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.
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.
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.
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?
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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