Heartworm disease is caused by the parasite Dirofilaria immitis, a type of roundworm that lives in the heart and larger blood vessels of the chest and lungs. It is transmitted by mosquitoes and can cause serious heart disease if left untreated. Heartworm disease is most often seen in dogs but can rarely affect cats, in which case it causes severe inflammation of the lungs, resulting in an asthma-like condition. 


The early stages of heartworm are often symptom-free. As the number of worms multiples within the body, you may note:


Heartworm disease is diagnosed by a relatively simple and quick blood test. Most veterinarians routinely run this test on a yearly basis. If your pet is positive for heartworm, your veterinarian will likely also recommend:


There is currently only one recommended treatment option for canine heartworm disease, and it consists of a series of potent injections designed to kill adult heartworm. Treatment generally takes six months to complete, and your dog must be exercise-restricted during the course of treatment. Doxycycline and heartworm prevention are also important parts of treatment and work to prevent re-infection.

There is currently no approved treatment for cats. Most cats are able to clear the heartworm infection on their own, but those that go on to develop an asthma-like condition will need lifelong treatment to control the associated clinical signs.


Heartworm is 100% preventable with regular monthly medication. Monthly preventives are available in both oral and topical formulations. An injectable option, now approved for 12 months duration, is also available for use in dogs.