5 Myths and Misunderstandings of Feline Heartworm Disease

1. Dogs Vs. Cats

  • Heartworm larvae induce an intense inflammatory response in the blood vessels and tissues in the lungs.
  • Once an adult worm dies, after 1-2 years, there is an additional intense inflammatory reaction resulting in acute lung injury.

I thought heartworm was mainly a dog’s disease.

Heartworm disease is not just a canine disease. Heartworms affect cats differently than dogs, but the disease they cause is equally serious.

2. But my cat stays indoors, so she is safe, right?

  • It only takes one mosquito to infect a cat, and because mosquitoes can get indoors, both indoor and outdoor cats are at risk and should receive heartworm preventive medication. In a North Carolina study, 28 percent of the cats diagnosed with heartworm were inside-only cats.

The severity of heartworm incidence as shown in this map is based on the average number of cases per reporting clinic. Some remote regions of the United States lack veterinary clinics, therefore we have no reported cases from these areas.

3. How does it affect my cat?

  • The name “heartworm disease” is a misnomer, as the disease mostly affects the lungs and not just the heart. Signs are often mistaken for feline asthma, allergic bronchitis or other respiratory diseases.

In the x-ray, the cats lungs appear congested as a result of heartworm infection.

X-ray image courtesy of Dr. Ray Dillon, Auburn University College of Veterinary Medicine.

4. What if the heartworm larvae never develop into adult worms?

  • Cats do not need an adult heartworm to exhibit clinical signs; in fact, larvae are a main cause of the problems. Studies show 50 percent of cats infected with heartworm larvae have significant disease of the small arteries supplying blood to the lungs.

Effect of Heartworm on Arterioles

arteriolA: An arteriole (small artery) in the lungs of a cat receiving heartworm preventive medication. arteriolB: An arteriole from a cat infected with heartworm larvae, showing thickened walls which can cause complete obstruction.

Images courtesy of Dr. Ray Dillon and Dr. Byron L. Blagburn, Auburn University College of Veterinary Medicine.

5. Is it easy to test whether my cat has heartworm disease?

  • Diagnosis is difficult as negative antigen and antibody tests do not rule out heartworm disease. Positive tests, however, are significant.



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