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Feline Heartworm Disease Fact Sheet

  • Feline heartworm disease, caused by the parasite Dirofilaria immitis, was first reported in 1921 in Brazil.
  • Studies indicate that less than 5 percent of U.S. households with cats regularly administer heartworm prevention. In contrast, 59 percent of dog-owning households regularly use a heartworm prevention product.
  • Several of the monthly preventives have activity against zoonotic intestinal parasites, which infect 3 to 6 million people every year.
  • Cats typically have fewer worms than dogs, and the life span of the worm is shorter in cats, but the consequences can be much more serious.
  • Heartworms do not need to develop into adults to cause significant pulmonary damage in cats.
  • A cat contracts heartworm disease when a mosquito carrying microscopic-size heartworm larvae bites a cat. The larvae enter through the bite wound where they develop in the tissues.
  • The immature worms then enter a blood vessel and are carried to the arteries in the lungs where they cause an inflammatory reaction. Most worms die at this stage, causing even more inflammation.
  • The worms that progress to the adult stage may live undetected for a couple of years. But, when the adult worms die, the inflammation can be severe enough to cause death.
  • The respiratory signs associated with these reactions are called Heartworm Associated Respiratory Disease (H.A.R.D.).
  • Chronic signs of feline heartworm disease include difficulty breathing, coughing or gagging, heavy or fast breathing and vomiting.
  • More acute signs can be anorexia or weight loss, lethargy, seizures, fainting and loss of coordination.
  • Many cats with heartworm infection may exhibit no signs of disease.

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