ACVIM 2017: An Alternative Treatment for Heartworm Disease

For dogs with heartworm disease, the American Heartworm Society recommends adulticide treatment with melarsomine along with doxycycline and a macrocyclic lactone. However, melarsomine is not always accessible (in some countries it is unavailable), and the cost of this protocol can be prohibitive for some pet owners.

At the 2017 ACVIM Forum in National Harbor, Maryland, Marisa Ames, DVM, DACVIM (Cardiology), discussed an alternative to the melarsomine protocol: topical treatment with moxidectin/imidacloprid plus oral doxycycline. Dr. Ames stressed that melarsomine is the recommended treatment but acknowledged that other options are sometimes needed.

Moxidectin/imidacloprid (Advantage Multi, Bayer Animal Health) is labeled as a heartworm preventive and microfilaricide for dogs. Compared with macrocyclic lactones (such as ivermectin), said Dr. Ames, moxidectin/imidacloprid reaches higher concentrations in blood and tissues and clears heartworms more quickly.

Dr. Ames’ team enrolled dogs with positive heartworm antigen tests in a prospective study of moxidectin/imidacloprid plus doxycycline. Dogs included the study generally had no or mild clinical signs. Dogs received topical moxidectin/imidacloprid at the labeled preventive dose but at an increased frequency of every 2 weeks for 3 months, then once a month. Dogs also received oral doxycycline for 15 days. Dogs were not restricted from exercise during the study.

The research team conducted heartworm antigen tests on days 90, 120, and 150 and every 2 months thereafter. Conventional antigen tests that yielded negative results were repeated with heat-treated serum. Owners of dogs whose antigen tests were positive after day 210 were offered melarsomine treatment.

Of the 22 dogs enrolled so far, said Dr. Ames, 72% had negative conventional antigen tests by 300 days and 91% had negative antigen tests by 1 year. Some of the dogs with negative conventional antigen test results had positive results with heat-treated serum; these results tended to convert to negative after a few months.

Some dogs developed a cough during the study and were treated with corticosteroids and/or cough suppressants. One dog vomited, she said, although it was not clear whether this was related to moxidectin/imidacloprid. In response to an audience question about reactions to dying microfilariae, Dr. Ames suggested that dogs be monitored at home or in the clinic for the first dose or 2.

According to the preliminary data, concluded Dr. Ames, moxidectin/imidacloprid plus doxycycline appears to be safe and well tolerated by dogs with mild to moderate heartworm infection and may be an acceptable alternative when melarsomine cannot be used.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.