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Pet's Friend Animal Clinic

Heartworm Disease & Prevention

Heartworms are parasites of dogs and other canine species, such as foxes. Cats can also be affected, although they are more resistant to infection. Very rarely, a few cases have also been reported in people. Heartworms are transmitted by mosquitoes. Once limited to the southern regions of the United States, heartworms are now found in most areas of the United States, and are well-recognized in many other regions of the world.

Mosquitoes inject the parasite, Dirofilaria immitis, into the dog or cat when they feed. The larvae mature into thin, adult worms that are several inches long. Adult heartworms live in the arteries of the lungs and heart. By their physical presence, they cause harm in two ways: they block the normal forward flow of blood, causing an excessive workload on the heart, and they also damage the inner lining of the blood vessels, which gives rise to blood clots that cut off circulation to parts of the lungs. Adult heartworms reproduce and release the next generation of immature larval worms, called microfilaria, into the bloodstream. Mosquitoes feeding on an infected dog pick up microfilaria and transmit heartworms to yet more animals.

Cats typically are infected with only a few worms, often only one or two; however one or two worms is a substantial worm burden for such a small animal. Damage to the pulmonary arteries is similar to those in dogs.

Symptoms that could indicate heartworm disease: • Dogs: coughing, exercise intolerance, loss of appetite, swollen belly. • Cats: coughing, vomiting, breathing difficulty.


A simple blood test can determine whether heartworms are present in your cat or dog. Most veterinary clinics can run an Antigen test in the hospital to detect microfilaria in dogs; however, detection of heartworm is more difficult in cats and requires an Antibody test that is usually sent to an outside laboratory. Either way, results are typically available within 24 hours.

Prevention and Treatment:

Unfortunately, the treatment for adult heartworm infection is in VERY limited supply and hard to get. For this reason, prevention is key. Monthly prevention, often in the easy-to-give form of chewable tablets, is recommended for all dogs and for cats in high-risk areas. Since these preventives can also treat the early stages of heartworm in dogs, annual testing is recommended for all dogs.

For more information, please feel free to call Pet's Friend Animal Clinic about any questions or concerns you may have at 408-739-2688.