The Test That Came Back Positive
Jason Merritt had owned dogs in Denver for eleven years without ever worrying much about heartworm. It was a Gulf Coast problem, he figured — something for people in Louisiana to think about, not someone in a mile-high city with cold winters and dry air. When his veterinarian mentioned during a routine visit in early 2025 that she was recommending year-round prevention for all her patients now, he was surprised enough to ask why. She showed him a map. Colorado had spots on it he didn't expect. His three-year-old Weimaraner, Scout, tested negative. But Merritt left with a prescription for preventative medication and a different understanding of where he lived on the American disease map.
That map has been redrawn.
Curious about a specific dog? Explore our Breeds guide to learn what makes each one special.
What the 2026 Survey Found
In April 2026, the American Heartworm Society released its newest triennial heartworm incidence survey — conducted in early 2026 using data from heartworm testing conducted throughout 2025, drawn from veterinary practices and shelters across the country. The 2025 Heartworm Incidence Map shows the deadly parasite has expanded to areas that are cooler, drier and far from the warmer, humid Gulf Coast and Southeast. The AHS estimates that at least 1.1 million dogs in the U.S. are positive for heartworm.
Read also: Chronic Kidney Disease Affects 1 in 4 Senior Dogs. New Vitamin D Treatment Shows Promise
The survey did not identify a single heartworm-free state. Several hot spots showed notably higher heartworm infection density than in prior surveys, with more than 100 cases diagnosed per clinic, including east Texas, the Florida Panhandle, southwest Florida, the central Carolinas, and southern Illinois. Texas topped the national rankings, with Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, and Tennessee rounding out the top ten.
But it's what's happening outside the traditional South that is drawing the most attention from veterinary researchers.
Ready to give a dog a home? Browse adoptable pups at Shelters near you.
The Map Is Moving
The highest risk of heartworm infection is expected to remain in the southeastern United States, with continued spread northward along the Mississippi River corridor and up the Atlantic coast. This trend places much of the central and eastern U.S., from Kansas to Maryland, at increasing risk.
Among the mosquito species now expanding their ranges northward in the United States are the Asian tiger mosquito and the yellow fever mosquito, both of which have contributed to increasing heartworm risk for dogs in areas that were previously considered low-risk or non-endemic.
Read also: Short-Term Foster Increases Adoption Odds by 1,400%. Shelters Face Post-Pandemic Surge
Dr. Colleen Duncan, a professor of veterinary medicine at Colorado State University, has watched this shift happen in real time in Fort Collins. Two decades ago, she said, her clinic didn't test for heartworm and didn't recommend prevention. Now she does both. Climate conditions that prolong mosquito activity, urbanization creating new mosquito breeding habitats, and the movement of dogs from the South to cities in the North are all contributing to a parasite's expanding geography.
"Warmer temperatures contribute to the proliferation of mosquitoes," Duncan told Inside Climate News. The connection between a longer warm season and a longer transmission window is direct.
Want more tips and stories like this? Dig into our Blog for the latest on all things dogs.
How Heartworm Works — and Why Prevention Matters
Heartworm disease is caused by a parasitic worm, Dirofilaria immitis, transmitted exclusively through mosquito bites. A single infected mosquito deposits microscopic larvae into a dog's bloodstream during feeding. Those larvae travel to the heart and lungs, where they mature over six months into adult worms that can reach a foot in length. An infected dog may carry dozens of worms simultaneously. Symptoms — a persistent cough, decreased appetite, fatigue, difficulty breathing — often don't appear until the disease is already advanced. Treatment exists but is expensive, physically taxing on the dog, and requires months of strict rest. Prevention, by contrast, costs a fraction of that and takes thirty seconds once a month.
The 2026 forecast reinforces that vector-borne disease risk is dynamic, local, and changing. "Low risk" does not mean "no risk," especially for pets that travel or are relocated.
Not sure which breed fits your lifestyle? Take our Quiz and find your perfect pup match in minutes.
The Alaska Footnote
One detail from the 2026 survey is worth noting specifically. Alaska was omitted from the heartworm incidence map — not because the state is confirmed clear, but because the AHS received no data from hospitals and clinics in the state. As AHS representatives told dvm360:
"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." A disease that shows up in Alaska, however it got there, is no longer regional.
Jason Merritt in Denver renews Scout's prevention prescription every year now. He doesn't think of it as an unusual precaution anymore. He thinks of it as the same category as vaccination — something you do because the world the dog lives in has changed, even if the dog's daily life looks exactly the same as it always did.
