DENVER (July 28, 2014) — The Companion Animal Parasite Council (CAPC) has modified its heartworm guidelines to include the recommendation that veterinarians perform yearly microfilariae tests on all dogs, including those on heartworm prevention. This is in addition to the current recommendation that all dogs receive an annual antigen test. CAPC also recommends that veterinarians consider testing all dogs living in areas with a higher risk of exposure to heartworm twice a year.

This position comes in light of new evidence that underscores the importance of using both tests when evaluating dogs with suspected heartworm disease. This information was presented at the American Association of Veterinary Parasitologists Conference that was held in Denver this week.

As reported at the conference, some dogs appear to develop immune complexes that block antigen detection with commercial tests. Some of the CAPC parasitology specialists speculate that one of the reasons for the increase in these “false negative” test results may be related to the practice of placing heartworm-infected dogs on long-term regimens of macrocyclic lactone preventives and antibiotics rather than treating them with an approved adulticide.

“Dogs managed with these protocols may experience prolonged inflammation, which could result in the formation of immune complexes that mask detection of antigen on commercial tests,” according to Susan Little, DVM, PhD, DACVM-Parasit., a CAPC board member and incoming president. “In addition, the presence of persistent microfilaremia in dogs being managed with ‘slow kill’ could contribute to selection for resistance.”

The new study reported that more than 50 percent of dogs on slow kill were false negative on antigen tests. This phenomenon was explored by using heat treatment of samples prior to testing, which would denature the antigen-antibody complexes, allowing detection of the antigen. The test samples analyzed were from the southern region of the United States, where prevalence of heartworm disease is high.

Although this new data is important, at this time, CAPC does not currently recommend routine heat treatment of all samples prior to testing. Heat treatment is not in the manufacturer’s instructions for the antigen tests and will destroy antibodies, invalidating the test for detection of other agents, such as tick-borne infections. Practitioners should seek consultation from a parasitologist or diagnostic laboratory whenever patients are negative on antigen tests but are clinically suspicious for heartworm disease, or from areas with increased heartworm disease prevalence. Further controlled studies will explore the extent to which the slow kill therapy approach contributes to the false negative phenomenon being seen.

Infected dogs should be treated with labeled adulticide (melarsomine dihydrochloride) as soon as medically possible and microfilariae should be cleared. (moxidectin/ imidacloprid is label approved for this purpose.)
“The case for year-round heartworm prevention for pets should be built into a lifelong wellness and prevention program that also reduces the risk for gastrointestinal parasite infection,” said CAPC Board Member Dwight Bowman, MS, PhD. “CAPC encourages veterinarians to rely on their clinical skills in assessing pets for heartworm disease and to consider further diagnostics when encountering a negative antigen test result for suspicious cases.”
CAPC will continue to monitor this evolving situation and will modify its recommendations to veterinarians as needed. CAPC guidelines, which are updated regularly, are recognized as the benchmark in veterinary medicine, creating standards for the optimal control of internal and external parasites that threaten the health of pets and people.