Heartworm disease is caused by Dirofilarie immitus. Heartworm is a newer disease to cats, but a nasty disease none-the-less. Heartworm disease is usually contracted through a mosquito bite. Thus cats in hot, humid areas of the country where there are lots of mosquitos are at the highest risk.

So how does a cat get heartworm? When a cat has heartworm the worms inside the cat’s body produce ‘babies’ called microfilarias. The microfilarias then circulate through the animal’s bloodstream and thus can be picked up by a mosquito feeding on that cat’s blood. Upon biting another cat that same mosquito can then transmit the microfilaria to a new cat. These heartworms then travel to the heart and pulmonary vessels where they then grow, mature, and live. It used to be thought that only outdoor cats could contract heartworm. However, as mosquitos can be found inside and outside of the home, we now know this not to be the case. Some studies also show that cats can acquire immunity to heartworm through successive mosquitos bites. Indoor cats, as they come in contact with mosquitos less often, usually have no such immunity. There currently is no treatment or cure for heartworm so the best thing you can do for your cat is prevention. There are many new medications available for heartworm. Talk to your vet about this if you think your pet may be as risk.

Symptoms to look for:

  • Difficulty breathing
  • Coughing
  • Vomiting
  • Weight loss
  • Lethargy
  • Anemia
  • Collapse
  • Seizure
  • Blindness


If you suspect a heartworm you should take your cat to the vet asap. Although diagnosis is difficult your vet can perform antigen tests, x-rays, ultrasounds and/or angiograms to check for the presence of a heartworm.

Currently, however, there is no known cure for heartworm. This is because when heartworms die they must pass through the pulmonary arteries which introduces the possibility of arterial blockage and death. So the best course of action to date is prevention. If your cat does have heartworm the best thing to do is to treat the symptoms and to hope that the cat outlives the worm as killing the heartworm is very dangerous to the feline.

Precautionary measures:

There are heartworm prevention medications available; one medication available from Merck is Heartgard (Rx)®. Cats who already have heartworm can be given this medication to prevent introduction of new worms into the cardiovascular system. And kittens can be started on Heartgard® as early as 8 weeks of age. If your cat is in an area which makes the cat susceptible to infection he/she probably should take a preventative medication. Any cat, however, can take a heartworm medication just as a safety measure against heartworm.