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AHS Heartworm Hotline, Parasitology

Beyond the Map: The State of Heartworm Incidence in the United States

Beyond the Map: The State of Heartworm Incidence in the United States
Chris Rehm, DVM, President, American Heartworm Society Doug Carithers, DVM, American Heartworm Society Board Member and Symposium Co-Chair
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.
How many pets in the United States are infected with heartworms? The truth is that no one knows. Because heartworm diagnosis requires one or more blood tests that must be conducted in a veterinary clinic and/or testing laboratory, diagnosis is limited to patients seen in veterinary hospitals 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.


While an exact number of infected pets cannot be pinpointed, the American Heartworm Society (AHS) has tracked the incidence of heartworm diagnosis in the United States for the past 15 years. The first AHS Incidence Survey was conducted in 2002 and focused on heartworm diagnoses made in veterinary hospitals during the 2001 calendar year, including data from tests conducted both in-clinic and in reference laboratories. The AHS has continued to survey US veterinary practices every 3 years and, since 2007, has added animal shelters to the survey. The survey results are used to create the AHS Heartworm Incidence Map every 3 years. The 2016 map (Figure 1) was unveiled in April 2017.

FIGURE 1. 2016 AHS Heartworm Incidence Map.

Thousands of veterinary practices and shelters participated in the AHS survey in 2016, and because many have reported data survey after survey, the AHS has been able to use the information to track trends in participating practices, as well as the trajectory of heartworm incidence. This is especially important, given that in the past several decades, heartworms have spread from highly endemic regions of the United States, such as the Southeast and Mississippi Delta regions, to states once considered “non-endemic,” including California and Arizona. Because it is not possible to know the exact number of heartworm infections in the United States, the AHS survey focuses on incidence versus prevalence. Nevertheless, the AHS has been able to venture a conservative estimate of heartworm prevalence via the following calculations:
  • Of the thousands of 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 United States and, more importantly, to track trends.
  • Since 2001, the national estimate has increased from nearly 900,000 heartworm-positive dogs to 1 million in 2010 to 1.2 million in 2016.


Almost 5000 veterinary practices and shelters participated in the AHS survey in early 2017; the incidence numbers reported are based on the 2016 calendar year. Participating veterinarians reported numbers of patients tested for heartworm infection, as well as the number of positive cases from that total. In addition, participants responded to a brief survey designed to provide information on factors behind the data. The following findings were identified: Heartworm incidence is up. The incidence numbers reported by participants in the 2016 AHS Incidence Survey indicated that the average number of positive cases per veterinary clinic has been climbing, rising by 21.7% over 2013 numbers; this was due to a small increase in both the absolute number of positive cases per reporting clinic and the percent positive per clinic. The average number of heartworm-positive states rose in 26 states, as well as the District of Columbia; stayed the same in 3 states; and dropped in 22 states (Figure 2). Of survey respondents, 23.3% reported seeing more heartworm cases in 2016 compared with 2013, while 19.8% reported a decline in their practice area. Veterinarians who reported an upward trend in heartworm diagnoses cited several reasons for the change, according to the AHS opinion survey. Almost half (47.8%) cited failure to give preventives, skipping doses. or failing to give preventives year-round as contributing factors. Other factors believed to contribute to incidence increases in certain regions were weather conditions conducive to heartworm transmission in 2016 (weather may also have been a factor where numbers of heartworm-positive dogs fell; regions such as the Southwest and states such as Alabama suffered from drought in 2016) and the movement of infected dogs into practice areas. Insufficient efficacy of heartworm preventives was viewed as a factor by only 3.3% of respondents who saw incidence rise.

FIGURE 2. Heartworm incidence trends: 2013 to 2016

  • Heartworm infection was diagnosed nationwide. No state in the country is heartworm-free. According to the AHS survey, the top 5 states in heartworm incidence were Mississippi, Louisiana, Arkansas, Texas, and Tennessee—all states that have been in the top tier since the AHS began tracking incidence data in 2001. Rounding out the top 10 were South Carolina, Georgia, North Carolina, Alabama, and Florida. 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 states where few cases have historically been diagnosed (for instance, heartworm-positive dogs in Alaska have, thus far, been non-native), many practitioners reported that the infected dogs originated from out of state. This is an important point, however, because wherever positive dogs exist, the potential for mosquitoes becoming infected and, in turn, infecting other animals also exists. Cool or dry weather does not necessarily eliminate transmission; it simply slows the process.
  • Compliance is key to reducing heartworm incidence. While it was disappointing to see the average number of heartworm-positive cases per clinic rise between 2013 and 2016, the good news is that the factors involved are controllable. Among veterinarians who reported a drop in heartworm incidence since the 2013 survey, 64% attributed the change to owner behavior, including increased usage of heartworm preventives and improved owner compliance.
  • Veterinarians are using AHS resources. According to the AHS survey, 77% of veterinarians reported that they follow the AHS guidelines on prevention, diagnosis, and treatment of heartworm disease, while roughly half of practitioners use resources on the AHS website. In most cases, the utilization of AHS resources was higher than that reported in the 2013 survey (Table 1).
TABLE 1 Veterinary Usage of AHS Resources Percentages are of Respondents by Survey Year [ultimatetables 18 /]  


The results of the AHS Incidence Survey represent a mix of bad and good news. On one hand, heartworm incidence has increased in a number of states, especially those states in the most heartworm-endemic areas of the Southeast, Mid-South, and Delta regions. On the other hand, it is clear that there is a straightforward answer to this: (1) convince more pet owners to use preventives and (2) convince them to protect dogs and cats year-round—with no lapses. The AHS Heartworm Guidelines recommend annual heartworm testing and year-round heartworm prevention for all canine and feline veterinary patients. As the results of this survey suggest, compliance with these guidelines has the potential to dramatically alter the trajectory of heartworm incidence in pets. Chris Rehm, DVM, President of the American Heartworm Society (AHS) board of directors, is a veterinarian and graduate of Auburn College of Veterinary Medicine. He began practicing in Mobile, Alabama, more than 30 years ago. What began as one veterinary clinic, Rehm Animal Hospital, PC, has grown to 4 American Animal Hospital Association–certified hospitals in 2 counties in lower Alabama. He employs 13 veterinarians and more than 70 support staff.     Doug Carithers, DVM, has held various roles with Merial (now Boehringer Ingelheim) since leaving private mixed-animal practice in 1993. He is responsible for North American postapproval studies and publications, accounting for more than 115 clinical and field studies involving nearly 20,000 animals. In addition, as a committee and board member for the AHS, he conducted the 2001, 2004, 2007, 2010, 2013, and 2016 AHS heartworm incidence surveys. He has authored/co-authored over 20 peer-reviewed papers and 30 scientific articles, and co-authored a best-selling companion animal parasitology atlas. He has edited over 10 book chapters and acted as guest editor of special editions of refereed journals. Dr. Carithers lectures nationally and internationally and has provided over 900 hours of CE. He is known internationally for his interest and expertise in parasitology, currently serving as an officer of the AHS (Symposium Chair) and of the American Association of Veterinary Parasitologists (Secretary Treasurer), as well as a board member of the National Center for Veterinary Parasitology.