Every winter, the same reasonable-sounding idea talks pet owners into a risky decision: the mosquitoes are gone, so my dog is safe, so I can pause the heartworm meds until spring. It feels like common sense. It’s also based on a misunderstanding of what those meds actually do, and that misunderstanding is what makes it dangerous.
Here’s the correction that reframes the whole question. A heartworm preventative doesn’t keep mosquitoes off your pet. It doesn’t repel anything. What it does is kill the heartworm larvae already circulating inside your pet, larvae that a mosquito deposited weeks earlier and that are still developing right now, cold weather or not. Once you understand that, “the mosquitoes are gone” stops being a reason to stop. In many ways it’s the moment the medicine matters most.
A quick refresher on the disease
Heartworm disease (dirofilariasis) is caused by the parasite Dirofilaria immitis, and its larvae are transmitted through mosquito bites. When a mosquito carrying larvae bites a dog, the larvae enter the bloodstream. Over months, they can mature into adult worms that inflame and obstruct the heart and lungs, unless a reliable preventative kills them first. Mosquitoes are of course more common in summer, but that seasonal pattern is exactly what lulls people into a false sense of winter safety.
Yes, a dog can be diagnosed in winter
It takes roughly six months for heartworm larvae to mature into adults, so a dog infected earlier in the year may not show a thing until the temperature drops. Some dogs with only a few worms have no symptoms at all, which is precisely why vets push yearly testing, to catch infections early rather than by accident. The most common first sign owners notice is coughing. As the disease progresses, you may also see:
- A productive cough
- Exercise intolerance
- Rapid or labored breathing
- Weight loss
- Ascites (fluid buildup in the abdomen)
- Discolored urine
- Weakness
- Fainting (syncope)
What treatment involves, and why you want to avoid it
Treating heartworm in a dog is a serious undertaking. After a positive blood test, vets run further diagnostics, bloodwork, urinalysis, chest X-rays, to gauge how much damage the heart, lungs, and vessels have taken. From there, treatment centers on:
- Stabilizing life-threatening symptoms, which can mean oxygen or care for congestive heart failure
- Killing the adult worms with melarsomine, the only drug that does it, given by injection in a two- or three-dose protocol and often uncomfortable for the dog
- Killing the larvae by continuing heartworm prevention so they can’t mature
- Supportive care, typically antibiotics, steroids, and pain control
- Strict exercise restriction, typically across a six-to-eight-week stretch, because a dog’s cardiovascular system is dangerously fragile during treatment
It’s arduous and expensive, which is the whole argument for year-round prevention: the treatment you’re avoiding is far worse than the pill you’re skipping.
Cats need it too, and can’t be treated at all
Cats absolutely need heartworm prevention year-round. Their infection rates are lower than dogs’, but because cats aren’t a natural host, adult worms don’t survive long in them, and it’s the immature worms and the die-off of any adults that inflame a cat’s lungs and vessels. That inflammation is heartworm-associated respiratory disease (HARD), which brings coughing, wheezing, labored breathing, and respiratory distress. Crucially, there is no approved treatment for heartworm in cats, so for them prevention isn’t the best defense, it’s the only one.
Five reasons to keep dosing through winter
The meds kill larvae, and larvae don’t take winter off
This is the big one. Heartworm preventatives work by killing larvae, and larvae from a mosquito bite can sit in that stage for about two months. That warm afternoon in early December was a genuine opportunity for infection, and those larvae keep developing well into winter. Each monthly dose kills them and stops that development cold.
Many preventatives do more than one job
A lot of products protect against more than heartworm, covering intestinal worms, fleas, ticks, and ear mites as well. Depending on which one you use, dropping the heartworm coverage in winter may also drop your pet’s defense against parasites that don’t clock out for the season.
”Mosquito season” isn’t a fixed date
As the climate shifts and weather grows less predictable, the calendar turning to winter no longer guarantees mosquito-free conditions. Mosquito eggs are hardy, surviving low temperatures and waiting for a warm enough stretch to hatch. The clean summer-to-winter boundary people picture doesn’t really exist.
Consistency keeps the routine intact
Routines survive on consistency. It’s far easier to give the dose every single month than to stop in November and then, sometime in the muddle of spring, suddenly realize you never restarted. Owners who dose year-round also don’t have to think about it when they travel somewhere warm to escape the cold with their pets.
Some products come with a guarantee
Even diligent owners can hit bad luck, and rarely, a pet on prevention still contracts heartworm. Many manufacturers offer financial help with treatment in that case, but typically only if you used their product as directed, which means giving it year-round. Skip a few winter months and you may forfeit that safety net.
But what if your winters are genuinely brutal?
There’s a real nuance here. Heartworm is less prevalent in regions with long, hard freezes. Research in Ontario, Canada, spanning the 1950s through the 1980s found no heartworm infections diagnosed after October, which led to a regional recommendation for seasonal prevention from June through November. In extremely cold climates, some vets still follow a seasonal protocol.
Many others, along with the American Heartworm Society, recommend year-round coverage regardless. If you live somewhere with harsh winter freezes, this is a conversation to have with your vet, who will weigh factors like the specific product you use, how common heartworm and other parasites are locally, your pet’s health history, your lifestyle, and your own tolerance for risk.
“When an owner asks whether they can pause over winter, I walk them through the larval math, not the mosquito math,” says Dr. Ravi Mehta. “The medication isn’t a bug spray. It’s clearing larvae that are already inside the animal and still growing. Stop for two months and you’ve handed those larvae the exact window they need to become adult worms.”
What newer research adds
The case for year-round prevention keeps getting stronger. The American Heartworm Society’s 2025 incidence survey found heartworm spreading beyond its old strongholds into states like Illinois, Michigan, Rhode Island, and Maine, with claims data showing cases up roughly 201 percent between 2020 and 2025, and diagnoses occurring across all seasons rather than clustering in summer.
Veterinarians have largely converged on the same answer. In a 2025 survey, about 94 percent of vets reported recommending year-round prevention for their canine patients. That near-consensus, paired with a disease that’s expanding geographically and showing up in every month of the year, is a strong signal that the winter pause belongs to an older, colder, more predictable climate than the one we actually live in.
References
- American Heartworm Society. “Current Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm Infection in Dogs.” Link
- Slocombe, J. O. D., et al. “Heartworm in Dogs in Canada.” Canadian Veterinary Journal. Link
- AAHA. “Latest AHS Heartworm Incidence Map Shows Troubling Trends.” Link
