Revised Recommendations for Annual Heartworm Testing

Photo Credit: Susannah Kay

Testing dogs annually for heartworm disease is not a new recommendation. What is new is the type of testing being recommended. Until recently, performing a simple blood test to screen for the presence of heartworm antigen was the test of choice. We now know that antigen testing produces ever-increasing numbers of false negative results (the test result is negative even though the dog has heartworm disease). A study in the March, 2014 edition of Veterinary Parasitology documented that, amongst a population of shelter dogs in the southeastern United States, 7.1 percent had false negative heartworm antigen test results.

The Companion Animal Parasite Council (CAPC) and the American Heartworm Society (AHS) recently revised their recommendations regarding annual heartworm testing. They now recommend that microfilaria testing along with antigen testing be performed. Combined, these two tests reduce the possibility of missing a heartworm-positive dog.

Antigen testing

Heartworm antigen testing detects protein particles within the reproductive tract of adult female heartworms. Here are some reasons why a dog with heartworm disease could have a negative antigen test:

  • The dog is infected with male worms only.
  • The dog has a very low worm burden- too few for the protein secreted by the adult females to be detected.
  • The dog was infected less than 5-6 months prior to testing, and not enough time has lapsed for the immature stages of the parasite transmitted by the mosquito to mature into adult worms.
  • Antigen detection can be suppressed in dogs that have been receiving treatment with certain heartworm preventive medications, particularly when administered at the higher dosages needed to treat rather than simply prevent heartworm infection.

Treating heartworm disease with heartworm preventive medication is referred to as the “slow kill” method. Doing so became popular when melarsomine, the preferred drug for treatment of heartworm infection, was in short supply. The slow kill method has remained popular because it is less expensive than the melarsomine protocol.

Parasitologists believe that dogs treated via the slow kill method may form immune complexes in which antibodies (the body’s immune system foot soldiers) bind with the antigens, thereby preventing them from being detected by heartworm antigen testing.

In addition to sometimes producing false negative antigen results, the slow kill use of heartworm preventives may also be contributing to the development of resistant heartworms- those that laugh in the face of exposure to heartworm preventive medications. For these reasons, both the AHS and CAPC recommend against use of the slow kill method for the treatment of heartworm disease.

Microfilaria testing

Microfilariae are immature (baby) heartworms that circulate within the bloodstream. Mosquitoes consume them during a blood meal, so microfilariae are considered to be the “contagious stage” of heartworm disease. Microfilariae are also the developmental stage that has the ability to develop resistance to commonly used heartworm prevention medications.

Like the heartworm antigen test, microfilaria testing can also produce false negative results. Reasons include:

  • A low worm burden (few adult heartworms present).
  • The presence of a “single-sex” infection- even in the world of parasites it takes two to tango.
  • The dog was infected less than 6-7 months prior to testing, and not enough time has passed for baby worms to be produced.
  • Heartworm preventive medications have the potential to reduce or eliminate the population of circulating microfilaria.

Current recommendations

While neither the heartworm antigen nor microfilaria tests are perfect, using the two in combination is currently thought to be the most reliable way to screen dogs for heartworm disease. Both are simple to perform, and all that is required is a small blood sample.

Annual heartworm screening is recommended for all dogs, even those receiving preventive medication. Lapses in administering the medication as scheduled and the existence of resistant heartworms are the basis for this recommendation.

Is your dog tested annually for heartworm disease? If so, do you know if your veterinarian is using the antigen test, the microfilaria test, or both?

Best wishes,

Nancy Kay, DVM

Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
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Please visit to read excerpts from Speaking for Spot and Your Dog’s Best Health.   There you will also find “Advocacy Aids”- helpful health forms you can download and use for your own dog, and a collection of published articles on advocating for your pet’s health. Speaking for Spot and Your Dog’s Best Health are available at,, local bookstores, and your favorite online book seller.


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10 Comments on “Revised Recommendations for Annual Heartworm Testing

  1. Dr. Kay, we just got back from CSU for Wyatt’s checkup; they gave him just the antigen test, and they didn’t perform any other tests to check for tick borne parasites so our total test cost was what I thought, a reasonable $21. The resident and student who checked out Wyatt hadn’t heard of the new heartworm testing recommendations and said their standard procedure is to only do both tests if there’s a suspicion the dog is infected.

  2. Re: Microfilarial testing.
    My vet (I’m in FL) told me he’s never found microfilaria when doing the Knott’s test.
    Also the level of microfilaria in the blood can vary. To see research, Google – microfilarial periodicity heartworm. Both a study in Korea and Tanzania show lowest levels at 11 AM and highest at 9 PM. This study shows a peak between 6 and 9 PM. So time of day might affect even finding microfilaria.

    The above study (with link) also notes some possible abnormalities in bloodwork, so perhaps this might be another clue if HW antigen test keeps coming up negative.

    And lastly, the whole issue of resistance sort of drives me nuts. You get resistance when you put continual pressure on a population with either the same drug/pesticide or a drug/pesticide with the same mode of action. In this case the pressure is continued use of ivermectin, many times year round, even when it’s not needed. A dog could throw up the pill unnoticed, get exposed to a mosquito with ready to go L1 heartworms and end up with a fertile female producing microfilaria, which get exposed to ivermectin each month. Dogs aren’t the only hosts for heartworms. It’s quite possible that year round use of heartworm has circulated resistant HW back into a wild population, only to come back at us. And since there’s no approved drug for killing off microfilaria, what is used? Ivermectin. To toss the whole resistance horror show on people using slow kill is a bit dishonest. The longer a pesticide is in use the higher the chances of resistance down the road.

    The standard heartworm killing drug, melarsomine, can cause neurological damage. Standard instructions do not include putting pressure on the injection site during and after injection, which keeps the drug from coming back out the needle hole, sometimes causing skin ulcers.

    And if they’re so worried about resistance, why do they put a wormer in the plus products, when perhaps the dog has no worms? Do they not think it’s possible that resistance will arise to our most common intestinal worm killers?


  3. Hi Lyn,
    Thanks very much for taking the time to voice your comments, concerns, and opinions. I want to clarify a few points based on the concerns you raised.

    – Circulating microfilariae do not become adults in the dogs in which they reside. They must go through two stages of development within the mosquito to then become the infective stage for another dog.
    – Allowing a “female only” infection to go untreated would be a detriment to the dog. The worms live for years during which time they can cause a variety of problems within the heart and the lungs.
    – You are correct, there are other reasons besides cost that some veterinarians and their clients choose the “slow kill” method of heartworm treatment. Whether this method or the melarsomine method is chosen, doxycycline is part of the therapy to treat the bacterial organism called Wolbachia that lives within the adult heart worms.
    – Heartworm antigen testing is quite inexpensive and can be ordered through any diagnostic laboratory. If there is no need to screen for tick borne diseases I encourage you to decline such testing.
    – I believe that the people who rate on committee for the American Heartworm Society truly know what they are doing and that their recommendations are those that serve the best health interests of dogs and cats. Their recommendations make good sense to me.

    Hope this helps.

    Dr. Nancy

  4. Dear Dr. Kay,
    I am concerned that you have seemed to become the advocate of the veterinarian and pharmaceutical companies in this debate rather than our dog’s advocate. I don’t want to sound argumentative! I have followed you since your first book came out – eager for our dogs to have a public voice.

    I am certainly no professional – so I would like to hear both sides of this discussion.
    First, It would seem that we are already paying a very high price for a test that really costs very little to produce and for the vet to purchase. It is also one of the few examples of an instance where we must pay to prove some product really works.

    On top of the test for heartworms which could reasonably be charged at $5.00 per test, out tick borne tests have routinely been added to this “standard” test and suddenly the price has become $45-$50.

    Second, looking at the reasons to ask for 2 tests – according to your statements:
    1. Both tests might “fail” because of a low worm burden.

    2. Both tests are not accurate if the dog has been infected within 5-7 months of the test.

    3. A dog infected with male worms only – would also include “same sex infestation”.

    4. A good heartworm preventative will kill microfilaria so that is a positive thing, isn’t it? Those “baby worms” will never grow into big worms.

    5. In antigen testing, the female proteins are detected. If there are no males and no microfilaria, the females will eventually die on their own. They will not reproduce.

    And finally, the “slow method” of killing an existing heartworm infestation, includes using the antibiotic doxycycline which kills the wobaccia that aids the female reproduction. It is not necessarily chosen just because it is cheaper.

    The “slow method” is effective. Although it may not be as effective as the melarsomine protocol, it is definitely much less harsh on the animal and causes no destructive side effects. As an advocate for our pets, I would like to see you devote an article or two comparing the both sides of this debate

    In advocating 2 tests now to validate a commercial product which we are already paying for – which has additional costly things added to it, why not make a simple test available at a very reasonable price to the public so we can be responsible for our own choices. Of course, then we would have to ask why we are not allowed – even encouraged – to use the simplest and most economical preventative that is available.
    Thank you for your time!
    Lyn McLaughlin

  5. Thanks for your input Carolyn. Good communication skills truly enhances the relationship between veterinarians and their clients!

  6. Thanks for an explanation of why testing needs to be done annually. I have kept my dog on the HW prophylaxis 12 mos. of the year owing to being in a warm climate in the winter where mosquitoes are still active. I have tried 2 vets, paid for wellness exams, and both refuse to renew the HW prescription.

    Both insist that my dog gets tested and I argued: why? if she is on this expensive preventative and I have been diligent about giving it to her on the proper schedule, isn’t she covered? I wish someone had taken the time to explain this to me as I’ll confess to some confusion and ill feelings over this.

    Maggie, my previous dog, was adopted in Central America with active heartworms. Our vet used the slow kill method and “real” ivermectin liquid administered by the milliliter based on weight. She said she found slow kill far better and more effective, even for dogs with serious infestations “on their last legs.”

    Dr. Sheila sold ivermectin in vials with a syringe and explicit directions on how to draw the appropriate amount out of the vial, then remove the needle and either squirt it under the dog’s tongue or put it on bread for the dog to eat.

    She charged all of $3 for a 12 mos. supply. I far prefer it to the
    the fancy packaging of all the expensive name brands here. My husband cured himself of whipworms with ivermectin, readily available w/o a prescription in developing countries.

    Dr. Sheila was US trained, still practices with more than 30 years under her belt. What she lacked in modern equipment, she made up for in practical experience, straight talk, compassion, and a desire to make medicines as available as possible without a huge markup in cost.

    I’ve been back in the US for 2.5 yrs. The longer I am here, the more I miss Dr. Sheila’s simple, pragmatic manner and desire to help any animal, any person, regardless of their ability to pay. I especially appreciated her willingness to discuss or explain until thoroughly understood. She often found abandoned and ill animals, brought them back to health, even if it required surgery paid for from her own pocket, and then found homes for them, or adopted them herself. Yes, I miss her!

  7. I look forward to hearing what the folks at Colorado State have to say!

  8. I look forward to hearing what the folks at Colorado State University have to say!

  9. Thanks for this Dr. Kay. Our Wyatt is going in for his annual exam on Friday and I’m curious to see which tests the folks at Colorado State’s Community Practice vet clinic are doing. I was told they do require annual heartworm testing AND a full exam each year, even if your dog is on preventive year-round, which is great. I’ll ask about the two tests and let you know.