Heartworms are one of the most common parasites that we see in dogs, and they are an increasing problem in cats. As the name implies, heartworms actually live inside the heart and the blood vessels surrounding the heart, and they can be up to several inches long. These parasites are transmitted by mosquitoes; a pet that has adult heartworms in it's heart will have immature heartworms throughout it's bloodstream. When a mosquito bites that pet and takes in blood, it also takes in those immature heartworms and then spreads them to another animal. All dogs and cats are at risk for heartworms, because even indoor pets can be exposed to mosquitoes.

Heartworms cause damage in two main ways. One, just the physical presence of the worms acts as a barrier to the flow of blood out of the heart, so the heart has to pump harder. Over time, this can lead to heart failure. There is also an increased risk of blood clots forming, which can cause a syndrome similar to a stroke involving the lungs. If the clot is large, this can be fatal and even cause sudden death with no warning symptoms.

For many years it was believed that cats were not susceptible to heartworms, but in recent years there have been reports that as many as 10% of all cats may be infected. The difficulty with cats is that since their hearts are so much smaller that dogs, if they are infected they may have only one or two worms, which makes the infection very difficult to diagnose even though the worms may cause significant damage. In addition, the medication that is used to treat dogs with heartworms cannot be used in cats, so once they are infected there are limited options for treatment.

Heartworms are treatable in dogs if diagnosed in time, though the treatment can be risky especially if there has already been damage to the heart. The best option by far is to prevent your pet from becoming infected in the first place. Heartworm preventatives are a tablet given by mouth or a liquid applied to the back of the neck once a month; this medication will kill any immature heartworms they get exposed to before they can get into the heart and cause damage. The topical liquid medication also helps with flea and tick control. All dogs should be tested to make sure they do not already have heartworms before they are started on prevention.