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Ask A Vet: Heartworm Testing 101: What Does A Heartworm Test Really Do?

| Published on October 10, 2015


Heartworm testing has become very routine in a veterinary clinic. There are in-house test kits and we can have results quickly. The American Heartworm Society (AHS) is a group that helps veterinarians make unified recommendations to promote eradication of this damaging parasite. AHS recommends at least once yearly testing. But when the veterinary staff member says there will be blood drawn for a heartworm test, do you really know what he/she means?

What does a heartworm test really test for?

An in-clinic heartworm test is a kit that utilizes similar science as a human pregnancy test. Human pregnancy can be confirmed by the presence of certain hormones only produced in pregnancy. In a comparable fashion, the test kits can detect the presence of antigen to Dirofilaria Immitis (heartworm). If there is antigen, there are adult worms. The specific antigen detected by the test kit is one produced by the ovary of the female, so if your dog has an all male heartworm infection, the test could miss it. However, female worms are required to reproduce so this is not likely. Today’s test kits are very sensitive and even a small number of worms will appear as a positive result.

What does it NOT test for?

Clients sometimes think that drawing blood for a heartworm test is a more comprehensive test than it really is. A heartworm test is only a test for heartworms. It is not to be confused with other diagnostic testing, like serum chemistry testing or blood sugar evaluation. When your vet staff tells you that the test result was negative, they only mean that the presence of ADULT heartworms was not detected. No commercially available test will detect the babies until they become adults, so if your dog is not on heartworm prevention, you will have to test again when those have had time to grow up.

What does a positive test mean for my dog?

A positive heartworm test means that there are adult female heartworms in your dog’s blood stream and they are present in sufficient numbers to trigger a positive result. It is critical to know that if there are undetected heartworms in your pet’s heart and lungs, they will cause irreparable damage that can lead to heart failure and other dangerous consequences.

How important is this test?

People question the necessity of heartworm testing, feeling that if they faithfully give the heartworm pills and believing the test will be negative. Their faith in heartworm prevention products is not unjustified, but blind faith in anything does not allow for any situation that falls outside the norms.

A positive heartworm test means that your dog has adult heartworms. Left untreated, he will progress to heart failure and die, spreading heartworm larvae to every mosquito that bites him along the way, potentially infecting other unprotected dogs. It does not test for any other parameters of his health, like a serum chemistry panel (although those tests will be a part of his staging for the treatment to kill the heartworms). A negative test indicates that there are no adult female heartworms in your dog’s blood stream at that time.

Only a combination of strategic testing and preventive strategies, following your vet’s recommendations, can make sure that your dog tests negative and stay negative for a healthy worm free future.

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