American Heartworm Society releases 2018 Canine Heartworm Guidelines

Revisions stress risk analysis in prevention, testing, and treatment decisions

The American Heartworm Society (AHS) has released 2018 Canine Heartworm Guidelines, which focus on reducing heartworm transmission, clarifying testing recommendations, and avoiding shortcuts in treatment.

The 2018 guidelines reflect the following updates to the AHS’ recommendations:


Given the highly preventable nature of heartworms, prevention practices are the cornerstone of any practice’s heartworm management program, said Chris Rehm, DVM, president of the AHS. “Unfortunately, the latest AHS survey found that incidence has been trending up rather than down, with the number of infected dogs per clinic rising by 21 percent in the U.S. and its territories between 2013 and 2016.”

Environmental and climatic changes, as well as the relocation of microfilaremic dogs and the expansion of microfilaremic wild canid territories are considered to be contributing factors to both incidence numbers and the spread of heartworms to areas once considered nonendemic, according to the AHS.

“For these reasons, we continue to stress the importance of year-round administration of macrocyclic lactone preventives, along with practical steps to reduce mosquito exposure, such as eliminating standing water on the property and keeping pets indoors during peak mosquito times,” said Dr. Rehm. “Year-round prevention is the single most important step owners can take to reduce the risk of heartworms to their pets.”

In an update to its prevention recommendations, the guidelines state that veterinarians should also consider the use of EPA-approved mosquito repellents/ectoparasiticides to control the mosquito vector and reduce heartworm transmission if the risk of heartworm transmission is high.

“The use of repellents is not a blanket recommendation, nor should repellents ever be used in place of macrocyclic lactone preventives,” said Rehm. “In regions with relatively low heartworm incidence numbers and few mosquitoes, use of heartworm preventives alone can be sufficient to safeguard patients. Where mosquito proliferation and heartworm incidence numbers are high, however, additional measures may be warranted on either a year-round or seasonal basis. Individual veterinarians are in the best position to assess the risk for their practices as well as for individual patients.”


Studies have been conducted over the past decade to better understand the potential for heat treatment of serum samples to unmask blocked antigen, raising questions about optimal heartworm testing methods, stated the AHS. While noting that further study of this effect is warranted to better understand the mechanisms in play, the high sensitivity of antigen and microfilaria tests make heat treatment unnecessary for routine heartworm screening, according to the guidelines.

“This doesn’t mean there isn’t a time and place for this practice in heartworm diagnosis,” Rehm said. The AHS Guidelines recommend veterinarians consider heat treating serum when either the presence of circulating microfilariae is detected or the veterinarian suspects active clinical disease in the absence of a positive antigen test.


“One of the most frequent questions we hear—especially from pet owners—is about the need from adulticide treatment for infected dogs. It’s understandable when you consider the expense of treatment and the need for multiple veterinary visits,” said Rehm. “We also 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 three doses of melarsomine on days 60, 90, and 91.

“Heartworm disease is a complex disease, and there are no shortcuts to appropriate treatment,” he added, noting that the AHS protocol was designed to kill adult worm infections with minimal complications while stopping the progression of disease. “Skipping any one of these steps can affect both the safety and efficacy of heartworm treatment.”

Nonarsenical treatment protocols, including the “moxy-doxy” combination of moxidectin and doxycycline, have been studied in both Europe and the U.S. to better understand how to manage heartworm-positive dogs that aren’t candidates for melarsomine treatment, Rehm said.

“Because some dogs are simply not candidates for adulticide treatment, there is a place for alternatives such as these,” he said. “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. This also greatly increases the length of time the pet needs strict exercise restriction, which is problematic.”

In a 2017 AHS survey of approximately 5,000 veterinarians, three-quarters of practitioners stated they follow the AHS heartworm guidelines.

“As the primary heartworm resource for veterinary practitioners and the public, it’s our job to continually assess new information on heartworm management and adjust our guidelines accordingly,” Rehm said. “Our hope is that the 2018 updates will clarify what veterinarians can do day-to-day to reduce the threat of this significant disease.”

To access the complete set of AHS canine and feline heartworm guidelines, visit

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