By Julie Augustine, MS, DVM

When you bring your dog or cat into the veterinarian’s office, several of the many questions that are asked when you first arrive revolve around a parasite that most pet owners know of but may not know as much about: heartworms. We ask about heartworm testing and heartworm preventative, but what does all of that mean? And what is all of that needed for your pet?

What Heartworm Is

Dirofilaria immitis, or as it is better known, heartworm, is a parasite that lives in the blood stream and affects the heart (primarily the right side), lungs and their associated blood vessels of affected pets, causing heart failure as the adult worms clog the heart, respiratory difficulties, and possible death. Heartworm disease affects dogs, cats and ferrets, but heartworms also live in other species and may not cause disease, including coyotes and foxes. Because these wild species live close to many urban areas these days, they are considered important resevoirs of the parasite for the mosquito to pick up and transfer the heartworm to other animals.

Heartworm is transmitted by mosquitos which deposit the parasite through a bite to your pet; it has a complicated life cycle which requires many steps and 5 stages of the parasite as it grows. The newer heartworm medications (selamectin, milbimycin oxime and ivermectin) are applied or given once a month and are inexpensive and very well tolerated (and many times happily eaten!) by both cats and dogs, and their method of action is to break the life cycle at certain points to keep your pets heartworm free. As an added protection for your pets, some of the current flea and tick medications also have a repellent property to keep the mosquitos away entirely so they do not have the opportunity to even have a chance to bite!

What Heartworm Isn’t

Heartworms are not an intestinal parasite and cannot be detected with routine testing of your pet’s feces; the testing requires a small amount of blood yearly and is a very simple testing procedure.

Heartworm disease is never a mild condition which can be managed simply; if your pet is diagnosed with heartworm disease, it is a painful, risky, and expensive treatment and also will require up to 3 months of very restricted activity as the medication takes effect to kill off the heartworms slowly as to not cause damage to the lungs and cause sudden death. The medication to treat heartworms is an arsenic-based medication (melarsomine hydrochloride) and requires hospitalization when it is administered.

Heartworm disease is not prevented by pets being indoors; mosquitos can find their way into houses through open windows and doors and infect your pet with the parasite as easily as it can if your pet were outside. It only takes one bite from one mosquito to start the long trail of testing and treatment. Even indoor cats can become infected with heartworm!

Heartworms are not less dangerous in the winter time; with warming trends and milder winters, mosquitos do not go away as completely as we would like. This is why we encourage owners to continue with heartworm preventative year round.

On the bright side, heartworm disease is not a difficult disease to prevent! One chew or application, once a month, year round can prevent heartworm infection almost up to 100% if given correctly.

A great website to visit for more information is the American Heartworm Society, or, and they have information on the heartworm life cycle, heartworm preventative and treatment, and FAQs, and don’t forget, you can always ask one of the doctors at Pet Dominion as well if you have any questions, we’re always glad to help!

Sources: American Heartworm Society