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Heartworm Disease

Heartworm disease is caused by an infestation with a worm-like parasite called Dirofilaria immitis. Dogs and, less commonly, cats are infected with heartworms through the bite of an infected mosquito. Through this bite, immature heartworms are transmitted into the animal where they burrow into tissue underneath the skin. The juvenile heartworm goes through several stages, as these stages progress the parasite makes its way into the bloodstream of the host (dog or cat) and eventually settles in the heart and/or vessels leading into the lungs. As this migration progresses and comes to completion, the heartworm has reached adulthood. As the adult heartworm reproduces, more immature worms are released into the bloodstream. These juvenile worms are picked up by mosquitoes and are used to infect another host. Heartworms must go through a mosquito to become infective, they cannot spread directly from animal to animal.

Clinical Signs

Many animals will not show any clinical signs, especially early on in the infestation. The longer the duration of infestation, the more likely it is for clinical signs to be apparent. Irreversible damage to heart, lung and vascular tissue is more common as the severity and duration of the infestation increases. Some common clinical signs are as follows:


Exercise intolerance

Weight loss

Decreased appetite

In advanced cases, congestive heart failure may develop


Diagnosing heartworm infestation requires a small blood sample. The heartworm test identifies the presence of adult heartworms in the body. The adult worms take up to 6 months to mature past the point of infection. Therefore, the test will only be positive if the animal was infected more than 5-6 months ago. We utilize an in-house test which takes only 10 minutes to get results. Once a positive result is uncovered, the next step is to stage the disease.

Staging the disease involves gathering as

much information as possible about how

the disease has affected the body. A complete

blood count, chemistry profile, chest radiographs,

urinalysis and a microfilaria test are performed.

The microfilaria test tells us if there are juvenile

stages of the heartworm circulating in the blood.


Once we have staged the disease we will recommend the best method of treatment. The treatment of choice for the majority of animals is to rid the pet of the heartworms. There are many protocols for heartworm treatment but they all include killing the adult worms as well as the juvenile stages. Immiticide is the drug used to kill the adult worms within the pet, it is an injectable drug administered in the muscles along the spine.

The most common protocol for killing the adult heartworms is as follows: one Immiticide injection is given initially, one month later a series of two Immiticide injections are given 24 hours apart. Four months after treatment, another heartworm test is performed to ensure that all the adult heartworms were killed by the treatment.

Worm death (brought on by Immiticide treatment) has the potential for causing side effects, such as: pulmonary embolism (blood clot in the lungs), anaphylaxis (severe allergic reaction to the adult worms dying), inappetence and lethargy. Some of these can be quite severe.

Animals that have severe clinical signs, other major health concerns, etc. may not be the best candidates for Immiticide therapy. These patients may benefit from year-round Heartgard administration. This prevents reinfestation with heartworm disease and bides time until the worms die naturally. Obviously, this is not the best method for most pets because the longer the worms are present in the body, the more likely irreversible damage will occur in the heart and lungs.

All pets should then be put on year-round heartworm preventative to prevent re-infestation.


Prognosis is extremely dependent upon many factors: severity of clinical signs upon diagnosis, changes already apparent on chest x-rays/bloodwork, other medical problems, worm burden, etc. The majority of animals that undergo Immiticide therapy do very well but there is the potential for very serious side effects not only from treatment itself but from the presence of the heartworms in the body.

Cats & Heartworm Infestation

The prevalence of heartworm disease in cats is less than that of dogs but it does occur and cause potentially life-threatening illness in our feline patients. About 1/3 of heartworm positive cats are strictly indoors.

Heartworm disease in cats is very different from HW disease in dogs. Cats have a very small number of worms but are extremely sensitive to the worm itself. This hypersensitivity leads to major inflammation in the lungs and associated vessels. Cats with HW will often display labored breathing, persistent intermittent vomiting or coughing.

Diagnosing HW disease in cats is more difficult. Two tests are assessed at an outside laboratory to help us determine if heartworm infestation is a possibility. Chest radiographs, bloodwork, etc. is also recommended in our feline heartworm patients. Treatment for feline HW disease is very different. Immiticide is not tolerated by cats so the adult heartworms are not killed. The focus is on decreasing/eliminating clinical signs with anti-inflammatory agents. Generally prednisone or Medrol are used long term in these patients.


We are very lucky in that there are safe, effective heartworm preventative medications on the market today. It is recommended by the American Heartworm Society that all dogs be put on year-round heartworm preventative. Most heartworm preventatives also serve as intestinal parasite control agents as well. Heartworm preventatives are given on a monthly basis. There are also heartworm preventatives available for cats.

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