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Heartworm Disease in Cats

Feline Heartworm


Heartworms are 9-11" long worms that live in a cat's heart or in the arteries leading to the lungs known as pulmonary arteries. Although heartworms occur commonly in dogs, most people do not consider them a problem for the cat. However, recent studies of cats with heart and respiratory diseases have found an incidence of heartworms that is far greater than previously thought. In Ontario, cats are at risk for contracting heartworm disease during mosquito season. Prevention for the disease begins on June 1st.

How are heartworms transmitted to a cat?Heartworms are transmitted by mosquitoes. When an infected mosquito bites a cat, it deposits baby heartworms (larvae). The larvae migrate and mature for several months, ending up in the right side of the heart and pulmonary arteries. They mature into adult heartworms about six months from the time they enter the cat. Shortly thereafter, they begin to release immature heartworms, known as microfilaria. Microfilariae live in the cat's blood for about one month. They are ingested by mosquitoes feeding on the cat. However, most mosquitoes acquire microfilaria by feeding on heartworm-infected dogs. Because of their life cycle, a cat must be bitten by a mosquito in order for it to become infected with heartworms. Heartworms are not transmitted directly from one cat to another or from a dog directly to a cat.

How are heartworms diagnosed?Detecting heartworm disease in cats is difficult and can be unreliable. Usually when a cat is infected with heartworm disease, 1-2 worms are present. The current feline heartworm antigen test looks for the presence of adult female heartworms. A heartworm positive cat can yield an inaccurate negative heartworm result. A cat must have at least 2 adult female worms present to make this test positive and a negative test may mean that the cat may only have a small number of worms or that all the worms present are male. Due to this unreliability of heartworm testing in cats, we do not recommend yearly heartworm screens as we do in dogs.

Jane Animal Hospital recommends treating every cat with Revolution, a seasonal prevention against feline and canine heartworm disease starting on June 1st. Preventing feline heartworm diseases in cats is safe and easy. The reasons that heartworm prevention should be considered in your cat are:

  1. Diagnostic difficulty - diagnosing heartworms is not as easy in cats as it is in dogs. A simple and reliable blood test in not yet available. Unfortunately, many cats are diagnosed with an autopsy following sudden death.
  2. Unknown incidence - heartworms are not nearly as common in cats as they are in dogs. However, they are probably more common than we realize. As we look more aggressively for heartworms in cats with better and better tests, we expect to find that the incidence is greater than previously thought.
  3. No good treatment - there is no good treatment for heartworm infected cats. Effective drugs are not yet available and treatment is dangerous.
  4. Prevention is easy Preventing heartworm feline disease is as simple as giving a monthly topical application of Revolution.
  5. Indoor cats get heartworm too - Exposure to mosquitoes is required for transmission. Cats do not have to be exposed to other cats or dogs infected with heartworms. Obviously cats that go outdoors are more likely to be exposed, however, about 25% of cats that are diagnosed with heartworms are reported by their owners to be indoor only. This simply means that mosquitoes that come into your house are just as dangerous as the ones outdoors.

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