Roughly 1 in 100 dogs tested at veterinary clinics in the United States is positive for heartworm, but that national average masks enormous variation. An estimated 1.2 million dogs in the U.S. are currently infected, up from about 800,000 two decades ago. For dogs that never receive preventive medication, the odds are far worse: approximately 85% of unprotected dogs will eventually contract heartworm disease.
The National Picture
The American Heartworm Society surveys veterinary clinics and shelters every three years, and every survey shows the number climbing by more than 50,000 diagnosed cases. The most recent data, collected through 2025, confirms that heartworm is not only holding steady but expanding into areas where it was once rare.
Globally, the numbers are higher. A systematic review pooling data from studies around the world found an overall heartworm prevalence of about 11% in dogs, reflecting the fact that many countries have less widespread access to preventive medications than the U.S. does.
Where Heartworm Is Most Common
Mississippi, Louisiana, Texas, Alabama, and Arkansas consistently report the highest concentration of heartworm cases. These states combine warm temperatures, high humidity, and large mosquito populations, which is exactly what the parasite needs to complete its life cycle. Heartworm larvae require about two weeks of temperatures consistently above 57°F to mature inside a mosquito and become capable of infecting a dog. In the Deep South, those conditions exist for most of the year.
What’s more alarming is where heartworm is showing up now. The 2025 survey flagged new hot spots in places without a historically high incidence: east Texas, the Florida Panhandle, southwest Florida, the central Carolinas, and southern Illinois all reported more than 100 cases diagnosed per clinic. States like Washington, Oregon, Kansas, North Dakota, Massachusetts, and Connecticut have also seen unexpected increases. Climate shifts, dog transport from southern shelters, and gaps in prevention all contribute to this spread.
Unprotected Dogs Face the Highest Risk
The 85% infection rate in unprotected dogs is the number that puts heartworm prevalence into perspective. The relatively low national average exists largely because millions of pet owners keep their dogs on year-round preventive medication. The remaining 15% of unprotected dogs that don’t get infected appear to have a natural resistance to the parasite. For everyone else, living in a mosquito-prone area without prevention is essentially a matter of when, not if.
Prevention costs a fraction of treatment. Monthly or annual preventive options keep larvae from maturing into adult worms after a mosquito bite. Treatment for an established infection, by contrast, is expensive, requires weeks of strict exercise restriction, and carries real health risks, particularly if the worm burden is high.
Shelter and Stray Dogs Are Hit Hardest
Dogs in shelters, strays, and those displaced by natural disasters consistently test positive at much higher rates than owned pets. A Canadian study illustrates the gap clearly: among dogs surrendered locally, heartworm prevalence was just 0.3%. Among dogs relocated from other regions (often southern U.S. shelters), it jumped to 6.6%. In one underserved community in Ontario, 36.5% of homeless dogs tested positive.
These disparities reflect access to veterinary care more than anything biological. Dogs that see a vet regularly and receive preventive medication rarely test positive. Dogs without that access, whether strays, shelter animals, or pets in communities with limited veterinary resources, face dramatically higher infection rates. If you adopt a dog from a shelter, especially one that transports animals from the southeastern U.S., a heartworm test is one of the first things to confirm has been done.
Why Reported Numbers Likely Undercount Reality
The standard heartworm test used in veterinary clinics detects proteins shed by adult female worms. These antigen tests are highly specific, meaning a positive result is almost certainly a true positive (specificity ranges from 94% to 98% depending on the brand). But their sensitivity, the ability to catch every infected dog, drops when the worm burden is low. In dogs carrying fewer than 10 worms, sensitivity ranges from roughly 52% to 76%. That means a dog with a light infection can test negative and go undetected.
Male-only infections also produce false negatives, since the test targets proteins from female worms. Heat-treating blood samples before testing can improve detection, and some veterinarians use this method when clinical suspicion is high. The practical takeaway: a negative test is reassuring but not a guarantee, particularly in dogs with inconsistent prevention histories or recent exposure to high-risk areas.
What Drives the Rising Numbers
Three factors explain why heartworm cases keep climbing despite effective preventives being widely available. First, compliance is lower than most owners realize. Missing even one or two doses of monthly prevention during peak mosquito season can leave a window for infection. Second, the movement of rescue dogs from high-prevalence southern states to adopters in the North and West seeds heartworm into regions where local veterinarians historically saw very few cases. Third, warmer average temperatures are extending the mosquito season in northern states, giving larvae more time to develop and increasing transmission windows in places that once had reliable cold-weather breaks.
The geography of heartworm is no longer limited to the humid South. Every U.S. state has reported cases, and the trend lines continue upward. Year-round prevention, regardless of where you live, remains the most reliable way to keep your dog out of the statistics.