What are the first signs of heartworm in dogs?

Heartworm starts quiet. Those tiny parasites, carried by mosquitoes, sneak into your dog’s bloodstream and grow—slowly, silently—until they’re causing real damage. By the time obvious symptoms hit, they’ve often been at work for months. Knowing the first signs can mean catching it early, when treatment is easiest. Let’s learn what to watch for.​

A soft, persistent cough is usually the first clue. Not the hacking “something’s stuck” cough after drinking too fast, but a low, raspy one that lingers, especially after play or exercise. It happens because worms irritate the lungs as they grow, making your dog struggle to catch their breath. You might notice it during walks—suddenly, they stop, hack a little, then keep going like nothing’s wrong. Don’t brush this off as “just getting older.”​

Lethargy creeps in next. Your once-energetic pup, the one who sprinted to the door at the word “walk,” now hangs back. They tire mid-play, or curl up instead of greeting you at the gate. It’s not laziness—heartworms rob them of stamina by blocking blood flow and straining the heart. Even short trips to the park leave them panting heavily, long after they’ve caught their breath before.​

Loss of appetite often follows. They pick at food they used to scarf down, or turn up their nose at treats. Weight loss happens gradually, not overnight—you might notice their ribs feel more prominent when you pet them, or their waist looks thinner. This isn’t about dieting; it’s their body diverting energy to fight the invader, leaving less for hunger.​

Watch their coat. A dog with early heartworm might develop a dull, dry coat that sheds more than usual. Their skin might flake, even with regular brushing. It’s a subtle sign, easy to blame on the weather or a change in food, but worth noting alongside other symptoms.​

Here’s what’s tricky: some dogs show no signs at all in the early stages. That’s why annual heartworm tests—usually a simple blood draw at your vet’s office—are non-negotiable. Many areas require these tests as part of routine care, and for good reason: mosquitoes thrive everywhere, from city backyards to rural fields, making every dog at risk.​

Mild exercise intolerance is another early indicator. Your dog might hesitate to jump into the car, or lag behind on walks they once led. They might stop to rest more, even on flat ground. These aren’t signs of “getting old”—they’re your dog’s body signaling that something’s off with their heart or lungs.​

Puppies and small breeds can show signs faster. Their smaller hearts and lungs have less room to accommodate growing worms, so symptoms like rapid breathing or weakness might appear sooner. Don’t assume youth protects them—mosquitoes don’t care how old your dog is.​

If you notice these signs, call your vet. Early treatment uses fewer medications and has shorter recovery times. And remember: prevention is cheaper and easier than cure. Monthly chewables or topical treatments, prescribed by your vet, keep those mosquitoes from passing heartworm larvae to your dog. Skipping them isn’t worth the risk—not when a simple pill can keep your buddy safe.​

Your dog can’t tell you when they’re hurting, but their bodies send signals. Catching heartworm early turns a scary diagnosis into a manageable one, letting you get back to the good stuff—fetch, belly rubs, and all the silly moments that make them family.