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Heartworm Disease in Dogs

By: Dr. Nancy Laste

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Diagnostic Tests

To establish the diagnosis, there are two type of tests used.

  • Microfilaria tests. These tests can either be a filter test or a modified "Knott's" test; these tests are indicated to see if an infection is "patent" resulting in circulating microfilaria, offspring of reproducing mature adult heartworms in the heart and pulmonary arteries.

  • Heartworm serology. This test should be done in all dogs to rule out heartworm infection, because heartworm infections are frequently occult ("hidden"). The term occult used here specifically means positive results on serology occurring without circulating microfilaria.

    Causes of Occult Disease

  • The heartworms are young and sexually immature.

  • There is infection with a single sex: all male or all female worms.

  • The dog's immune system is actively destroying the microfilaria as they are being produced.

  • The dog has been exposed to medications which kill microfilaria (milbemycin, higher dose ivermectin).

  • The dog has been exposed to medications which result in infertility of the female heartworms (this is reported in dogs taking monthly low-dose ivermectin). Finally, if a dog is positive on a filaria test but negative on the serological tests, this could suggest infection with Dipetalonema, a harmless blood parasite which resembles D. immitis. Treatment for heartworm would be inappropriate in this circumstance. A pathologist can identify Dipetalonema on a blood sample.

    Tests To Stage Heartworm Disease

  • Complete physical examination

  • Full blood evaluation. A CBC, biochemical profile and urinalysis are usually run to be sure the dog is otherwise healthy. Specific abnormalities that may be noted on the CBC with heartworm disease are mild anemia or an increase in eosinophils (a white blood cell type that is frequently present in the bloodstream of animals with parasitic infections or allergies). The combination of eosinophils with basophils (an uncommon type of white blood cell) is suggestive of heartworm disease. Some dogs may also have an increase in neutrophils, another type of white blood cell which increase in number related to the inflammation in the blood vessels related to heartworm disease.

    The platelet count may be mildly decreased in dogs with HWD. Platelets are particles made in the bone marrow which circulate in the blood to seal off any leaks in the blood vessels, controlling small bleeding problems. Platelets are decreased in dogs with HWD, due to the platelets clumping in the areas of inflammation in the pulmonary arteries. Mild liver enzyme elevations are often noted in dogs with HWD. Evaluation of the CBC and profile pre-treatment allows the veterinarian to be sure the dog is healthy and free of other diseases which could complicate the treatment of the HWD.

  • Chest X-rays. Radiographs (X-rays) of the chest are taken to examine the appearance of the heart, the pulmonary arteries and the lung fields. Dogs with mild infections may have normal X-rays, dogs with more severe infections may have a change in appearance of the pulmonary arteries, enlargement of the pulmonary arteries and the heart and changes in the lung texture. Looking at the radiographs allows your veterinarian to decide how severe the disease is, 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. An echocardiogram is done on patients who have signs of congestive heart failure on physical examination, on patients who appear to have heart disease (abnormal heart sounds on physical examination or heart enlargement on the chest X-rays) or on patients showing signs of caval syndrome (anemia, jaundice, right-sided heart failure). An electrocardiogram (ECG) is indicated in any patient with signs of an arrhythmia (abnormal heart rhythm). The ECG may help identify patients with severe right heart disease. An echocardiogram and ECG are not routinely done in healthy young patients with normal heart appearance on chest X-rays. Because heartworms actually live in the smaller pulmonary arteries, they are not typically seen on echocardiograms of most infected dogs. In dogs with caval syndrome, the worms are readily imaged in the vena cava and right heart using echocardiography.


    After evaluation, treatment is generally recommended.

  • 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. Eliminating the Wolbachia disrupts the heartworms' life cycle. It is also believed that treatment with Doxycycline also results in less inflammation in the lungs when the heartworms are killed with an adulticide.

    Adulticide Therapy

    Killing of the adult heartworms. One hopes to accomplish complete death of the heartworms, but slowly enough to allow the body time to break down and absorb the dying heartworms.

  • Melarsomine (Immiditicide®). This arsenic-based 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 allows for slower death of the heartworms, which may cause fewer complications in dogs with more severe infections. Immiticide is the most commonly used adulticide.

  • Thiacetarsemide (Caparsolate®). This drug is given by injection into the vein. The most widely accepted protocol is two injections given eight hours apart on two consecutive days, for a total of four injections. It is extremely important that this drug not spill outside the vein or there will be severe inflammation and injury to the tissues, resulting in swelling of the leg and possible death of muscle and skin in the area. Caparsolate causes liver toxicity in many dogs. Before each injection, the veterinarian checks to be sure that the patient is eating and feeling well, and doesn't have any yellow color to the eyes and gums (icterus or jaundice). If any of these occur, the treatment must be temporarily suspended and tried again one month later. Please note that thiacetarsemide (Caparsolate®) is currently unavailable/off the market.

    Microfilaricide Treatment

    Killing the circulating baby worms. This is unnecessary in dogs with occult heartworm infections. Microfilaria can be killed in a few ways:

  • Milbemycin (Interceptor®). Will kill microfilaria within a few doses.

  • Ivermectin (Ivomec®). Ivermectin is administered in a concentrated form either orally or by injection under the skin in low doses to kill the microfilaria. The dose of ivermectin used to prevent heartworm infection (Heartgard®) is not high enough to kill the microfilaria.

    Sometimes microfilaria treatment needs to be repeated (one month after the first treatment) before complete elimination is achieved.

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