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. It’s now believed that the antigen test alone is not enough. 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. It’s not that antigen tests are any less reliable than they used to be; rather, heartworm disease and heartworm treatments are better understood now. For example, the “slow-kill” treatment, which I will discuss later in this blog is now recognized as having the potential to change a positive into a false negative result1.
As a result, the Companion Animal Parasite Council (CAPC) and the American Heartworm Society (AHS) recently revised their guidelines pertaining to annual heartworm testing. They now recommend that microfilaria testing, along with antigen testing, be performed. Combined, these two tests reduce the possibility of missing heartworm-positive dogs.
Heartworm antigen testing detects protein particles that are produced within the reproductive tract of adult female heartworms. Here are some reasons why a dog with heartworm disease might 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 may 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 to kill adult heartworms, 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 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 the microfilariae are considered to be the “contagious stage” of heartworm disease. This is 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 heartworm testing 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.
If you have any questions or concerns, you should always visit or call your veterinarian -- they are your best resource to ensure the health and well-being of your pets.