Map Shows New Heartworm Hotspots for Dog Owners Across US

Heartworm disease is spreading into new parts of the United States, according to a newly released nationwide map that veterinarians say should serve as a warning for dog owners who may wrongly assume they are not at risk..

The latest incidence map from the American Heartworm Society (AHS), based on 2025 data from veterinary clinics and animal shelters, shows rising infection rates and emerging hotspots well beyond regions traditionally associated with the mosquito-borne disease.

“It's not just a Florida problem or a Southeast problem anymore,” Dr. Seth Colman, medical director at VEG ER for Pets in Jacksonville, Florida, told Newsweek via email.

For the first time, Texas leads the country in heartworm incidence, followed closely behind by Louisiana, Arkansas, Mississippi, South Carolina, Tennessee, Alabama, North Carolina, Florida and Georgia. Those areas with especially high concentrations of cases have clinics reporting more than 100 heartworm diagnoses.

However, new areas of concern have also emerged in parts of California, Colorado, Wisconsin, Virginia, Maine, Washington, Idaho, Nevada and Wyoming, where clinics reported roughly six to 25 cases each. In some states, infections have risen sharply compared to the last report three years ago, increasing from fewer than one case per clinic to between three and five annually.

Colman said the map sends owners a clear message: if there are mosquitoes, there is a risk of heartworm.

Heartworm Is No Longer Confined to the South

Heartworm is a serious and potentially fatal disease caused by parasitic worms spread through mosquito bites. Once inside a dog’s body, the worms can grow in the heart, lungs and blood vessels, causing coughing, breathing problems, heart failure and lasting organ damage.

The AHS estimates that more than 1 million pets in the United States are infected with heartworms, with diagnoses in all 50 states, despite the disease being largely preventable.

Why Cases Are Spreading

Experts say several factors are driving the spread into areas once considered lower risk.

One major factor is the expansion of mosquito populations. Species like the Asian tiger mosquito have spread farther north and west in recent years, increasing the reach of transmission, the American Animal Hospital Association reports.

Climate also plays a role, as warmer temperatures, humidity and urban “microclimates” allow mosquitoes to survive longer throughout the year, extending the transmission season.

“Mosquito seasons are not as predictable as many people assume,” Colman said. “Warm spells can happen earlier or later in the year, and mosquitoes can survive in small pockets around homes, apartments, storm drains, wooded areas and urban neighborhoods.”

Dogs are also moving more frequently than in the past through relocations, vacations and interstate rescue transports. If a heartworm-positive dog enters a new community, local mosquitoes can continue the cycle, Colman said.

Despite the growing risk, many infections still occur because of inconsistent prevention. Colman said most cases are not caused by medication failure, but by missed or delayed doses.

The Biggest Mistakes Dog Owners Make

“The biggest misconception is probably, ‘My dog is mostly indoors, so I do not need to worry about it,'” he said. “Indoor dogs still go outside to use the bathroom, go on walks, visit parks, travel or sit on patios. And mosquitoes get inside homes all the time.”

Many owners also assume flea and tick prevention automatically protects against heartworm, though not all products do. Colman suggests confirming with a veterinarian whether a product specifically prevents heartworm disease.

The AHS recommends year-round prevention for dogs nationwide, even in lower-risk states. Colman said consistency is critical because these medications work by eliminating immature heartworms before they can develop into adults.

“Instead of trying to guess when mosquito season starts and stops, the dog stays protected consistently,” he said.

Why Prevention Matters

Colman stressed that the prevention is significantly less expensive and less traumatic than treatment. Preventive medications may come as monthly chews, topical products or long-lasting injections administered by a veterinarian.

Treatment, however, can involve extensive testing, blood work, injections to kill adult worms, multiple veterinary visits and months of restricted activity while the worms die off.

“Costs vary depending on the dog's size, where the family lives, and how advanced the infection is, but heartworm treatment can become a major expense very quickly,” Colman said.

As an emergency veterinarian, Colman often sees the most severe cases after significant damage to the heart and lungs has already occurred, with dogs coughing and struggling.

“Heartworm disease is not theoretical to me,” he said. “It is very real, and it can be devastating.”

2026 NEWSWEEK DIGITAL LLC.

This story was originally published May 10, 2026 at 4:30 AM.