Feline Heartworm Disease

symptoms

treatment

testing

prevention

Heartworms and heartworm disease in cats

Because cats are not a natural host, heartworm larvae usually die before reaching adulthood and this natural "immunity" was thought to protect cats.  We know now that this isn’t true.

A single mosquito can inject dozens of larvae into the body. When these larvae reach the lungs, most of them are killed, but the inflammatory immune reaction that kills the worms also damages the lungs. In cats, rather than being a cardiac problem, heartworm disease is mostly a respiratory problem. It’s not less serious than in dogs, it’s just different.

symptoms

Usually there aren't any symptoms at all. When we do see symptoms, they mimic other more common problems. 

• wheezing, choking
• vomiting and gagging
• fainting spells
• hindquarter paralysis
• sudden unexplained death

treatment

We do not have a satisfactory drug for heartworm treatment in cats.   If we did, using it would be risky because cats do not deal well with dead heartworms.  They may absorb the dead worms successfully; or they may die from arterial obstruction or allergic reaction to the dead worms.  So, if we could kill feline heartworms, it is not clear that we ought to. As it stands now, the best treatment for feline heartworm disease is to start on prevention (so they don't get more) and try to control the symptoms, if there are any.  Eventually the heartworms will die on their own, one at a time, which gives the cat its best chance of dealing with them successfully. 

testing

Primarily because of the low heartworm numbers found in cats and the fact that heartworms do not reproduce in cats, testing is much less reliable than in dogs. Although the American Heartworm Society considers it "good medical practice" to test cats before starting on prevention and it is hard to argue with this, we feel that effort and expense required to test the entire cat population would be better devoted to doing something more useful.  After all, even when know that a cat  has heartworms, our treatment is exactly the same as for cats that do not have heartworms:  we start it on prevention and treat the symptoms as necessary. 

If your cat has unexplained vomiting and gagging symptoms, or especially if there is difficult or noisy breathing, heartworm testing is an important part of the diagnostic workup.  If you would like to know if your cat has heartworms, that is also a good reason for testing.   However, we do not ordinarily test before starting on prevention. 

prevention

Heartworm can be prevented either by giving a once-a-month pill or by applying a liquid medication called Revolution to the skin.  Revolution also kills most intestinal worms, fleas, ticks, and ear mites, and also prevents heartworm infection.

Feline Revolution handout
acrobat download

In cats, it’s heartworm larvae that most often cause damage. Pills kill larvae only once a month, and because mosquitoes bite whenever they feel like it, damage can happen between treatments. Revolution is better. As cats groom themselves, Revolution recirculates throughout the body, continuously killing larvae before they cause damage. We urge everyone in our area to protect their cats from heartworm disease.  If your cat  is entirely indoors and you don't get mosquitoes in the house, she may not need heartworm medication, although a surprisingly high proportion of feline heartworm cases occur in "indoor" cats.