September 14, 2018. American Veterinarian By Maureen McKinney
Today, the American Heartworm Society (AHS) released a revised version of its highly regarded canine heartworm guidelines—the first update since 2014. Reducing disease transmission, clarifying testing recommendations, and avoiding treatment shortcuts are priorities in the new guidelines. Based on the 2016 Triennial Symposium of the AHS, as well as new research and clinical experience, major changes to the guidelines include the following.
Heartworm Prevention: Use of Mosquito Repellents
According to Dr. Rehm, the incidence of heartworm disease in the United States and its territories rose by 21% between 2013 and 2016. Prevention is the cornerstone of any practice’s heartworm management program, yet compliance continues to be problematic and is believed to be 1 of the key roadblocks to reducing incidence rates.
“Right now, roughly two-thirds of pets are not on prevention,” Dr. Rehm said in a previous interview with American Veterinarian®. “Sometimes I think we take for granted that our clients know more about it or have some more parasite common sense, if you will, than they actually do. And we don’t probe enough to find out where our clients are on the learning curve about parasite prevention.”
Year-round use of macrocyclic lactone (ML) preventives and environmental control of mosquito populations continue to be the basis of prevention, but AHS now recommends the use of Environmental Protection Agency–approved mosquito repellents/ectoparasiticides to control the mosquito vector and reduce heartworm transmission in high-risk areas.
“The use of repellents is not a blanket recommendation, nor should repellents ever be used in place of ML preventives,” Dr. Rehm said. “In regions with relatively low heartworm incidence numbers and few mosquitoes, use of heartworm preventives alone can be sufficient to safeguard patients.”
Heartworm Testing: Putting Heat Treatment in Perspective
AHS continues to recommend annual heartworm screening for all dogs over 7 months of age with both an antigen and a microfilaria test. Because of the high sensitivity of these tests, however, the guidelines recommend against routine heat treatment of blood samples for heartworm screening because “it is contrary to the label instructions for commonly used in-house tests and may interfere with the accuracy of results of not only heartworm testing but also the results of combination tests that include antibody detection of other infectious agents.”
Instead, the guidelines recommend that veterinarians consider heat treating serum when circulating microfilariae are detected or when active clinical disease is suspected but an antigen test has returned a negative result.
Heartworm Treatment: Use of Non-Arsenical Protocols
While no changes were made regarding the best treatments for canine heartworm, AHS is using the updated guidelines to double-down on its stance that the society’s current protocol remains the best method for successfully and safely treating infected dogs.
“We get questions from veterinarians about the AHS protocol itself, which includes pretreatment with an ML and doxycycline, followed by a month-long waiting period, then 3 doses of melarsomine on days 60, 90 and 91,” Dr. Rehm explained. “Heartworm disease is a complex disease, and there are no shortcuts to appropriate treatment. Skipping any 1 of these steps can affect both the safety and efficacy of heartworm treatment.”
For dogs that are not candidates for melarsomine treatment, Dr. Rehm noted, alternatives such as the non-arsenical combination of moxidectin and doxycycline may be appropriate. “However, it’s also important for veterinarians to understand that these non-arsenical protocols have serious disadvantages, the most important of which is the length of time required to kill adult worms, during which time heartworm pathology and damage can progress,” he said. “This also greatly increases the length of time the pet needs strict exercise restriction, which is problematic.”
To view the updated guidelines, as well as the AHS feline heartworm guidelines, visit heartwormsociety.org.