Heartworm Test

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Heartworm tests are blood tests run to determine if an animal is infected with heartworms. The test may be run in-house or sent to a reference laboratory. If run in-house, results are often available in as little as 10 minutes.

In dogs, heartworm tests work by detecting antigens of the heartworm in the dog’s blood. A few drops of blood are mixed with the test solution, and then poured into the tester. The tester contains antibodies that bind to heartworm antigens (a protein produced in the female worm’s uterus). When this binding occurs, a colored dot develops indicating the animal is infected with heartworms. Because infected dogs have a large number of heartworms in their system, these tests are very accurate. However, in order for a dog to test positive on the test, it must have mature female worms in its system. Heartworms take heartworm-interpreting-results courtesy of Idexxapproximately 6 months to reach maturity after infection, so dogs with very early infections may still test negative on the test.

There are two types of tests available for cats. One is an antigen test similar to the one used in dogs. This test will turn positive if it detects antibodies of three or more adult female worms. However, most infected cats only have a few adult worms in their system (1-7 worms), and chances are high that all or most may be male. If this is the case, the test will be negative. Thus, a positive antigen test in a cat indicates it is truly infected with heartworms, but a negative test does not rule out the possibility of infection.

The second type of test available for cats is an antibody test. This test detects heartworm-specific antibodies circulating in the cat’s blood stream. Antibodies are produced by the
cat’s body when heartworm larvae reach the L4 stage. However, these antibodies are transient, lasting only a few months. Thus a positive antibody test indicates a cat has been infected with heartworm recently (it does not tell us if the cat has adult worms in its system or not) and a negative test doesn’t really indicate much at all. Running both the antibody and antigen tests in cats is the best way to detect heartworm infection, but even the combination of tests may miss infection in some cats.