Heartworm guidance could change

Diagnostic, vector control, and adulticide protocols might be adjusted
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In an interview following the recent American Heartworm Society triennial symposium, Dr. Christopher J. Rehm, president, said he expects the AHS executive board will discuss whether changes need to be made to the AHS recommendations for veterinarians. Among the issues on the table are diagnostic testing for heartworm disease, vector control, and adulticide protocols for treatment of heartworm infection. He described any potential recommendation changes as adjustments versus wholesale changes but noted that the symposium included a number of compelling presentations from leading experts on heartworm disease.

The society hosted the 2016 symposium this September in New Orleans. Findings presented during the meeting will be published in the journal Parasites and Vectors in early 2017, Dr. Rehm said. While the AHS awaits publication of the symposium topics, he said, the board will discuss whether the new information affects the AHS guidelines and how to best use the information in the society’s mission of education.

Dr. Christopher J. Rehm (Courtesy of the American Heartworm Society)

An AHS announcement indicates the meeting’s lectures provided information on the connections between persistence of heartworm disease and factors such as climate, vectors, diagnostic challenges, prevention practices, and treatment protocols. Dr. Clarke Atkins, co-chair of the symposium program committee, said in the announcement that heartworm disease remains one of the most important and complex diseases addressed in veterinary medicine, and that that complexity was reflected in the presentations.

Dr. Rehm became AHS president during the meeting. He is the owner of Rehm Animal Clinics in southern Alabama.

Heartworm testing

The AHS guidelines currently advocate use of microfilaria testing in tandem with heartworm antigen testing, especially when veterinarians suspect their patients have had intermittent heartworm preventive use or the patient’s preventive history is unavailable, as in the case of dogs from shelters, rescued or found pets, or any new pet without a history of consistent heartworm preventive usage.

“It has come to light that in some dogs infected with heartworms, antigen–antibody complexes may lead to false-negative antigen test results,” the guidelines state. The guidelines cite a study (Vet Parasitol 2014;206:67–70) conducted on shelter dogs in the southeastern United States that identified this occurring at a rate of 7.1 percent. Dr. Rehm said the problem was discovered when tests were repeated, with the addition of heat-fixing or immune complex dissociation techniques.

“Interestingly,” he said, “some of the dogs with false-negative antigen results were found to be microfilaria-positive, which implies that some infections can be identified with microfilaria testing even when antigen–antibody complexes lead to false-negative antigen tests.”

Unstained microfiliariae

Dr. Rehm said research indicates veterinarians can eliminate some of those false negatives through immune complex dissociation techniques, and he expects more information on use of those techniques will be presented at the North American Veterinary Community Conference in February 2017. But he expressed doubt that those techniques are ready for clinical practice, since they measure antigen presence without distinguishing whether the heartworms are alive.

“The AHS board will be discussing ICD, what the results may mean, and when it may be appropriate to use such techniques,” he said.

Further study also could help determine whether macrocyclic lactone administration is connected with immune complex development, he said.

“I think that it’s obvious, with the studies that have been done, that more immune studies need to be done in heartworm disease and how the immune system works,” he said.

Dr. Rehm added that the AHS may consider placing more emphasis on its recommendations for routine microfilaria testing. He noted that one presenter, Dr. Ray Kaplan, had indicated an absence of microfilaria is likely to correlate with an absence of heartworm resistance to preventives.

“I know the guidelines suggest microfilaria testing, but we need to discuss the need to encourage and do more of it. We need the profession to equate heartworm testing with both antigen and microfiliaria testing and consistently perform both every time,” Dr. Rehm said.

Dr. Rehm performs an eye examination on his patient Winston. (Photo by Laurel Fleming)

In July 2014, the Companion Animal Parasite Council changed its guidelines to recommend veterinarians perform annual microfilaria tests on dogs in addition to annual antigen tests because of the potential for false negatives on antigen tests. Similar to the AHS guidance, the CAPC announcement notes that some dogs appear to develop immune complexes that block antigen detection through commercial tests, and suggests that these false negatives could be related to the practice of treating dogs long-term with macrocyclic lactone preventives and antimicrobials, rather than with adulticide treatments. Dr. Rehm stressed that melarsomine is the only treatment for adult heartworm disease, and the use of macrocyclic lactones and doxycycline is recommended by the AHS as a preparation for melarsomine treatment—not as an alternative to it.

Heartworm treatment, doxycycline, and vector control

Dr. Rehm said he expects the AHS board also will discuss the society’s guidelines regarding heartworm disease treatment.

When a dog is positive for heartworm infection, the AHS guidelines recommend administering macrocyclic lactones in conjunction with doxycycline for two months before starting melarsomine treatment. This regimen is intended not only to prevent new infections but also to eliminate susceptible larvae while 2- to 4-month-old worms become susceptible to adulticide therapy. It also allows Wolbachia organisms to diminish and adult worm biomass to decrease, both of which appear to decrease pulmonary complications during treatment, he said. Restricted activity remains an extremely important recommendation for decreasing treatment complications.

Dr. Rehm also anticipates discussion on whether more specific guidance and information are needed on vector control. The symposium included presentations on factors that increase the prevalence of heartworm infection among mosquito populations, how microclimates affect mosquito populations, and the potential to reduce infection through multimodal approaches.

In conclusion, Dr. Rehm noted that prevention is the best heartworm intervention. He noted that the disease can have devastating effects, causing lung lesions that can be detected more than a decade later in those dogs that survive.

Related JAVMA content:

Report: Heartworm disease decreasing, diabetes increasing (June 15, 2016)

Heartworm preventive resistance is real (Nov. 1, 2013)