Most people feel that their indoor cats are sealed in a vacuum and shielded from parasites. Unfortunately that is not the case. No residence is 100% protected from the outside.

We should not forget that some “indoor” cats do get outside time. Some of the cats do get time on a balcony or time on the deck.

Heartworm disease is a mosquito borne disease. Although primarily considered a dog disease, heartworms can be spread to a cat from an infected mosquito. For every 100 dogs that are positive there will be 10 positive cats since they are more resistant. Cats are considered aberrant host for heartworms.

So what does that mean for my cat?

Cats usually have a much smaller heartworm load since they are not the definite host. So instead of hundreds of heartworms, they may only have 2 or 3.

That does not sound so bad. What harm can 2 or 3 heartworms do?

Unfortunately for cats it is not the heartworm that causes issues but their immune system’s response. Heartworm disease in cats is primarily a lung disease.

Heartworm Associated Respiratory Disease (HARD). Many cats may cough and be misdiagnosed with asthma. More information about HARD.

Some cats may vomit. Another clinical sign is sudden death.

Since cats have so few heartworms, diagnosis can be very challenging. Since dogs have so many it is much easier to diagnosis. Cats rarely have microfilaria (immature heartworms). This is the easiest method of detection in dogs.

If there is no microfilaria, an antigen test can give a diagnosis. Unfortunately, you need to have pregnant females to get a positive response. With only a few heartworms, this does not happen often in cats.

There is no treatment for cats if they get heartworms. So prevention is the easiest solution to the problem. The American Heartworm Society (AHS) has not recommended a testing protocol for cats at this time – see the FAQ from AHS.

Is heartworm disease common in the United States?

Here is a map from AHS which illustrates the extent of heartworm disease in the US.


For more information, the Companion Animal Parasite Council (CAPC) has prevalence maps for your state.

But wait, my cat is indoors and I live in an apartment building.

So my cat is safe?

Your cat may be at less risk, but still at risk. Please keep in mind any outside time puts your cat at risk, and mosquitos can get inside your residence.

Asian Tiger Mosquitoes have been found breeding in tree holes over 40 feet above ground. Mosquitos have been found in apartments 21 stories above ground.
The current recommendation from AHS and CAPC is for cats to be on year round prevention in the United States.

Revolution is a topical preventative and cost $0.41 per day or $12.50 per month. Revolution also prevents fleas and deworms. Heartgard is an oral preventative and runs $0.21 per day or $ 6.45 per month.

You should check with your veterinarian about the need for heartworm prevention in your part of the world.

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