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How Testing for Heartworm Changes with Life Cycle

Why Multiple Tests Will Probably Be Needed

By Kat Sherbo. August 08, 2013 | See Comments

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    PetCareRx Staff Veterinarian

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How Testing for Heartworm Changes with Life Cycle

Heartworms are tricky to test for because the diagnostics we have only detect specific stages in the heartworm life cycle, making a false negative a frequent occurrence. Find out when and how to best test for heartworms.

Heartworms are a dangerous parasite, transmitted via mosquito bites, that frequently infect cats and dogs. These nasty creatures can clog your pet's arteries and heart, causing all sorts of problems, sometimes even leading to death.

And just to keep things interesting, testing your pet for heartworm infection is a bit tricky. Here we break down how to test for heartworm, and how the tests results may be skewed based on the life stage of the parasite.

The Heartworm Life Cycle

1. An infected mosquito, carrying heartworm larvae, bites a pet. As the mosquito feeds, it leaves the larvae behind in the pet's bloodstream.

2. The larvae mature within the pet's bloodstream over several months, eventually becoming a fully grown, adult heartworm.

3. After three to four months the adult heartworm can give birth to microfilaria, which is the first stage of  heartworm life.

4. When another mosquito bites the pet, that mosquito ingests the microfilaria.

5. Inside the mosquito, the microfilaria matures to a larval stage, and is then passed along to other pets.

Testing for Heartworm

Veterinarians test for heartworm by taking a small blood sample from your pet and using it for one, or both, of these types of tests:

  • Antigen Test: Veterinarians will check the blood sample for antigens within the bloodstream left by female heartworms.
  • Microfilaria Test: Veterinarians will test for the presence of microfilaria, or the young heartworms, within the pet's blood.

Both types of tests have a the risk of a false negative, meaning that your pet could in fact have heartworm even if the test reveals no evidence.

Why Would a Heartworm Test Show a False Negative?

Heartworm tests look for antigens from female heartworms or for microfilaria. It's possible that your pet could be infected only with pre-adult heartworms, with only male heartworms, or with a low number of worms such that neither antigens or microfilaria are found. Depending on the life stage of the parasites within your pet, one test may prove more useful than the other.

Could use a visual? Check out this video from Bayer -- the people who brought you K9 Advantix, Advantage, and Seresto -- on the heartworm life cycle.

When to Test for Heartworm

All pets should be tested for heartworm at least once a year. This is the best way to ensure that your pet remains healthy.

More on Pet Parasites

5 Things You Need To Know About Roundworm
What To Do When Your Pet Gets Tapeworm
Parasites and Worms in Dogs and Cats

This information is for informational purposes only and is not meant as a substitute for the professional advice of, or diagnosis or treatment by, your veterinarian with respect to your pet. It has, however, been verified by a licensed veterinarian for accuracy.

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Heartworm Tests at a glance

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  • 1The effectiveness of heartworm tests relies on the heartworm life cycle.
  • 2It's possible that a heartworm test will give a false negative.
  • 3Heartworm tests look for antigens from female heartworms or for microfilaria - young heartworms.
  • 4All pets should be tested for heartworm yearly.