Heartworm medication is necessary for dogs in every region of the United States. Heartworm disease is caused by parasitic worms that grow up to a foot long inside a dog’s heart and lung arteries, and a single missed dose of prevention can leave a dog vulnerable to infection. The worms are transmitted exclusively through mosquito bites, which means any dog with even minimal outdoor exposure is at risk.
What Heartworms Do Inside a Dog
When an infected mosquito bites a dog, it deposits microscopic larvae into the skin. Those larvae spend the next several months migrating through the dog’s body, molting twice as they develop into adults. The adult worms take up residence in the pulmonary arteries and sometimes the right side of the heart itself, where they can live for five to ten years.
A single dog can harbor dozens of adult worms. Over time, they cause inflammation and scarring in the blood vessels of the lungs, force the heart to work harder to pump blood, and eventually lead to heart failure. By the time most dogs show obvious symptoms like coughing, exercise intolerance, or a swollen belly, significant damage has already been done. Treatment at that stage is expensive, painful, requires months of strict rest, and carries real risks. Prevention costs a fraction of what treatment does and avoids the organ damage entirely.
Why Year-Round Prevention Is Recommended
Both the American Heartworm Society and the FDA recommend giving heartworm preventives every month, all twelve months of the year. That might sound excessive if you live somewhere with cold winters, but there are practical reasons behind it.
Mosquito activity does drop in colder months, but urban microclimates (warm basements, subway tunnels, standing water near heated buildings) mean transmission risk never truly hits zero. Year-round dosing also acts as a safety net. If you accidentally skip or delay a dose, having the previous months covered reduces the window of vulnerability. Missing just one dose during active mosquito season can be enough for a dog to become infected.
Most heartworm preventives also protect against intestinal parasites like roundworms and hookworms, some of which can spread to humans. Staying on a twelve-month schedule keeps those covered too.
Heartworm Is Spreading to New Areas
If you live outside the traditional “heartworm belt” of the southeastern U.S. and think your dog is safe, the latest data suggests otherwise. A 2026 survey based on 2025 testing data found heartworm showing up in new hot spots across the country. Texas topped the list of leading states for heartworm incidence for the first time. East Texas, the Florida Panhandle, southwest Florida, the central Carolinas, and southern Illinois all reported more than 100 cases per clinic, in areas without historically high rates.
Dog transport and rescue operations move infected animals across state lines regularly, introducing heartworm to regions that previously saw very few cases. Climate shifts are also expanding the range and active season of mosquitoes. Wherever mosquitoes exist, heartworm can follow.
How Preventives Work
Heartworm preventives don’t create a force field against infection. Instead, they work retroactively. Each monthly dose kills any heartworm larvae that entered your dog’s body during the previous 30 days, before those larvae can mature into adults. This is why timing matters: a gap in coverage gives larvae a chance to develop past the stage where preventives can eliminate them.
There are three main ways to give heartworm prevention:
- Monthly oral chews or tablets: The most common option. Many combine heartworm prevention with protection against intestinal worms, fleas, or ticks.
- Monthly topical treatments: Applied to the skin between the shoulder blades. A good alternative for dogs that won’t take oral medications.
- Injectable prevention: Given by a veterinarian every six or twelve months. This eliminates the risk of forgetting monthly doses.
All FDA-approved heartworm preventives require a prescription, which means your dog needs a heartworm test first. This isn’t a formality. Giving a preventive to a dog that already has adult heartworms won’t kill the adults, so the dog stays infected and gets progressively sicker. Worse, if the dog has baby worms (microfilariae) circulating in the bloodstream, the preventive can cause those microfilariae to die all at once, potentially triggering a shock-like reaction that can be fatal.
Drug-Resistant Heartworms Are Real
Some heartworm strains in the United States have developed confirmed resistance to the standard class of drugs used in preventives. Most resistant cases have been concentrated in the lower Mississippi River Valley, but resistance can emerge anywhere. This doesn’t mean preventives are useless. It means the margin for error is shrinking.
Among available options, moxidectin-based preventives have shown stronger effectiveness against resistant strains in laboratory studies compared to other drugs in the same class. If you’re in a high-risk area, this is worth discussing with your vet. Regardless of which product you use, consistent, on-time dosing remains the single most important factor. Noncompliance with the dosing schedule, not drug resistance, is still the leading contributor to heartworm diagnoses in dogs on preventives.
The Real Cost of Skipping Prevention
Monthly heartworm prevention typically costs between $5 and $25 per month depending on your dog’s size and the product you choose. The injectable option runs higher upfront but covers six to twelve months at once. Heartworm treatment, by contrast, can cost $1,000 to $3,000 or more, requires multiple vet visits, involves injections to kill the adult worms, and demands weeks of strict exercise restriction to prevent dead worm fragments from causing dangerous blockages in the lungs.
Some dogs, particularly those with heavy worm burdens or advanced disease, suffer permanent heart and lung damage even after successful treatment. There is no scenario where skipping prevention and treating later comes out ahead, financially or medically. The preventives themselves carry a very low risk of side effects. Serious reactions are rare and almost always linked to giving the medication to a dog that was already infected and hadn’t been tested.