What are the symptoms of a dog with heartworm disease?

Heartworm disease creeps in quietly, like a storm that builds over weeks instead of hours. Those tiny parasites, spread by mosquitoes, can settle into a dog’s heart and lungs without making a peep—until suddenly, your energetic pup starts slowing down in ways that feel off. Knowing the signs early could mean the difference between a tough treatment and something far worse.​

One of the first hints might be a cough that won’t quit, especially after play or a short walk. It’s not the dry, hacking sound of kennel cough; this one feels deeper, like they’re struggling to clear something from their chest. You might notice they tire faster, too—hesitating halfway up the stairs or lagging behind on your usual neighborhood loop. That boundless energy you love? It fades, replaced by a quiet reluctance to move.​

As the disease progresses, their ribs might start showing more than usual, even if they’re eating normally. Weight loss sneaks in because their body’s working overtime to fight the worms, burning calories faster than they can take them in. Their gums might look pale or even a little blue-tinged, a sign that oxygen isn’t moving through their blood like it should.​

Watch for signs of labored breathing, especially when they’re resting. A dog with heartworm might sit with their elbows out, shoulders hunched, as if every breath takes extra effort. Some pups get restless at night, pacing or struggling to get comfortable—their bodies are telling them something’s wrong, even if they can’t put it into words.​

Mosquitoes don’t care about fences or zip codes, which is why prevention matters so much. In many places, vets recommend year-round heartworm meds, not just during warm months—those tiny bugs can linger longer than you’d think, especially with milder winters. Skipping doses isn’t worth the risk; treatment for advanced cases is expensive, painful, and hard on their bodies.​

Here’s the tricky part: some dogs show no symptoms at all until the disease is well advanced. That’s why regular vet checkups, including blood tests, are non-negotiable. Catching it early can keep the worms from damaging their heart or arteries, which can lead to lifelong issues even after treatment.​

If you notice any of these signs, don’t wait to call your vet. Heartworm isn’t something to “wait and see” about—those worms keep growing, and every day counts. Your pup can’t tell you when they’re hurting, but their body will show you. Being their advocate means paying attention to the little changes, even when they seem like no big deal.​

Prevention is simpler than you might think: a monthly chew, a topical treatment, or a shot from the vet. It fits into your routine, just like their flea meds or morning walk. And since mosquitoes thrive in damp, shaded areas, clearing standing water from your yard—birdbaths, clogged gutters—can lower the risk, too. It’s a small step that keeps both your dog and your community safer.​

Your dog relies on you to notice when something’s wrong. Those subtle shifts in their energy, that new cough, the way they pause mid-play—they’re all clues. Catching heartworm early isn’t just about their health; it’s about keeping that tail wagging, that nose sniffing, and those paws moving for years to come.