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Heartworm Disease in Dogs

By: Dr. Nancy Laste

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Canine heartworm disease (CHWD) is a common condition in many regions of the world. It is caused by the filarial (threadlike) worm Dirofilaria immitis that lives in the pulmonary arteries (blood vessels leading from the heart to the lungs) in most infected dogs. The name "heartworm" is a bit misleading because only in very heavy infections do the worms actually reside in the heart itself.

Nonetheless, the presence of these worms causes strain to the heart and an intense reaction in the blood vessels, resulting in problems for the dog. D. immitis is injected into the dog by infected mosquitoes. For this reason, CHWD is more common in tropical regions of the world, where the conditions of warmth and moisture are ideal for mosquitoes to live.

Predispositions for CHWD include living outdoors. Male dogs may be more likely to be infected than female dogs. The most important predisposing factor is failure to receive heartworm preventative medication. All dogs living in an area where heartworm disease exists are at risk, even if they live entirely indoors.

Transmission occurs when a mosquito bites an infected dog and ingests heartworm larvae (baby heartworms), which live in the bloodstream. When the insect bites another dog or cat, some of the larvae are injected under the skin. The larvae grow for 3 to 4 months and eventually make their way into the heart where they develop into adults, and the process is ready to repeat itself.

Impact on the pet is variable. Dogs can be without symptoms if the infection is light or has occurred recently. In some cases HWD can cause severe debilitation and eventually may be fatal. Because heartworm disease is easily preventable, it has become a less common diagnosis in pets that receive regular veterinary care.

What To Watch For

Symptoms don't usually develop until damage has already occurred to the heart. Dogs can have a wide range of symptoms, with some dogs being completely asymptomatic (no symptoms at all). Symptoms usually occur because of heart failure. These include:

  • Coughing
  • Coughing up of blood (hemoptysis)
  • Heavy breathing
  • Unwillingness to exercise
  • Signs of right sided congestive heart failure, which include fluid distention of the belly, pulsation of the jugular veins in the neck when the dog is sitting or standing and heavy breathing.


    Two types of blood tests are used to establish the diagnosis:

  • Microfilaria screen. These are screening tests to detect the baby worms (microfilaria) circulating in the bloodstream (produced by the adult heartworms living in the pulmonary arteries). This test can either be a filter test, or a "Knott's test" (spinning down a sample of blood mixed with formalin and looking for the microfilaria with a microscope).

  • Heartworm serology. This test checks for proteins in the bloodstream of the dog that are produced by the heartworms. These tests are very sensitive and accurate.

    Once diagnosed, to stage the heartworm disease:

  • Complete physical examination. A physical examination will detect signs of disease or may suggest a co-existing disease condition.

  • Laboratory evaluation. A CBC, biochemical profile and urinalysis may be run as a general health screen, and to assess stability of patient for treatment.

  • Chest x-rays. Looking at the radiographs (X-rays) allows your veterinarian to decide how severe the disease is, and to decide how to treat the dog, and to warn dog owners about the individual dog's susceptibility to complications with treatment of the heartworms.

  • Evaluation of the heart. This specialty examination would include an ultrasound examination of the heart (echocardiogram) to look for enlargement of parts of the heart, or the presence of the heartworms within the heart itself. An electrocardiogram (ECG) maps out the electrical activity of the heart and determines the heart rhythm. An ECG may be recommended if there are any signs that the heartworm disease is advanced. The echocardiogram and ECG are not part of the routine evaluation of heartworm disease.


  • Doxycycline is also used to treat Wolbachia, an organism that infects heartworms and is required for heartworm survival, in an effort to treat heartworm disease.

    Killing of the adult heartworms (adulticide therapy)

    There are two main agents administered by veterinarians for adulticide treatment:

  • Melarsomine (Immiticide®): This drug is given by injection deep into the muscles of the back as either a two-day series (one injection per day, two consecutive days) or as a split protocol (one injection, followed one month later by two consecutive injections one day apart for a total of three injections). This split protocol approach has become more common Immiticide® is currently the most commonly used type of treatment. Melarsomine is considered superior in its safety and efficacy.

  • Sodium thiacetarsemide (Caparsolate®). This drug is given by injection into the vein. The most widely accepted injection routine is two injections, eight hours apart on two consecutive days, for a total of four doses. Please note that thiacetarsemide (Caparsolate®) is currently unavailable/off the market.

    Killing of the baby worms (microfilaricide)

    This is done only in dogs with a positive filarial test. It is given one month after the adulticide treatment is completed:

  • Milbemycin (Interceptor®). A monthly heartworm preventative, effective at killing the microfilaria.

  • Ivermectin (Ivomec®). A concentrated form of this drug is given to kill the microfilaria.

    Home Care

  • Strict exercise restriction. THIS IS THE MOST IMPORTANT PART OF THE TREATMENT PROCESS. Dogs who have been treated for heartworm disease should receive the absolute minimum of exercise for four weeks following the treatment. Ideally, cage rest should be provided. In very hyperactive dogs, a sedative may need to be used to help achieve this very important goal.

    Exercise restriction is so important, because as the drugs are killing the worms in the arteries, the worms can break off and travel to block parts of the blood vessels (pulmonary thromboembolism or PTE). PTE results in obstruction of the blood flow to parts of the lung (pulmonary infarction). Keeping the dog quiet allows the body time to slowly break down and absorb the dying worms.

  • Aspirin. Low dose aspirin is prescribed in some cases of heartworm disease as an anti-inflammatory to minimize the reaction in the lungs to the death of the worms.

  • Prednisone. Administered pre-treatment only if there is intense lung inflammation and severe coughing. Administered post-treatment only to treat the complications discussed below.

  • Watching for complications. The major complication to treatment is PTE. PTE is associated with signs of fever, heavy or fast breathing and coughing. Observation of any of these clinical signs would necessitate a visit to your veterinarian or local veterinary emergency center. In its worst form, PTE can result in sudden death. Back pain is often noted as a minor complication secondary to the Immiticide® injections into the muscles of the back.

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