April 2023

In this VETgirl online veterinary continuing education blog, we discuss the incidence of heartworm disease in dogs.

Can We Win the War on Heartworms? The New AHS Heartworm Incidence Survey Offers Clues

by Jenni Rizzo, DVM, President, American Heartworm Society

Visit the American Heartworm Heartworm Society (AHS) website and you’ll see the Society’s vision statement: “A world without heartworms.” While aspirational, this vision is not a fantasy. Heartworms are almost 100% preventable with the products we have today. The challenge is that we as veterinary practitioners are not consistently getting the commitment and prevention compliance we need from pet owners to turn what’s possible into a reality.

Where are the worms?
The AHS recently conducted a nationwide survey of veterinary practices and shelters to gather antigen testing data for the creation of a new Heartworm Incidence Map. This survey was conducted in early 2023 and reflects the results of heartworm tests conducted throughout 2022. Along with submitting their numbers, practitioners also completed a brief, multiple-choice survey on heartworm-related factors they have noted since the last AHS survey was conducted three years ago.

The 2022 survey and map reveal some troubling trends in the three years since the survey was last conducted, as well as insights on how these trends might be reversed. Following are the latest findings:

  • Unfortunately, heartworm incidence is on the rise. Heartworm continues to be diagnosed in all 50 states of the U.S. And while roughly half of the survey’s respondents (53%) said heartworm rates have remained stable since 2019, significantly more practitioners (29%) reported that they saw rates rise in their practice areas than fall (17%).
  • The worst areas are getting worse. It is no secret that the South and Southeastern regions of the U.S. have the highest rates of heartworm incidence. Warm temperatures and precipitation produce conditions ripe for mosquito proliferation in this part of the country. Meanwhile, the continuous presence of untreated heartworm-positive dogs and wildlife that serve as reservoirs for infection means that unprotected pets here are persistently at high risk for infection.

Year after year, the convergence of these factors has resulted in large sections of the South and Southeast appearing dark red in color on the AHS heartworm incidence maps. The lower Mississippi Delta states have historically been hotbeds for heartworm infection, and Mississippi, Louisiana, Texas, Alabama, and Arkansas led the pack in this year’s survey, too. What’s especially concerning is that the size of some of the darkest regions—which indicate the highest incidence—have increased over the past three years. The Gulf Coast between the Southern tip of Texas and the western edge of the Florida panhandle now appears as a solid swath of dark red. Meanwhile, hotspots in the top five states, along with others in Georgia, Florida, and the Carolinas, are also noticeably larger than they were in 2019.

2022 AHS Incidence Map

  • Heartworms are turning up in expected AND unexpected places. Since the AHS began tracking heartworm incidence in 2001, the picture of heartworm infection has changed significantly, with infections spreading into what were once considered non-endemic areas. Animal movement, increased heartworm testing and shifting human populations are considered contributing factors. Where people and their pets move, heartworms invariably follow. For example, the populations of Idaho, Arizona, and Washington State grew, respectively, by 46%, 40% and 30% between 2001 and 2022, and heartworm incidence has clearly risen in those states as well, with the most recent survey showing markedly higher incidence rates in urban centers such as Tucson, Boise, and Seattle.

The same holds true for southern states such as Texas, Florida, and Georgia, which already had relatively high rates of heartworm. These states experienced population increases of 41%, 35% and 30% between 2001 and 2022. Not surprisingly, regions in these states with high rates of heartworm incidence have expanded significantly during the same time frame.

Other phenomena contributing to heartworm spread include the movement of heartworm-positive pets into new areas, whether due to family travel and/or relocation or via the transport of rescue animals. Unfortunately, a history of low infection rates sometimes means that routine heartworm prevention is not the norm in “newer” areas. Meanwhile, practices like backyard watering and the presence of golf courses, fountains and even birdbaths in arid urban areas have rendered these environments more mosquito-friendly than they once were. More than 20 species of mosquitoes are now known to transmit heartworms in the U.S., and some species are capable of breeding in very small areas of standing water.

Can these heartworm trends be reversed?
While these are sobering statistics, additional insights from the AHS survey offer clear evidence that the answer is “yes.” Veterinary respondents who saw heartworm incidence rates rise over the past three years stated that the leading reasons were (1) the influx of heartworm-positive pets into the practice area and (2) poor compliance with heartworm preventive administration. Conversely, among practitioners who saw rates drop, the two most frequently cited reasons were (1) increased numbers of pet owners administering heartworm preventives and (2) improvement in prevention compliance.

Reversing the heartworm incidence does not require new technology; it requires education. Veterinary practice data tell us that more than two-thirds of canine patients today are leaving veterinary practices without a single dose of heartworm preventive. Imagine how quickly heartworm incidence could fall if pet owners simply understood the risk of heartworms to their pets and became motivated to protect them.

In my conversations with clients (I’ve practiced in both Texas and Florida), I strive to make heartworm messages as relatable as possible. If I’m explaining heartworm transmission, I might start with a question like “When was the last time you killed a mosquito in your house?” A little anthropomorphizing can also go a long way. I’ll tell them, “Heartworms grow to the size of spaghetti noodles and live in the blood vessels of the body. Can YOU imagine having spaghetti-sized worms crawling around inside you?”

It’s also important to listen to the client and give them agency. After discussing the pet’s most serious parasitic risks, I let the client choose from among the preventives that meet their pet’s needs while also selecting a product that would be easy and cost effective for them to administer. Sometimes it’s a monthly oral preventive. Sometimes it’s a topical preventive. Sometimes it’s an annual injection that we pair with flea/tick prevention that they can take home. Working with the client’s preferences and budget helps ensure the pet will be protected.

Dr Jenni Rizzo heartworm exam

Finally, let’s circle back to that AHS vision statement. Our leadership will be sticking with “a world without heartworms” because it’s a long-term, aspirational goal that inspires us to continue the drumbeat of heartworm education. In the near term, however, I suggest that we, as veterinary professionals, agree to start by working towards “a world with fewer heartworms”. When we publish our next map three years from now, let’s show we can make a difference.

This VETgirl online veterinary continuing education blog is sponsored by the American Heartworm Society, whose mission is to lead the veterinary profession—and the public—in the understanding of heartworm disease. If you’re one of thousands of busy practitioners who have recently diagnosed a dog with heartworms, visit heartwormtoolkit.com. With this treatment calculator, you can instantly generate a customized treatment timeline for your patient, then email the treatment plan to your client and save it in your medical record. Visit heartwormtoolkit.com today!

This VETgirl online veterinary continuing education blog is sponsored by American Heartworm Society. Please note the opinions in this blog are the expressed opinion of the author(s), and not directly endorsed by VETgirl.

  1. I am hopeful that we, as an industry, can continue to improve client compliance with heartworm prevention through education and really solid communication skills. There are great tips in here for talking to your clients.

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