New heartworm incidence map shows shifting hot spots

Texas is leading the US in heartworm incidence for the first time, according to results from a new survey by the American Heartworm Society (AHS). Overall, incidence rates remained steady over the past 3 years, with the Southeastern states and Gulf Coast region continuing to see some of the highest rates of heartworm infection in the nation.¹

Marisa Ames, DVM, DACVIM (Cardiology), president of the AHS and an associate professor of cardiology at the University of California, Davis, School of Veterinary Medicine in California, said that seeing Texas lead the top areas of heartworm incidence is “very much of interest” but not a huge surprise. ”Texas has consistently been in the top 10 states for heartworm incidents in the past 2 decades of this survey, so in that case, we know that they have the climatological and geographic setup for heartworm disease,” Ames said in a dvm360 interview.

Every 3 years, the AHS conducts its nationwide canine heartworm incidence survey, which collects data from more than 1000 veterinary practices and shelters. The data are derived from heartworm antigen tests run over the previous year. The current survey was conducted in early 2026 using data from heartworm testing conducted throughout 2025.¹ The AHS incidence map—created using data from the triennial survey—was last updated in 2023.²

The 2023 heartworm incidence survey, based on 2022 data, found that infection prevalence was notably high in the Mississippi Delta. This trend was echoed in the new survey, which showed that Louisiana, Arkansas, and Mississippi ranked among the top 5 states with the highest heartworm incidence, along with Texas and South Carolina. Tennessee, Alabama, North Carolina, Florida, and Georgia rounded out the top 10.¹

According to Ames, human movement and climate changes are potential factors for why Texas now leads US states in heartworm disease incidence. “It was predominantly eastern Texas…. More suburban areas [are] spreading out into new areas where incidence was already high, but now we have a lot more pet animals in areas like this in the Southeast. So, potentially, it's the movement of people,” Ames said.

Ames also noted that because of the length of the heartworm life cycle, changes in weather patterns that affect mosquito populations usually take at least a year to affect upward or downward shifts in heartworm incidence.¹ “There have been some small floods or rain events,” she said. “That's enough to set up some good mosquito habitats to create an increase in incidence in [Texas].”

Besides the ongoing prevalence of heartworms in the Southeastern states and Gulf Coast region, the AHS has also identified new trends outside known “heartworm epicenters.” Infection density was notably higher in some new hot spots than in previous surveys, with an average of more than 100 cases per clinic diagnosed. These areas include East Texas, the Florida Panhandle, Southwest Florida, the central Carolinas, and southern Illinois.¹

Meanwhile, newly expanded areas with moderate infection rates (about 6-25 cases per clinic) included southern California, southwest Colorado, east-central Wisconsin, western Virginia, and southern Maine. In the Western and Mountain states, such as Washington, northern Idaho, northwest Nevada, and southeast Wyoming, cases increased from fewer than 1 per clinic in the 2023 survey to 3 to 5 per year in the recent survey.¹

Survey respondents also reported trends in incidence they observed since the 2023 survey. Among those surveyed, 59% of practitioners said heartworm rates had remained the same, 19% said rates had increased, and 22% said rates had decreased. Only 10% of survey participants reported seeing no heartworm cases.¹

Ames noted that Alaska was omitted from the 2025 heartworm incidence map because the AHS did not receive any data from hospitals and clinics in that state. “It is a colder climate, but heartworm disease occurs in all 50 states. There are cases popping up in Alaska at this time. Potentially, they’re thought to be imported cases,” Ames said in the interview.

“While the bad news is that heartworm continues to be a serious and nationwide threat to pet health, the good news is that heartworm disease continues to be almost 100% preventable with year-round heartworm prevention,” Ames said in an AHS news release.¹ “We encourage veterinarians and shelters to leverage the new AHS incidence map in client outreach about heartworm disease—and to remind their clients that pets need protection from this devastating disease.”

The FDA recommends prescription heartworm preventives approved by the agency as the best way to combat heartworm disease in dogs. The FDA has also approved injectable melarsomine dihydrochloride for killing adult heartworms in canines and a topical combination of imidacloprid and moxidectin for ridding a dog’s bloodstream of microfilariae.³

“There’s little room for error. Missing a single dose [of parasite prevention therapies] can set you up for an infection,” Ames said in the interview.

Ames noted that “indoor cats” are not immune to heartworm disease and still need parasite protection, despite some clients’ view of infestation risk being an “outdoor” threat. Christopher Lee, DVM, MPH, DACVPM, DACVM (parasitology), an assistant professor of microbiology at Lincoln Memorial University College of Veterinary Medicine at Orange Park in Jacksonville, Florida, and host of dvm360’s Vet Watch digital program, also addressed indoor parasite risk in a recent interview.⁴

“Clients who say their pet lives indoors and therefore does not need heartworm or flea protection often change their minds when I explain that indoor pets still get exposed through cockroaches, flies, and mosquitoes. Once clients understand their pets can act as a bridge to the outside, they are much more likely to agree to year-round protection. That protects both the animals and the people in the household,” Lee said.⁴

Ames also suggested training the entire practice staff on heartworm disease so they can answer common pet owner questions about prevention strategies. “Training them that year-round heartworm prevention is recommended—[that] because of the difficulty in predicting seasonality, it's better to just protect year-round and not guess about mosquito prevalence. There's also some thought that there's improved [adherence] if a client is giving it year-round and doesn't have to start and stop with the season,” she said.

References

1. American Heartworm Society releases new heartworm incidence map. News release. American Heartworm Society. April 7, 2026. Accessed April 10, 2026. https://www.heartwormsociety.org/in-the-news/977-american-heartworm-society-releases-new-heartworm-incidence-map
2. New heartworm incidence map shows increase in parasitic cases. dvm360. April 20, 2023. Accessed April 10, 2026. https://www.dvm360.com/view/new-heartworm-incidence-map-shows-increase-in-parasitic-cases
3. Keep the worms out of your pet’s heart! the facts about heartworm disease. FDA. Updated December 22, 2022. Accessed April 10, 2026. https://www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease
4. Alaburda R, Lee C. How to talk to clients about isoxazolines. dvm360. December 22, 2025. Accessed April 13, 2026. https://www.dvm360.com/view/how-to-talk-to-clients-about-isoxazolines