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What Every Pet Owner Should Know About Heartworm Disease

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Imagine—your unsuspecting pet is bitten by a pesky mosquito, and becomes infected with microscopic worms that parasitize their heart and lungs. The larval worms grow to foot-long adults, and eventually cause deadly heart failure.

This is not a horror movie plot, but a real-life situation that occurs all too often. Heartworms infect dogs and cats in all 50 U.S. states, and constantly threaten your furry family member.

Now for the good news. Heartworm disease is completely preventable, and you can take steps to keep your pet safe from this devastating disease.

What is heartworm disease in dogs?

Heartworms are parasitic worms transmitted by infected mosquitoes. When a mosquito bites, larval worms enter a dog’s bloodstream and migrate to the heart and nearby blood vessels. Over the next six months, the worms grow to 12-inch-long adults that can reproduce. An individual worm can survive for five to seven years, and a single dog can host up to 250 worms. The growing worm burden eventually impairs heart and lung function, which can be deadly.

What is heartworm disease in cats?

Cats are not the preferred host for heartworms, so the disease follows a different course. Larval heartworms transmitted by a mosquito rarely reach adulthood in cats, and the worms cannot reproduce. However, that does not mean the disease is less dangerous for your feline friend. Immature heartworms—and worms that reach adulthood—can trigger significant inflammation that can damage your cat’s heart and lungs, or cause sudden death.

What are heartworm disease signs in pets?

Early heartworm disease causes few, if any, symptoms, which makes diagnosis tricky. As the disease progresses, you may notice the following signs in your dog:

  • Lethargy

  • Appetite loss

  • Weight loss

  • Coughing

  • Vomiting

  • Tiring easily

The parasite causes a syndrome known as heartworm-associated respiratory disease (HARD) in cats, who may develop the following signs:

  • Coughing

  • Wheezing

  • Asthma-like attacks

  • Appetite loss

  • Vomiting

  • Weight loss

How is heartworm disease diagnosed in pets?

All pets should undergo annual heartworm screening, which is a simple test that requires only a few drops of blood. In dogs, the test detects proteins in the blood produced by adult female worms. In cats, heartworms are more difficult to detect, and the feline test instead detects antigens made by the cat’s immune system in response to the parasite’s presence.

Annual testing is critical for all pets, since only the advanced disease causes signs when significant damage may already be present. If your pet’s heartworm test is positive, your veterinarian may perform additional blood work, X-rays, and diagnostic tests to determine disease severity and assess your pet’s overall health before beginning treatment.

How is heartworm disease treated in pets?

Heartworm positive dogs will be treated with several medications to accomplish the following goals:

  • Kill adult heartworms

  • Kill microfilariae (i.e., circulating immature heartworms)

  • Decrease inflammation

Heartworm treatment in dogs involves multiple injections of the worm-killing medication over several months. During treatment, exercise restriction is critical to prevent dead worms from lodging in your pet’s lung vessels and causing a potentially deadly pulmonary embolism. Without treatment, heartworm disease will progress to heart failure, and eventually cause death.

Medications to kill adult heartworms are not available for cats. Instead, cats must be administered medications that control heartworm-associated inflammation and support the cat while their immune system slowly eliminates the infection, which may take years.

How can I prevent heartworm disease in my pet?

Fortunately, heartworm disease is easily prevented in dogs and cats by administering prevention medications. Preventives come in many forms, including:

  • Monthly oral medications

  • Topical medications

  • Six- or 12-month injectable medications administered by your veterinarian

Every dog and cat should receive year-round preventives to ensure they are fully protected. Mosquitoes are active whenever temperatures reach 50 degrees, which means that infections can easily occur any time of year.

If your pet is due for their annual heartworm test or preventive refill, use this locator to find a veterinarian near you.

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