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Why is My Cat Coughing: Heartworm Associated Respiratory Disease (HARD)

Reviewed by Missy Beall, DVM, PhD on Friday, July 24, 2015
Posted July 24, 2015 in Cat Diseases & Conditions A-Z

Cat coughingYears ago, veterinarians thought that cats developed heartworm disease far less frequently than dogs did. That misconception was due in large part to two factors. First, heartworms produce a very different disease in cats than in dogs. Second, testing for heartworm disease in cats is performed differently than in dogs. For instance:

  • In dogs, according to the American Heartworm Society, at least 60% of infective larvae develop into adult heartworms causing disease in the heart and lungs. In cats, very few infective larvae develop into adult heartworms (<10%), and as these immature parasites die, a severe inflammatory disease develops in the lungs.
  • Because dogs typically have multiple adult heartworms, antigen tests are used to identify the presence of infection. Cats, on the other hand, can have lung disease even without any adult worms so it is important to use both antibody tests and antigen tests to determine if a cat’s clinical signs are due to heartworm disease.
  • Although cats don’t develop adult heartworm infections at the same rate as dogs, according to CAPC, studies suggest that cats are infected with immature stages of heartworms at an infection rate similar to that of dogs.

Today, veterinarians know that just like the adult worms, very young worms can also cause a severe and sometimes acute, life-threatening disease in cats called Heartworm Associated Respiratory Disease (HARD).

What is HARD?
HARD describes a lung disease caused by heartworms in cats. Yes, you read that correctly— lung disease. Adult worms, if present, tend to reside in the blood vessels of the infected cat’s lungs.  Additionally, developing immature worms can set off a severe, inflammatory response in the smaller lung blood vessels, in the airways and in the lung tissue itself1.

What are the symptoms of HARD?
Unfortunately the symptoms of HARD can be vague and varied. The American Association of Feline Practitioners says they may include:

That means that the signs of a cat with HARD can be confused with other illnesses from primary gastrointestinal disease to respiratory disease due to bronchitis or asthma.

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Mike has more than 35 years of experience in companion animal veterinary practice and is a valued member of IDEXX’s Pet Health Network team since 2013.