Symbiosis in parasitology: Heartworm recommendations updated


The National Center for Veterinary Parasitology and Companion Animal Parasite Council address emerging needs in understanding and managing heartworms and other complex parasites.

Like a lot of specialists, I love nothing more than learning something new or different about my passion-parasitology. But I also realize that the complexity of parasite systems and the seemingly relentless barrage of information can be confusing. Fortunately, a couple of my favorite organizations-the National Center for Veterinary Parasitology (NCVP) and the Companion Animal Parasite Council (CAPC)-are doggedly working to enhance our profession's understanding of clinical parasitology.

Getty ImagesThe NCVP is a relatively new organization that supports clinical veterinary parasitology research, diagnostics, consulting, and advanced training of residents and other scientists. The CAPC has been around for several years, fostering understanding and adoption of best practices in managing companion-animal parasites. The two groups complement one another, working to solve key issues in veterinary parasitology and translating those research findings so that veterinarians can use the information in practice.

New research, new guidelines

As an example of this collaborative effort, NCVP research recently documented that antigen-antibody complexes may prevent detection of heartworm antigen in some samples, regardless of the test used. This finding is not new and was discussed in the research literature through the 1980s, but because modern antigen tests are so highly sensitive and specific the phenomenon had been largely forgotten until recently. Interestingly, antigen-antibody complexes appear to be particularly common in dogs receiving slow-kill heartworm disease treatment. In one study, more than half the dogs converting to antigen negative status while receiving slow-kill treatment had false negative results and were found to be antigen positive after heating and retesting of the samples.

After learning about this research, CAPC updated their canine heartworm recommendations (available at, stressing that all dogs should continue to be tested at least annually for heartworm but that care should be taken in interpreting test results in dogs receiving slow-kill treatment. CAPC also issued a press release ("CAPC modifies its heartworm guidelines to recommend both antigen and microfilariae testing," also available at to the veterinary community.

It is important to realize that the antigen tests haven't changed-they are every bit as reliable as they have been for many years. What has changed is our understanding of what happens when dogs are treated with the slow-kill method or have other conditions that cause prolonged, systemic inflammation. Both CAPC and the American Heartworm Society recommend against slow-kill treatment. This new data from the NCVP, which suggests that slow kill may cause antigen tests in some dogs to convert from positive to false negative results without necessarily eliminating the heartworms, helps us understand another piece of the heartworm parasite puzzle.

Progress in the making

Parasites such as heartworms present real challenges to achieving our profession's goal of healthy animals and healthy people. But heartworms are by no means the only parasites our profession combats. For comprehensive parasite information, including images, videos, and recommendations for control, please visit and Thanks to the efforts of these active parasitology organizations, we are making progress.

Susan E. Little, DVM, PhD, DACVM (parasitology), is a Regents Professor and Krull-Ewing Chair in Veterinary Parasitology at Oklahoma State University. She serves as co-director of the National Center for Veterinary Parasitology and president-elect of the Companion Animal Parasite Council.

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