The Animal House

21 Red Brook Road
Buzzards Bay, MA 02532




Heartworm is a fairly large worm, up to 14 inches long, that in adulthood lives in the heart and pulmonary arteries of an infected dog or cat. Dogs and cats acquire this infection through mosquito bites as mosquitoes pick up larval heartworms from infected dogs and carry them to new dogs and cats.

Before reviewing the clinical signs seen in heartworm disease, an important distinction must be made between heartworm disease and heartworm infection. Heartworm infection by definition means the host animal (usually a dog) is parasitized by at least one life stage of the heartworm (Dirofilaria immitis). Dogs with heartworms in their bodies do not necessarily have adult worms in their hearts; they may have larval heartworms in their skin only. Dogs with heartworms in their bodies are not necessarily sick, either. Dogs with only larvae of one stage or another are not sick and it is controversial how dangerous it is for a dog to have only one or two adult heartworms. These dogs are certainly infected but they do not have heartworm disease.

On the other hand, dogs with heartworm disease are sick. They not only have the infection but they have any of the problems because of it. Fortunately, heartworm disease is both treatable and preventable.

Heartworm disease in cats is quite a bit different from heartworm disease in dogs. Cats are so small that only one adult worm could be enough to cause heart failure, plus there is much more inflammation involved with immature worms in cats.

Prevention is recommended with year round administration of a monthly preventative.


If heartworm preventative is being given year round, it is recommended that a heartworm test be done every 2 years. We must look for some surety of compliance, either sufficient amount purchased here, or notes that a prescription was written along with receipts from another facility, to cover the period of compliance.

If heartworm is not given year round, a heartworm test is required yearly.