Maryland Veterinary Medical Association

Heartworm disease is an infestation of a pet’s heart and blood vessels by long, thin worms. The scientific name for the heartworm is Dirofilaria immitis. A single worm can grow up to 14 inches long.

Heartworms cause damage to the arteries of the heart, the lungs, the heart itself and other organs. Left untreated, heartworm disease is usually fatal. Dogs are the most common pets affected by the disease, although it is possible for cats or even ferrets to get heartworm disease as well.

The cycle begins when a mosquito feeds on an infected dog and picks up immature heartworms that are circulating in the dog’s blood. Over the next two to three weeks, these develop into infective larvae inside the mosquito. When the mosquito bites another pet, it injects the larvae into the pet's bloodstream. The larvae grow into adult heartworms within six months.

It only takes one infected mosquito to land on and infect a vulnerable dog or cat, so even if your pet is indoors 100 percent of the time, it is still at risk from a mosquito that comes in an open door or break in a window screen.

Heartworm disease is a threat to dogs and cats nationwide. In Maryland, it is found in every county, especially in marshy areas along rivers and streams. It is particularly heavy on the Eastern Shore, in Southern Maryland and in the counties surrounding Baltimore and Washington, D.C.

Infection rates of unprotected dogs have been reported to be as high as 45 percent in areas up to 150 miles inland from the Atlantic Coast. Even small bodies of standing water, such as hubcaps, empty cans, tires and flower pots can increase the risk of heartworm disease in pets by giving mosquitoes a home.

Absolutely! Prevention is as easy as giving your pet a tasty treat or pill, or applying a liquid to the back of its neck once a month. Before your dog begins a preventative program, however, it must be tested to make sure it does not already have heartworms. Giving an infected dog a preventative pill can cause problems. Once your dog is diagnosed heartworm free, your veterinarian will prescribe one of several heartworm preventatives.

Because heartworm preventatives are strong medications, they can be purchased only from a veterinarian or by prescription.

Signs of heartworm disease in pets include a chronic cough, loss of energy, weight loss and loss of appetite. In cats, vomiting can be an early sign of infection. Most heartworm-infected pets have no symptoms at all until the disease is well advanced and the heart and lungs have already been damaged.

To test for heartworm disease, your veterinarian must collect a small sample of blood from your pet. The blood test can often be completed in your veterinarian’s office, giving results as quickly as a few minutes.

Heartworm disease in cats can also mimic the signs of feline asthma and can have serious effects on the lungs. Sometimes the first sign that a cat is infected with heartworm is sudden death. Diagnosis of heartworm disease in cats is more difficult than in dogs. Your veterinarian may run laboratory tests, x-rays, or an ultrasound of the heart to look for signs of heartworms.

Treatment of early infection in dogs is safer than waiting for the disease to progress, but it is not without risk. When the worms have already done damage to the heart, lungs or other organs, however, treatment is more risky. Regardless of the stage of the disease, there is the potential for serious complications including death.

There is no safe and effective treatment for heartworms in cats. Your veterinarian may recommend using medication to protect the lungs while waiting for the heartworms to die on their own.

Any heartworm infection poses a serious health risk to your pet. Prevention is always a better choice than treatment.