Heartworm disease once had a reputation of just being a "southern" problem that dog owners in the Gulf Coast and southeastern regions of the U.S. needed to worry about. There was also a time when heartworm disease in cats was not thought to be much of a concern and, therefore, was rarely discussed during a veterinary visit. Times, however, have changed dramatically and some beliefs of the past no longer hold true. Heartworm is now considered a potential threat to the health of pets regardless of where they live.

The American Heartworm Society's most recent (2025) nationwide heartworm incidence survey¹ revealed that heartworm disease no longer respects the regional boundaries it used to — infections have now been diagnosed in all 50 of the United States. The Gulf Coast and southeastern U.S. remain the highest-incidence regions in the country for heartworm but, for the first time, Texas claimed the top spot for total heartworm cases diagnosed. Most concerning has been the sharp increases in heartworm diagnosis rates in states that traditionally were not considered as “high risk” for the disease as well as the increase in diagnoses of heartworm disease in cats. Some of these newly expanded areas documenting rising heartworm infection rates include parts of California, Colorado, Idaho, Maine, Nevada, Virgina, Washington, Wisconsin, and Wyoming.

Source: American Heartworm Society Heartworm Incidence Map

A separate analysis by Banfield Pet Hospital² — of medical record data over a five-year period (through 2023) — revealed the following increases in rates of heartworm diagnoses:

• Delaware — 299% increase
• Montana — 140% increase
• Nevada — 101% increase
• Idaho — 81% increase
• Utah — 57% increase
• Washington — 55% increase
• Arizona — 52% increase
• Iowa — 35% increase
• Oregon — 32% increase
• Rhode Island — 30% increase

What’s notable in their data are the areas that the American Heartworm Society has historically classified as low-risk (e.g. Oregon and Washington).

While we see these increases in infection rates, the data cannot tell us exactly where an individual pet may have contracted heartworm. Pets acquire heartworm infection from mosquitos and could have become infected during the course of travel (e.g. out of state or to a different part of the state) or from a different point of origin (e.g. if they’d been obtained by adoption from another state). It's also important for everyone to understand that the American Heartworm Society’s incidence distribution map is not static. A new survey is conducted every three years, and it can continue to shift meaningfully from year to year. Geographic areas that feel "safe" today could become a “hot spot” for heartworm over time. There are several reasons why:

1. Major weather events. Severe weather events (e.g. hurricanes and flooding) can foster an explosion of mosquito populations in their aftermath. Mosquitos spread heartworm by feeding on an infected dog and then moving on to another animal to feed on – whether it’s a dog, cat or ferret, etc.
   
   These events can also lead to the displacement or relocation of animals. We’ve seen this happen in association with catastrophic hurricanes of the past (e.g. Hurricane Katrina). Heartworm-positive animals may wind up in geographic locations where the prevalence of heartworm had been low, and now they become a source for infection to other animals.
   
   
2. Climate patterns in general are extending mosquito season. The trend of milder winters or warmer, humid conditions extending longer into the fall and winter months give mosquitoes more time and more territory to transmit the parasite. Some mosquito species have also adapted to survive winters indoors.
   
   
3. Wildlife carriers are moving into more populated areas. Coyotes, foxes, and wolves can all carry heartworm, and these animals, increasingly, are appearing in suburban and even urban areas. As carriers, these animals can introduce the parasite to places it wasn't previously established.
   
   
4. Pets are being relocated more than ever. Adoption and rescue transport programs — while doing important work — can unintentionally move heartworm-positive dogs from high-prevalence regions into areas where the disease was previously rare.

The bottom line is that no matter where you live – whether it’s Florida or Idaho or anywhere in between — the safest approach is to follow the recommendations set forth by the American Heartworm Society. Pet owners should consult with their veterinarian about annual heartworm testing and year-round preventive strategies.

Resources:

1. The American Heartworm Society - 2025 Heartworm Incidence Survey (released in April 2026).

2. New pet owner survey suggests common myths fueling significant gaps in prevention of deadly parasite for U.S. dogs and cats. PR Newswire. April 2024.