Signs of Heartworms in Dogs: Behavior Clues

As a veterinary assistant here in Frisco, Texas, I can tell you one of the trickiest things about heartworm disease is how quietly it can start. Many dogs look “mostly fine” at first, and the earliest hints are often changes in behavior: they slow down, seem less playful, or get tired faster than they used to. Because heartworms live in the heart and blood vessels of the lungs, symptoms often show up most during activity, excitement, or any time your dog needs more oxygen. That said, some dogs can cough at rest, and some may look completely normal for a long time.

Let’s walk through the most common behavior changes that may signal heartworm disease, what they can mean, and when to call your veterinarian.

Quick refresher: what heartworms do

Heartworm disease is caused by Dirofilaria immitis, a parasite spread by mosquitoes. Over months, worms mature and live in the pulmonary arteries and sometimes the right side of the heart. This can trigger inflammation, reduce blood flow, and make exercise feel harder for your dog. Over time, it can lead to serious lung disease, heart failure, and in severe cases a life-threatening collapse.

The important takeaway is this: early heartworm disease can be subtle, but it can still be causing internal changes. That is why prevention and routine testing matter so much.

Early behavior changes to watch

Many families notice a “personality shift” before they ever notice a dramatic cough. These behavior clues are worth taking seriously, especially if they persist or seem to be getting worse.

• Less interest in play or walks: Your dog hangs back, quits early, or does not bring you toys like usual.
• Exercise intolerance: They seem fine at the start, then tire quickly, lie down, or move slowly afterward.
• More sleeping or resting: A dog who used to follow you around may start choosing naps over normal activity.
• Reluctance to climb stairs or jump: Some dogs avoid anything that raises their breathing rate.
• “I’m not myself” mood: Less social, less enthusiastic, or mildly irritable. Not every dog does this, but it can happen with chronic discomfort and low stamina.

These signs are not unique to heartworms, but they are classic “pay attention” signals. When they show up together, especially with coughing or quick fatigue, heartworm disease should be on the list of possibilities.

Later signs: behavior plus symptoms

As heartworm disease progresses, behavior changes often become more obvious, and you may also notice physical signs.

• Ongoing cough: Often worse after activity. Some dogs may also cough at rest.
• Getting winded easily: Faster breathing, heavy panting, or needing longer to recover after excitement.
• Decreased appetite: Some dogs become pickier or eat less as they feel unwell.
• Weight loss and muscle loss: You might notice a slimmer look, especially along the back and hind end.
• Low motivation: Dogs may stop greeting you at the door or choose to stay in one spot.

If your dog is showing these signs, do not “wait it out.” A veterinarian can run a heartworm test and help you sort out whether this is heartworms, another lung or heart issue, arthritis, allergies, a respiratory infection, or something else entirely.

Emergency red flags

Some symptoms suggest severe disease or complications and should be treated as urgent.

• Collapse or fainting
• Labored breathing (open-mouth breathing at rest, struggling for air)
• Sudden weakness
• Pale gums
• Swollen belly (possible fluid buildup)
• Dark, tea-colored urine (a possible sign of a serious complication)

One especially dangerous condition is called caval syndrome, which can occur when a large number of worms obstruct blood flow. It is a true emergency and requires immediate veterinary intervention.

Why these changes happen

Behavior is often your dog’s first way of communicating, “This feels hard.” With heartworms, activity can become uncomfortable because:

• Oxygen delivery may be reduced due to inflammation in lung vessels and impaired blood flow.
• The heart works harder, especially during exertion.
• Inflammation can build over time, leading to fatigue and reduced stamina.

That is why a dog may look normal at rest, but seem “off” during walks, playtime, or excitement.

How vets diagnose heartworms

Diagnosis is not based on symptoms alone. Many conditions can mimic heartworm disease, and many heartworm-positive dogs do not look sick early on.

In clinic, we commonly use a combination of:

• Heartworm antigen test: A blood test that detects proteins from adult female heartworms.
• Microfilaria test: Checks for microfilariae (immature heartworms) in the bloodstream.
• Chest X-rays: To assess lungs and heart changes.
• Bloodwork: Helps evaluate organ function and inflammation.

One extra nuance that is helpful to know: after a new infection, there is a window of time when tests may still be negative. If your dog has had a recent lapse in prevention or a possible exposure, your veterinarian may recommend repeat testing based on timing and risk.

If your dog has never been on prevention, has missed doses, or you are unsure of their history, it is worth talking with your vet about testing, even if symptoms are mild.

What to do right now

If you suspect heartworm disease, your most helpful next steps are calm, practical ones.

• Call your veterinarian and schedule a test. Mention the specific behavior changes you are seeing and when they started.
• Keep activity gentle. Until you know what is going on (and especially if heartworm disease is suspected or confirmed), avoid intense exercise and high-excitement play. Exercise restriction is commonly recommended because heavy activity can worsen stress on the heart and lungs and increase the risk of complications.
• Start tracking symptoms now. While you are arranging your visit, jot down coughing frequency, stamina on walks, appetite, and energy, even if it is just for a few days.
• Check prevention history. Write down the product name, last dose, and any missed months.

If you only remember one thing: heartworm disease is far easier to prevent than to treat. Consistent prevention plus testing at least annually (or as recommended by your veterinarian) can protect your dog’s heart and lungs for life.

Prevention basics

Heartworm preventives are typically monthly chewables or topicals, and there is also a long-acting injectable option available through your veterinarian. These medications work by killing immature larvae acquired from mosquito bites.

Because mosquitoes can pop up in warm spells and many regions have long mosquito seasons, prevention is usually recommended year-round in most parts of the U.S. The best plan for your dog depends on your location, lifestyle, and medical history, so your veterinarian is your best guide.

This article focuses on dogs. Cats can also get heartworm disease, but it looks different and is diagnosed and managed differently, so it is worth asking your veterinarian what prevention is best for any cats in the home.

FAQ

Can heartworms cause anxiety or restlessness?

They can. Some dogs become restless because they feel short of breath or uncomfortable, especially after activity. Others become quieter and withdraw. Behavior shifts are common with chronic illness.

My dog is coughing, but still playing. Could it still be heartworms?

Yes. Some dogs push through symptoms, and early disease can be intermittent. A simple test is the only way to know.

Do indoor dogs need heartworm prevention?

In many areas, yes. Mosquitoes get indoors, and it only takes one bite. Indoor lifestyle lowers risk but does not eliminate it.