Many owners believe their cat does not need heartworm protection, and while cats are atypical heartworm hosts, the parasites can still cause your feline friend significant, potentially life-threatening, health complications. To learn the importance of year-round heartworm prevention, read our Just Cats Clinic team’s valuable information about just how deadly feline heartworms can be.

Are indoor cats at risk for heartworms?

Mosquitoes transmit heartworms, and while you may think that outdoor cats have a higher infection risk, indoor living does not guarantee your cat’s immunity. In fact, according to the American Heartworm Society, indoor and outdoor cats have similar infection risk. Consider these feline heartworm transmission factors:

  • Mosquitoes can get indoors — According to the Mosquito Control Association, mosquitoes are extremely adept at entering homes through screens, open doors, attic soffits, and bathroom exhaust vents. In addition, these pests often congregate in garages and near doors, trying to get in your house. 
  • Indoor cats may sneak outdoors — Some indoor cats spend time outdoors on balconies or screened patios, where mosquitoes can target them. In addition, cats are wily, and may sneak out where they have absolutely no mosquito protection.

How is heartworm disease different in cats and dogs?

Dogs are natural heartworm hosts, which means the parasite can grow to adulthood, mate, and produce baby heartworms while living in an infected dog’s heart. However, cats are atypical heartworm hosts, and the disease affects them differently. While dogs experience vascular complications associated with heartworm disease, according to the American Heartworm Society, cats can experience heartworm disease in two stages:

  • Stage one — After infection, immature heartworms migrate to the cat’s pulmonary arteries where many die, causing the acute, severe inflammatory reaction—heartworm-associated respiratory disease (HARD). As the living worms mature, the inflammation lessens, because the cat’s immune system is suppressed.
  • Stage two — When adult heartworms die, the host cat experiences an anaphylactic, often fatal, response. In addition, pulmonary blood clots frequently occur. Cats who survive adult heartworm death have permanent lung damage and chronic respiratory disease.

A cat’s heart is small, and the presence of one or two adult heartworms can block blood flow through the heart, resulting in caval syndrome. Unless your veterinarian surgically removes the worms, your cat will die from this condition. 

What are heartworm disease signs in cats?

Cats typically do not exhibit early stage heartworm disease signs. Unfortunately, an infected cat’s first sign may be sudden death. A cat who has a heartworm infection may exhibit these common signs:

  • Lethargy
  • Decreased appetite and weight loss
  • Increased respiration rate
  • Open-mouthed breathing
  • Difficulty breathing
  • Coughing
  • Vomiting
  • Heart murmur

How is feline heartworm disease diagnosed?

Because cats are atypical hosts, heartworm disease can be difficult to diagnose. Common tests used to detect the parasites in dogs are usually not helpful for diagnosing the condition in cats. To determine whether your cat has a heartworm infection, your veterinarian will likely use these diagnostic tools:

  • Microfilariae testing — Microfilariae are baby heartworms that are commonly seen in heartworm-positive dogs. However, cats rarely have circulating microfilariae, and although a cat has been infected, they often test negative.
  • Antigen blood tests — Detecting the antigens adult female heartworms produce, these diagnostics are typically used in conjunction with microfilariae testing to diagnose heartworm disease in dogs. Heartworms often don’t grow to adulthood in cats, and if they do, only one or two worms are usually present, which may be male. This means that infected cats usually test negative on an antigen test.
  • Antibody blood tests — Antibody tests detect a pet’s immune response to heartworms, and your veterinarian will likely perform this test in conjunction with an antigen test when trying to determine whether your cat has heartworm disease.
  • Chest X-rays — X-rays are often helpful to diagnose heartworm disease in cats. Changes that may be present include enlarged pulmonary vessels, lung inflammation, and lung edema.
  • Echocardiography — Your veterinarian may recommend taking an ultrasound of your cat’s heart to visualize live worms in the pulmonary artery and the heart’s right side.

How is heartworm disease treated in cats?

Medications used to treat dogs’ heartworm disease aren’t safe for cats. In some cases, cats spontaneously clear heartworm infection, and if your cat is not displaying signs, our Just Cats Clinic veterinary team may recommend close monitoring without treatment. However, if your cat needs heartworm treatment, our veterinarians may recommend any of the following strategies:

  • Exercise restriction — Physical activity can exacerbate the damage heartworm parasites cause, and your cat must be strictly confined to prevent exertion.
  • Supportive care — Supportive care, such as intravenous (IV) fluids and oxygen therapy, may be necessary to stabilize your cat’s condition.
  • Heartworm preventives —Preventive medication can decrease feline worm loads for two years or more, but inflammatory reactions are a potential side effect.
  • Steroids — Steroids can help control an infected cat’s strong inflammatory response.
  • Antibiotics — Bacteria (e.g, Wolbachia) often live inside heartworms, increasing inflammation. Antibiotics can help combat this infection.

What is the prognosis for a cat infected with heartworms?

If your cat tests positive for heartworms, your veterinarian will perform antigen and antibody testing, X-rays, and echocardiograms every 6 to 12 months to monitor your feline friend’s heartworm status. A cat is considered recovered from heartworm disease when their serologic tests are negative, clinical signs resolve, and imaging reveals an absence of heartworms. A cat infected with heartworms typically survives less than four years.

How is heartworm disease prevented in cats?

Preventing heartworm disease in cats is easy. To make monthly administration easy, heartworm prevention medications are available in chewable and spot-on (i.e., topical) preparations. Ensure your cat receives year-round, life-long heartworm prevention medications.

Contact our American Animal Hospital Association (AAHA)-accredited Just Cats Clinic team, so we can determine the best heartworm prevention plan for your feline friend.