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Heart with heartworm Heartworm disease is a serious, and often deadly disease if left untreated.  Unfortunately, most owners are unaware of the risks and pain associated with heartworm treatment. This page is meant to educate owners about the recommended treatment for heartworm disease and the side effects associated with ridding your beloved friend of these deadly parasites.  There are four major procedures associated with treatment for heartworm disease.  These include the pre-treatment evaluation, elimination of adult worms, elimination of microfilariae (baby worms), and effective pain management throughout the entire procedure.  Be sure to understand all aspects of the treatment process before taking your pet in for treatment.  Also, be certain that your veterinarian is using a three shot protocol which is safer and more effective, as well as the treatment recommended by the American Heartworm Society and many university teaching hospitals.

Pre-Treatment Evaluation

After being diagnosed with heartworm disease through the use of a simple blood test, the veterinarian will perform evaluation and stabilization procedures to assess the full extent of the disease and your pet's ability to withstand the treatment.  This involves X-rays, blood tests, heart evaluation, and any other tests indicated to completely evaluate the pet.  The veterinarian evaluates the over-all health of your pet, then determines how to best proceed with treatment.  Part of this evaluation is staging the severity of the Heartworm Disease in your pet.  The categories into which patients are grouped are as follows:

  • Group I: Lowest Risk. Young healthy dogs with minimal disease evident on radiographs, normal blood work, and no symptoms of illness.
  • Group II: Moderately Affected. Some coughing, moderate radiographic changes but normal blood work.
  • Group III: Severely Affected. Dog is suffering from weight loss, cough, difficulty breathing, blatant damage to the vasculature is apparent on radiographs, blood work reveals kidney and/or liver damage.
  • Group IV: Caval Syndrome. Dog is collapsing in shock with dark brown urine evident. Heartworms visible by ultrasound in the AV valve of the right side of the heart, very abnormal blood work. These dogs, as mentioned previously, are dying and can only be saved by the physical removal of adult heartworms via an incision through the jugular vein. If such a dog can be saved from this crisis, further heartworm infection treatment cannot be contemplated until the dog is stable enough to fit into one of the other categories above.

An important basic concept in the treatment of heartworm infection is that there are adult heartworms in the heart and pulmonary arteries to be killed and there are microfilariae circulating in the bloodstream to be killed and one medication cannot be used to do both jobs.  In addition, pain associated with the treatment must be managed throughout the treatment period.

Medications and Treatment


There is currently one drug approved by the FDA for use in dogs for the elimination of adult heartworms. This drug is an organic arsenical compound. This drug, called Immiticide (generic: Melarsomine dihydrochloride) is given deeply in the muscle by injection.  A three-injection treatment protocol of one dose initially, followed in four to six weeks with a two-dose treatment is the treatment of choice of the American Heartworm Society and several university teaching hospitals, regardless of stage of disease, due to the increased safety and efficacy benefits. By initially killing fewer worms and completing the treatment in two stages, risks associated with dying worms causing blockages in the lungs and other side effects can be reduced.

Pain and Anti-inflammatory Medications

As with any major medical procedure, treating heartworm is extremely painful.  Muscle soreness and inflammation is common after treatment. The primary post-adulticide complication is the development of severe pulmonary thromboembolism or blockage in the lungs. Pulmonary thromboembolism results from the obstruction of blood flow through pulmonary arteries due to the presence of dead heartworms and lesions in the arteries and capillaries of the lungs. If heartworm adulticide treatment is effective, some degree of pulmonary thromboembolism will occur.

As a result of these injuries, patients require both the administration of anti-inflammatory doses of corticosteroids and a strict reduction in exercise.  Pain medication is also advised to reduce pain associated with the swelling, injury as a result of pulmonary thromboembolism, along with pain at the injection site.  These protocols will help to keep your pet as comfortable as possible during his or her treatment period. 

When considering a veterinarian to treat your beloved pet for heartworm, be sure to ask about pain therapies associated with the treatment.  Just think about your reaction if you were taking your child to the hospital to have heart surgery. Would you leave your child in a doctor's care if the doctor said that he did not think pain medication was necessary?


The most effective drugs for ridding your pet of microfilariae are the macrocyclic lactone (ML) anthelmintics, i.e.,milbemycin oxime, selamectin, moxidectin and high doses of ivermectin. These drugs are the active ingredients in the commonly used heartworm preventives. Although their usage as microfilaricides has not been approved by the FDA, they are widely used as such because there are no approved microfilaricidal drugs currently available. It is recommended that microfilariae positive dogs being treated with these drugs be hospitalized for at least eight hours following treatment for observation of possible adverse reactions resulting from rapid death of the microfilariae.

Circulating microfilariae (baby heartworms) usually can be eliminated within a few weeks by the administration of the ML-type drugs mentioned above. Today however, the most widely used microfilaricidal treatment is to simply administer ML preventives as usual, and the microfilariae will be cleared slowly over a period of about six to nine months.

Follow-up Testing and Therapy

After completing treatment for heartworm, your pet will be tested for any residual affects of the disease.  Once it is established that your dog has been successfully rid of heartworm and microfilariae, the veterinarian will place your pet on a preventative such as Heartgard so as to prevent the future infection.  By giving your dog a tasty, once a month chewable tablet, you can effectively prevent your dog from contracting this deadly and painful disease.