The State of Heartworm Incidence in the U.S.
The 2019 AHS Heartworm Incidence Survey shows that heartworms maintain a stubborn hold in the United States.
The Heartworm Hotline column is presented in partnership between Today’s Veterinary Practice and the American Heartworm Society (heartwormsociety.org). The goal of the column is to communicate practical and timely information on prevention, diagnosis, and treatment of heartworm disease, as well as highlight current topics related to heartworm research and findings in veterinary medicine.
Changes in weather patterns. Lapses in preventive medication compliance. Pet relocation. These factors and more were cited in a recent survey of U.S. veterinarians as factors contributing to the rise and fall of heartworm incidence in their practice areas.
To help veterinary professionals, shelter personnel, and pet owners understand heartworm trends in their areas, the American Heartworm Society (AHS) began tracking U.S. heartworm incidence in January 2002. Since then, the AHS Heartworm Incidence Survey has been conducted every 3 years, using heartworm testing data submitted by veterinary practices, reference laboratories, and animal shelters. Following the analysis of survey results, a U.S. heartworm incidence map is generated to provide a visual representation of the spread and severity of heartworm infections.
The AHS released the results of the most recent survey, which reflects testing results from the 2019 calendar year. Just under 6000 veterinary practices and shelters submitted data from more than 5.5 million pets. The 2019 map (Figure 1) was unveiled in April 2020.
How Many Pets in the U.S. Are Heartworm-Positive?
While it is possible to use the survey data to make rough projections regarding heartworm prevalence, it is impossible to know exactly how many pets in the U.S. are infected with heartworms at any given time. An important factor is that heartworm diagnosis requires one or more blood tests that must be conducted in a veterinary clinic and/or reference laboratory, which limits diagnoses to patients seen in clinics and animal shelters. Dogs and cats not seen by veterinarians and/or not tested for heartworms during annual veterinary visits may be infected but go unrecognized for years. Unfortunately, the lack of veterinary care translates to a lack of heartworm prevention, and these pets are among those most likely to be positive, infecting mosquitoes and other dogs in the neighborhood.
While acknowledging these important caveats, the AHS hypothesized a conservative estimate of heartworm prevalence via the following calculations:
- Of the thousands of U.S. veterinary practices that report testing data to the AHS, a large proportion have reported data from their practice records year after year.
- Using an estimate of client base and combining these data with dog numbers from the American Veterinary Medical Association, it is possible to generate a rough estimate of heartworm cases for the U.S. and, more importantly, to track trends.
- In 2001, the national estimate for heartworm-positive dogs was just under 900 000, growing to 1 million in 2010 and 1.2 million in 2016. A rough comparison of the 2019 map (Figure 1) with the 2016 map (Figure 2) suggests that there has been little significant change in heartworm incidence at these time points, although variance occurs in the years between surveys.
- The 2019 estimate dropped slightly to approximately 1.1 million infected dogs. It is believed this change was triggered by drier-than-normal conditions in the western U.S. in 2017 and 2018 that led to lower mosquito populations.
Key Findings of the 2019 Heartworm Incidence Survey
Almost 6000 veterinary practices and shelters participated in the AHS survey in early 2020. Veterinarians reported numbers of patients tested for heartworm infection, as well as the number of positive cases from that total.
- Heartworm infection was diagnosed nationwide. No state in the U.S. is heartworm-free, according to the AHS survey. While Alaska has reported positive dogs in every AHS survey since 2001, to our knowledge, none of those cases occurred in native dogs without a history of travel outside the state. It was revealed the top 5 states for heartworm incidence were Mississippi, Louisiana, South Carolina, Arkansas, and Alabama—all states that have been in the top 10 states since the AHS began tracking incidence data in 2002. Rounding out the top 10 states were Texas, Tennessee, Georgia, North Carolina, and Oklahoma. Among the top 10 states, only Alabama, Louisiana, and Texas saw decreases in the average number of dogs diagnosed per practice, while increases in the average number of cases were noted in the other 7.
- In addition, participants responded to a brief survey designed to provide insights on factors behind the data. From the survey, the following findings were identified:
- While heartworm incidence dropped slightly in some parts of the country in 2019, more veterinarians overall noted an upward trend in heartworm incidence over the previous 3 years than a downward trend. Of respondents participating in the AHS survey, 26% reported seeing more heartworm cases in 2019 versus 2016, while 16% reported a decline in their practice areas. The remaining 57% perceived the prevalence in their areas to be “about the same.” These numbers were similar in 2016, when 23% of participants perceived a rise in incidence and 19% saw a drop.
- Among veterinarians who believe heartworm incidence increased, leading reasons cited were an increase in heartworm-positive pets coming from heartworm-endemic areas, poor compliance among pet owners, and weather trends that caused an increase in mosquitoes. Given the mobility of society today, as well as the need to relocate animals due to natural disasters or animal homelessness, the transport of pet animals is a fact of life in U.S. veterinary practices today. To help reduce the transmission of heartworm disease in animals, the AHS partnered with the Association of Shelter Veterinarians (ASV) in 2018 to develop recommendations for the safe transport of heartworm-positive pets (Figure 3).
- Compliance is the leading factor associated with downward trends in heartworm incidence. Among veterinarians who saw cases drop over the past 3 years, the top 2 reasons they gave were (1) improved compliance, which was defined as more year-round usage of preventives and more doses given on time, and (2) more users, defined as more pet owners giving preventives to their pets.
- Veterinarians are seeking educational resources on heartworm disease. According to the AHS survey, 82% reported that they follow the AHS guidelines on prevention, diagnosis, and treatment of heartworm disease, while more than half of practitioners utilize resources on the AHS website. In most cases, the utilization of AHS resources was higher than that reported in the 2013 and 2016 surveys (Table 1).
The results of the AHS Incidence Survey represent a mix of both bad and good news. On one hand, heartworms maintain a stubborn hold in the U.S., with the southeastern U.S. leading the nation in incidence. On the other hand, it is clear that there is a straightforward answer to this: persuade more pet owners to use preventives and convince them to protect dogs and cats year-round—with no lapses. In addition, adherence to the AHS/ASV recommendations on animal transport by veterinarians, shelters, and rescue organizations can help minimize the chance that heartworm-positive pets moved from one part of the country to another do not become reservoirs for heartworm infection in their new homes.
The 2019 AHS Heartworm Incidence map, as well as maps from 2001 to 2016, can be downloaded at heartwormsociety.org/veterinary-resources/incidence-maps.