Canine Heartworm Disease Incidence on the Rise

Canine heartworm disease is prevalent throughout the United States. The original heartworm prevention recommendation was daily diethylcarbamazine. Prevention then shifted to monthly administration of the macrocyclic lactone (ML)—containing drugs ivermectin and milbemycin oxime, which gained FDA approval in 1990.

Despite the many available ML-containing heartworm preventives and consistent efforts by the American Heartworm Society (AHS) and Companion Animal Parasite Council (CAPC) to promote heartworm testing and prevention, millions of dogs in the United States remain unprotected from heartworm disease. Of concern, a study investigating complaints submitted to the FDA in the early 2000s about the efficacy of heartworm preventives determined that Dirofilaria immitis is developing resistance to MLs.

Since 2001, AHS has conducted a triennial survey of US heartworm incidence. In the most recent survey, released in 2016, AHS reported a troubling 21.7% increase in the average number of positive heartworm cases per veterinary clinic since 2013.

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For the current study, published in Parasites & Vectors, a research team determined whether heartworm test results compiled by CAPC aligned with the 2016 AHS survey results.

Analysis

Using CAPC’s online maps, the researchers collected data on the total number of heartworm tests performed and total number of positive heartworm test results from 2013 to 2016, with the caveat that “these data do not represent the total number of positive tests,” according to CAPC. Special attention was paid to the 12 southeastern states that had the highest number of submitted ML efficacy complaints: Texas, Oklahoma, Arkansas, Louisiana, Missouri, Tennessee, Mississippi, Alabama, North Carolina, South Carolina, Georgia, and Florida.

Researchers also collected data from Vetstreet, LLC on prescribing habits for heartworm preventives from 2013 to 2016.

Results

Nationally, the total number of heartworm tests performed increased by 33%, from 7 million to 9.2 million. The increased number of positive test results, from about 78,000 to 119,000, represented a 53% increase, indicating that the rate of positive tests outpaced that of tests performed. The incidence of positive tests increased by 15.3%, from 1.1% to 1.28%, aligning well with the 21.7% increase in positive cases reported in the 2016 AHS survey results, the researchers noted.

Within only the southeastern United States, the incidence of positive tests increased by 17.9%, from 2.2% to 2.6%. Several states had notable trends in positive test incidence:

• Florida’s incidence decreased by about 5%.
• Mississippi had the highest incidence (8%) in 2013, but the incidence decreased by 10% in 2016.
• Missouri had the largest percent increase (53%) in incidence, from 0.99% to 1.5%.

An increased number of dogs received heartworm preventives (19.3 to 20 million), yet the proportion of dogs receiving preventives significantly decreased each year, representing a troubling downward trend in heartworm prevention.

Bringing it Together

Study results indicate a slow increase in positive heartworm tests nationally, despite concerted prevention efforts. The incidence of positive tests in the southeastern United States may be higher than the national incidence for several reasons, including poor guideline compliance and heartworm resistance to MLs. Importantly, movement of dogs across the country due to natural disasters in the southeast (eg, Hurricane Harvey) may increase heartworm incidence faster than by natural mosquito transmission.

Researchers advised veterinary professionals to continue promoting year-round heartworm prevention with ML-containing preventives and annual heartworm testing.

Dr. Pendergrass received her Doctor of Veterinary Medicine degree from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, a medical communications company.