If you live in Illinois, Maine, Colorado, or California, there’s a good chance nobody has ever sat you down and had a real talk about heartworm. That disease is a Southern problem, right? A hot, humid, mosquito-thick problem that belongs to states like Louisiana and Mississippi. You might be wondering why your vet keeps pushing year-round prevention when you live somewhere with actual winters. I get it. That reasoning made sense for a long time. It doesn’t anymore.

The American Heartworm Society released its 2026 incidence map in April, built from over one million tests collected across the country, and the findings are stark: for the first time in the survey’s history, there is no heartworm-free state in the United States. Not one. And then on June 30, 2026, Embrace Pet Insurance released claims data showing a 201% increase in heartworm diagnosis claims between 2020 and 2025, with 79% of those claims involving pets age four and younger. We’re right in the middle of peak mosquito season. This is exactly the moment to pay attention.

Texas Is Just the Headline. The Map Tells the Bigger Story.

Texas topping the nation in heartworm cases for the first time ever gets the dramatic news hook, and it deserves it. Texas clinics are averaging nearly 50 positive dogs each, with a positivity rate of 3.78%, up from 2.97% in 2022. That’s a significant jump in just four years. But if you don’t live in Texas, it’s tempting to scroll past that number and move on.

Here’s what I tell people: look at what’s happening around Texas, not just in it. The 2026 AHS survey identified new moderate-risk hot spots in southern California, southwest Colorado, east-central Wisconsin, western Virginia, and southern Maine. These are places that, until recently, vets weren’t trained to treat as heartworm territory. They weren’t. And the Embrace claims data backs this up with cold numbers: Illinois, Michigan, Rhode Island, and Maine all saw notable increases in heartworm insurance claims over that same five-year window. These aren’t statistical blips. They’re a pattern.

The AVMA’s coverage of the survey, published April 27, 2026, made it clear that this isn’t a localized Texas story. It’s a national redistribution of risk.

Why Is This Happening Now?

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A few forces are working together, and none of them are reversing anytime soon.

The mosquito species responsible for transmitting heartworm, primarily Culex mosquitoes historically, are being joined by expanding populations of Aedes albopictus and Aedes aegypti. These species are aggressive, adaptable, and they’re establishing themselves in geographic ranges that would have been inhospitable to them even fifteen years ago. Warmer winters and longer warm seasons mean mosquitoes are active for more months in more places.

Wildlife is the other piece people don’t think about enough. Coyotes are one of the most significant reservoir hosts for heartworm, and coyote populations have expanded dramatically across suburban and even urban areas. An infected coyote moving through your neighborhood doesn’t need to interact with your dog directly. It just needs to be bitten by a mosquito that later bites your dog. That’s the whole chain.

Rescue dog transport is also a real factor. Dogs moved from high-incidence regions in the South and Southwest into lower-incidence areas can bring heartworm into places that lack the local transmission history, and the infrastructure of regular testing, that high-risk areas have developed. This isn’t an argument against rescue adoption. It’s an argument for testing and prevention regardless of where your dog came from.

Finally, inconsistent preventive use remains one of the most controllable variables in this entire picture. Many owners in historically low-risk areas never started prevention, or they use it seasonally rather than year-round. As the risk geography shifts beneath them, that gap in protection becomes a real vulnerability.

What “Moderate Risk” Actually Means for Your Dog

MetricFigureSource
Heartworm increase in insurance claims (2020-2025)201%Embrace Pet Insurance, June 30, 2026
Claims involving pets age 4 and younger79%Embrace Pet Insurance, June 30, 2026
Texas positive dogs average per clinic~502026 AHS Survey
Texas positivity rate (2026)3.78%2026 AHS Survey
Texas positivity rate (2022)2.97%2026 AHS Survey
Heartworm treatment cost range$1,000-$1,500+Article
Annual prevention cost range$35-$120Article

When the AHS designates a region as moderate risk, it means the probability of your unprotected dog encountering an infected mosquito is now meaningful enough that a vet in that area should be seeing it clinically. It doesn’t mean your dog will definitely get heartworm. It means the old assumption that you’re safe without prevention is no longer valid.

Heartworm disease is worth understanding clearly, because it’s not a quick illness. The worms, Dirofilaria immitis, live in the heart, lungs, and associated blood vessels. Treatment exists, but it’s expensive (typically $1,000 to $1,500 or more depending on your area and the disease stage), physically hard on the dog, and requires weeks of strict exercise restriction. There’s no over-the-counter cure. Prevention, by contrast, costs somewhere between $35 and $120 per year depending on the product and your dog’s weight. The math is not complicated.

The Embrace claims data, reported by PR Newswire on June 30, 2026, is particularly sobering on one point: 79% of diagnosed pets were age four and younger. These aren’t senior dogs with complicated health histories. These are young, otherwise healthy animals whose owners, in many cases, probably didn’t think they were at risk.

What to Do Right Now, In July

Testing is the starting point. If your dog isn’t on prevention and hasn’t been tested recently, an antigen test at your vet is the first step. The AHS recommends annual testing even for dogs on prevention, because no preventive is 100% effective if doses are missed or vomited, and catching an infection early matters enormously for treatment outcomes.

If your dog has been on consistent year-round prevention, keep going. If you’ve been doing seasonal prevention or skipping altogether because you live somewhere that felt low-risk, talk to your vet about switching to a year-round protocol now. July, right in the middle of mosquito season, is not the moment to wait until fall.

Ask your vet specifically about your county or region. The CAPC (Companion Animal Parasite Council) maintains forecast maps that are updated regularly, and as dvm360 reported in late June 2026, the 2026 data is reinforcing what the AHS map already showed: risk is moving. Your vet should have a current sense of local incidence. If they don’t, the CAPC’s website at capcvet.org has publicly accessible maps.

Cats need to be part of this conversation too. Heartworm in cats presents differently than in dogs, is often misdiagnosed, and has no approved treatment. Prevention is even more critical for cats precisely because there’s no good treatment option if they get infected.

The geography of heartworm risk has changed faster than most pet owners have had a chance to update their mental maps. That’s not a failure of anyone who genuinely believed they were in a safe zone. But the data is clear now, and July 2026, with mosquitoes at their peak, is the right time to act on it.

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This article is for general informational purposes only and does not constitute veterinary advice. Pet health symptoms can have many causes and require professional evaluation. Always consult a licensed veterinarian for diagnosis and treatment specific to your pet.

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