Symptoms of Heartworms in Dogs

Heartworm disease is one of those conditions I wish every dog parent in Texas (and honestly, everywhere mosquitoes live) understood early. It can be quiet at first, then become life-changing once the worm burden grows. The tricky part is that many dogs do not look sick in the beginning, which is exactly why prevention and routine testing matter so much.

Quick note: Cats can get heartworms too, but this article focuses on dogs. If you have a cat at home, ask your vet what prevention makes sense for them.

What heartworms do

Heartworms (Dirofilaria immitis) are spread by mosquitoes. After a bite, immature worms develop over months. In many dogs, it takes about 6 to 7 months from infection for adult worms to be present and for common screening tests to reliably turn positive. Adult worms live mainly in the pulmonary arteries (the vessels going from the heart to the lungs). As the worms accumulate, they can trigger inflammation, restrict blood flow, raise pressure in the lungs, and strain the right side of the heart.

Because that process is gradual, symptoms often show up slowly. Some dogs look totally normal until the disease is advanced.

Early symptoms

In the early stages, signs can be mild and easy to mistake for “getting older” or “out of shape.” Watch for subtle changes like these:

• Soft, persistent cough, often worse after activity or at night
• Getting tired faster on walks or needing more breaks than usual
• Less interest in play or reduced stamina
• Mild breathing changes like light panting with little exercise

If your dog has any of these signs and is not on consistent heartworm prevention, it is worth calling your vet and scheduling a heartworm test.

Moderate to advanced symptoms

As heartworm disease progresses, the heart and lungs have to work harder. Symptoms can become more obvious, especially after activity:

• Frequent coughing and more noticeable breathing effort
• Exercise intolerance (your dog stops, sits, or lies down sooner)
• Rapid breathing or heavy panting even at rest
• Weight loss or muscle loss over time
• Decreased appetite
• Lethargy that is out of character

Some dogs will also develop signs related to heart strain, like weakness after exertion. If your dog seems winded just going outside to potty, that is not something to wait on.

Emergency warning signs

Severe heartworm disease can become an emergency. Get immediate veterinary care if you notice:

• Collapse or fainting
• Labored breathing or distress
• Coughing up blood (uncommon, but always urgent)
• Swollen belly (fluid buildup can be a sign of heart failure)
• Sudden weakness with pale gums

There is also a critical condition called caval syndrome (a heavy worm burden interfering with blood flow through the heart). It can cause sudden collapse and requires emergency intervention.

Why some dogs look fine

Dogs can have heartworms and look “fine,” especially early on, or if they are naturally calm and not very active. That is why veterinarians recommend:

• Year-round prevention (mosquitoes can be active in many regions for long stretches, and some can survive in mild winters)
• Regular testing, even if your dog is on prevention, because missed doses and vomiting after dosing can happen

It is much easier to prevent heartworms than to treat them.

What to do next

1) Call your vet for testing

The most common screening is a blood test that detects heartworm antigen. Your vet may also recommend a microfilaria test and additional diagnostics depending on results and symptoms.

Timing matters: If your dog is newly starting, restarting, or has had missed doses, ask your vet when to test now and whether a repeat test in several months is recommended. Because it can take months after infection for tests to turn positive, your veterinarian may build a follow-up timeline specific to your dog.

2) Restrict activity

When heartworms are suspected, keeping your dog calm is important. Strenuous exercise increases strain on the heart and lungs.

3) Ask your vet how to restart prevention safely

If you missed a dose, do not panic. In many cases, veterinarians recommend getting prevention back on board promptly. The safest move is to contact your vet for the right plan and testing timeline, especially if doses have been inconsistent or you are unsure how long the gap was.

How vets stage disease

If a test is positive, your veterinarian may recommend additional steps to understand how advanced the disease is and how to treat it safely. These may include:

• Chest X-rays to evaluate lungs and heart size
• Bloodwork to assess organ function and overall health
• Ultrasound in some cases to visualize heart changes or worms

This matters because treatment and risk level are not one-size-fits-all.

Treatment reality check

Heartworm treatment can be very effective, but it is not a casual process. Many dogs need a structured plan that may include multiple medications and strict exercise restriction for weeks to months. It can also be costly, and the risk of complications is one big reason vets push prevention so hard.

Prevention basics

Heartworm prevention is not just for “outdoor dogs.” Mosquitoes come inside, and it only takes one bite. Prevention typically includes:

• Monthly chew or topical, or an injectable option given at the vet
• Annual testing as recommended by your veterinarian
• Consistency, because missed doses can open a window for infection

If you are in a mosquito-heavy area like North Texas, year-round prevention is commonly advised. Your vet can recommend the best option for your dog’s age, lifestyle, and health history.

Quick symptom checklist

If you only remember one thing, remember this: cough + fatigue + reduced stamina deserves a call to your vet, especially if prevention has been inconsistent.

• Persistent cough
• Tiring easily on walks
• Breathing changes
• Weight loss or decreased appetite
• Collapse or trouble breathing (emergency)

A gentle reminder

If you are worried you might have missed a prevention dose, you are not alone. Life happens. The most helpful next step is simply to schedule a heartworm test and get a plan you can stick with. With early detection and consistent prevention, most families can avoid the heartbreak that heartworm disease can cause.

Note: This article is for general education and does not replace veterinary care. If your dog is coughing, struggling to breathe, collapsing, or otherwise seems in distress, seek urgent veterinary attention.