American Heartworm Society releases new heartworm incidence map

Three out of four veterinarians in the U.S. diagnosed patients with heartworm in 2013, and heartworm-positive pets were reported in every state in the country, according to the latest Heartworm Incidence Survey conducted by the American Heartworm Society (AHS). And while weather conditions favorable to mosquito proliferation and transport of infected dogs were viewed as contributing factors, client compliance in giving heartworm preventives was identified as the clear tipping point for incidence rates.

The AHS survey has been conducted every three years since 2001 in order to track trends in heartworm incidence and inform the veterinary profession and public about the need for heartworm prevention. The latest survey, which was fielded in March of 2014, represented 2013 testing data from 3.5 million patients from more than 4,300 veterinary clinics and shelters.

“Our survey determined that heartworm disease continues to be widespread in many parts of the country,” said AHS president Stephen Jones, DVM. “The silver lining to this unwelcome news is that a simple solution is in the hands of veterinarians and pet owners: getting more pet owners to give their pets heartworm preventives year-round.”

The American Heartworm Society recommends heartworm prevention 12 months a year and heartworm testing once a year. Seventy-two percent of veterinarians surveyed said they follow the AHS guidelines for prevention, testing and treatment.


Geographical trends in heartworm incidence

While veterinarians from the Gulf and Mississippi Delta states reported the highest incidence rates of heartworm disease in dogs in the country, relatively high rates were also noted in areas less well known for heartworm risk. These included the Upper Midwest—especially the states of Michigan, Illinois, Indiana and Ohio—and the western states of Arizona and California.

The top ten states in average rate of heartworm-positive cases/clinic in 2013 were (1) Alabama, (2) Louisiana, (3) Mississippi, (4) Texas, (5) Arkansas, (6) South Carolina, (7) North Carolina, (8) Tennessee and Georgia (tie) and (10) Florida. In the 2010 AHS survey, Louisiana led the nation, followed by Mississippi and Texas, while Alabama was number 5 in the ranking.

Factors influencing heartworm trends

Veterinarians that supplied heartworm data were also surveyed about the significance of heartworm disease in their practice areas and factors contributing to disease incidence. Among the factors evaluated were:

  • Incidence trends. Sixty percent of veterinarians stated that the incidence of heartworm disease in their practice areas had held steady since the last AHS survey was conducted three years ago. A roughly equal proportion of veterinarians said that incidence was up (19 percent) and down (21 percent) in their practice areas.
  • Importance of the disease. One-third of veterinarians stated that heartworm is either a significant disease problem (18 percent) or one of the most serious diseases affecting their patients (15 percent). Just over half of surveyed practitioners reported they see heartworm cases “occasionally,” while only 13 percent said they never see heartworm-positive patients.
  • Contributing factors. Veterinarians who witnessed an upward or downward trend in heartworm disease reported on multiple factors related to those trends. Findings included:
    • Poor compliance by clients, defined as not administering preventives year-round or skipping doses, was the most common factor (61 percent) cited by veterinarians who witnessed an upward trend. An influx of infected rescue dogs to local areas was also noted by approximately 40 percent of veterinarians in this group.
    • Heartworm preventive lack of efficacy (LOE) was not considered a major factor; among the 19 percent of veterinarians who reported that heartworm incidence was up, less than 6 percent of these cited LOE as a factor.
    • Among veterinarians who noted a downward trend in heartworm incidence, the leading reason given was more pet owners administering preventives (74 percent), followed by improved compliance (63 percent).

 “Heartworm is a devastating but preventable disease. If we make consistent, year-round prevention in dogs and cats a priority, we will see incidence go down,” concluded Dr. Jones.

For more information on heartworm disease and the AHS Heartworm Incidence Survey, visit

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