnose feline heartworm disease. Tests are not 100 percent accurate. For cats, an antibody blood test is more useful than the tests used for dogs. A negative antibody test result means the cat is probably not infected, but the result may be positive long after a cat has had heartworms.

If a cat has symptoms of heartworms, chest radiographs can help locate any worms, but this diagnostic procedure isn’t always accurate either.

Heartworm disease treatment for dogs

First, the veterinarian will assess and treat any conditions resulting from heartworm disease, such as heart, kidney, or liver problems. Once the dog is in good enough health to tolerate heartworm treatment, he is given a drug to kill the adult worms.

The drugs used for heartworm disease have side effects, and there are risk. Dogs undergoing heartworm treatment may have to spend the first few days in the veterinary clinic. For the next few weeks, dogs must be kept confined and in a quiet place to reduce the risks of heartworms forming a blood clot or of side effects from the drugs.

Because of the toxic side effects of heartworm treatment, older dogs that have tested positive for heartworms are sometimes given heartworm prevention medication instead. With this treatment, dogs don’t acquire new heartworms, and the adult ones eventually die off. The risk of heart and lung damage from the adult heartworms is weighed against the risk of more aggressive treatment.

Heartworm disease treatment for cats

There are no approved medications in the US to treat feline heartworm disease. Heartworm medication used to kill heartworms in dogs may cause severe reactions and death with cats.

Treatment with intravenous fluids as well as prednisone and other drugs can help support an infected cat’s health. While there’s a risk that the cat will die suddenly from the infection, the heartworms may also die off on their own.

Heartworm disease prevention

Your veterinarian can prescribe a monthly heartworm prevention drug for your dog or cat. Heartworm prevention drugs may have side effects, and some animals have adverse reactions to these drugs. Heartworm disease is a serious condition, though, and once contracted, the treatment carries more risks than the prevention drugs do. The usefulness of heartworm prevention medication needs to be weighed against the risks of contracting heartworm disease in your area.