A Veterinarian’s Perspective

I’ve been practicing as a small-animal veterinarian for over a decade, and if there’s one misconception I hear every winter, it’s this: “We can skip heartworm prevention until spring.” I understand where it comes from. Cold weather feels like a natural reset, a break from fleas, ticks, and mosquitoes. But heartworm doesn’t follow our assumptions—it follows biology.

And that’s where things get a bit uncomfortable.

The Short Answer: Yes, Dogs Can Get Heartworm in Winter

Heartworm disease, caused by Dirofilaria immitis and transmitted by mosquitoes, doesn’t magically disappear when temperatures drop.

In my experience, winter simply reduces risk—it doesn’t eliminate it.

I’ve treated dogs that likely became infected during what their owners thought was a “safe season.” And those cases tend to come with a mix of surprise and regret.

Why Winter Isn’t as Safe as You Think

Mosquitoes are the key factor here. Most people assume they vanish entirely in winter, but that’s not always the case—especially in regions with mild winters or fluctuating temperatures.

A young Labrador with a persistent cough was brought in after skipping heartworm prevention for three winter months. Unusually warm weeks allowed for mosquito activity, and the dog tested positive.

This case stood out—the owner simply followed what seemed logical, unaware of the risk.

What many people don’t realize is:

• Mosquitoes can survive indoors or in sheltered outdoor areas.
• Even brief warm spells can reactivate mosquito activity.
• It only takes one infected mosquito bite to transmit heartworm.

So while the risk is lower in colder months, it’s never zero.

The Hidden Timing Problem Most Owners Miss

Here’s where things get a bit more technical—but it matters.

Heartworm preventatives don’t stop infection instantly. They kill larvae acquired in previous weeks.

Skipping doses in winter may allow larvae to mature into adults.

I’ve had multiple conversations with clients who thought they could safely “pause” prevention and restart in spring. Unfortunately, by the time spring rolls around, the damage may already be done.

One case that comes to mind involved a rescue dog adopted in early spring. The previous owner had stopped prevention in winter. By the time we tested the dog months later, the infection had progressed enough to require a lengthy, expensive treatment.

What I Personally Recommend to My Clients

After years of seeing how this plays out, my recommendation is clear: keep your dog on heartworm prevention every month, year-round. This is the safest and most effective way to protect your dog against heartworm, regardless of the season.

Not because every winter is high-risk, but because the consequences of getting it wrong are significant.

Treatment for heartworm disease isn’t simple. It involves:

• Multiple injections
• Strict activity restriction for weeks
• Potential complications if the worms die too quickly

Energetic dogs can struggle through recovery due to preventable lapses in prevention.e.

From a practical standpoint, the best way to protect your dog is to keep them on a consistent monthly heartworm prevention schedule all year. This avoids confusion about timing and eliminates coverage gaps, making prevention simple and reliable.

Common Mistakes I See Every Winter

There are patterns that repeat themselves almost every year in my clinic:

Some owners stop prevention too early, assuming the first cold snap means mosquitoes are gone for good. Others wait too long to restart in spring, missing that early window when mosquitoes become active again.

And then there are those who rely on visible signs—thinking, “I don’t see mosquitoes, so we’re fine.” Unfortunately, heartworm transmission often happens quietly and invisibly.

A More Realistic Way to Think About It

Instead of asking whether dogs can get heartworm in winter, focus on this actionable question: Are you consistently giving heartworm prevention throughout the year to keep your dog safe? Consistency is key.

Is the risk lower in winter? Yes.

Is it predictable enough to rely on? Not really.

Weather patterns shift. Indoor environments stay warm. Mosquitoes adapt.

In my own practice, I’ve found that the simplest strategy—consistent, year-round prevention—is also the most reliable. It avoids the gray areas where most infections occur.

Final Thoughts from the Clinic

Consistency is the key to keeping a dog heartworm-free.

It’s not about timing or weather—just stay consistent.

Clients with strict, year-round prevention have avoided issues, while those skipping a couple of months have faced bigger problems..

Winter isn’t a break for heartworm—consistency prevents missed risks.d.