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Heartworm Test in Dogs

By: PetPlace Veterinarians

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A heartworm test is a test to check for the evidence of the parasite Dirofilaria immitis, more commonly known as heartworm, in your dog's bloodstream. The test should be performed on any dog showing signs of heartworm disease, e.g. exercise intolerance, coughing, loss of appetite, weight loss, labored breathing, or heart disease.

Antigens for the heartworm cannot be detected until seven months after initial infection. For this reason, testing animals less than 7 months of age is not indicated. Annual re-testing is recommended for dogs not receiving heartworm preventative year round. Testing every year is recommended for dogs on year-round preventative medication. Testing is also recommended when a pet owner switches between preventative medications.

There is no contraindication to performing a heartworm test. Negative results help determine your pet's good health and rule out the presence of heartworm disease.

What Does a Heartworm Test Reveal?

There are two type of tests used.

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

Heartworm serology: This type of test checks for proteins in the bloodstream that are produced by the adult heartworms. This test is very sensitive and accurate and should be carried out in all dogs to rule out heartworm infection. Some dogs that test negative for microfilaria may have adult heartworms detected on serology. This is referred to as occult, or hidden, heartworm infection. Heartworm infections may be occult for a number of reasons:

  • The heartworms are still 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 that kill microfilaria (milbemycin, medium dose ivermectin), but do not kill the adults.
  • The dog has been exposed to medications that result in infertility of the female heartworms (this is reported in dogs taking monthly low-dose ivermectin).

    If a dog is positive on the microfilaria test but negative on serological tests, this could suggest infection with Dipetalonema, a harmless blood parasite that resembles the microfilaria of Dirofilaria immitis. Treatment for heartworm would be inappropriate in this case. A pathologist can identify Dipetalonema microfilaria on a blood sample.

    A positive serology test result indicates heartworm disease. For more information, see Heartworm Disease in Dogs.

    How is a Heartworm Test Done?

    Your veterinarian will need to obtain a blood sample, which may be used for a microfilaria test or serology (antigen) test. The blood is immediately placed in a glass container with a substance that prevents clotting. For the microfilaria test, a specific amount of blood is combined with a specific chemical and the mixture is passed through a filter. The filter is then examined under the microscope for the presence of microfilaria (baby worms in the blood).

    For the serology test, a specific amount of blood is combined with a different chemical and the mixture is placed on a commercially available heartworm test filter paper. The fluid spreads across the filter paper, which is impregnated with heartworm specific antibodies. A positive result is indicated by a color change or line developing at a set point on the paper. This test works under the same principle as at-home pregnancy tests for women.

    Most veterinary hospitals have the kits and instruments to perform a heartworm test in their hospital, although some veterinarians prefer to send the blood to an outside laboratory for analysis. Heartworm testing generally take 10 to 20 minutes to perform. If the test is sent to a veterinary diagnostic laboratory, it usually takes 1 to 2 days to obtain results.

    Is a Heartworm Test Painful?

    A heartworm test is a blood test. In order to obtain a blood sample, a needle must be passed through the skin and into a blood vessel. As with humans, the pain involved in venipuncture varies from individual to individual, but is no more painful than any injection.

    Is Sedation or Anesthesia Needed for a Heartworm Test?

    Sedation or anesthesia is not needed for a heartworm test.

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