Published 09/02/2020

5 Signs Your Dog May Have Heartworms

dog with heartworms
Fatigue, weight loss, decreased appetite—these could be signs of a general ailment in your dog, or something worse: heartworm disease.

By the time heartworm symptoms appear, it often means the disease has spread. Heartworm signs in dogs tend to show up when the worm reaches maturity, typically around 6 months post-implantation.

This preventable condition starts when a mosquito bites an animal that’s already infected with heartworm. The mosquito takes in worms while ingesting the animal’s blood, then moves to another animal—like your dog—and bites him. When it does this, it transfers the worms from the previous animal.

The repercussions of a mosquito bite could result in a long, expensive treatment plan that’s emotionally draining for both you and your pet.

The kinds of symptoms your dog displays depend on the stage of the heartworm’s lifecycle. Keep in mind, these symptoms may also be signs of other conditions.

Early-stage symptoms of heartworm disease

If your pet hasn’t been consistently treated for heartworm disease, early detection is the next best defense. Unfortunately, treatment for heartworm disease can be difficult, costly, and painful for your dog. Here are five warning signs your dog may have heartworms.

1. Persistent cough

Unlike a regular cough or a kennel cough, which is strong and sporadic, a heartworm- related cough is dry and persistent. In the early stages, the cough may be induced by even small amounts of exercise, as the heartworm parasites make their way into the lungs, creating blockage and discomfort.

2. Lethargy

If your pet suddenly loses interest in going for walks or being active, it may be a sign of heartworm. As the condition worsens, doing any type of physical activity may become too strenuous for your pup.

3. Weight loss

When heartworm disease begins to spread, it becomes tougher for dogs to muster enough energy for even the simplest tasks. Routine actions like eating or enjoying a snack may prove to be too taxing, resulting in rapid weight loss.

Middle-stage symptoms of heartworm disease

Once the heartworms have matured, they inhabit the lungs and veins of the host, causing severe blockage and leading to the following symptoms:

4. Difficulty breathing

Along with coughing, breathing problems mimicking that of an asthma attack may occur in your dog. Fluid can also build around the blood vessels in the lungs, making it difficult for the lungs to oxygenate the blood.

5. Bulging ribs

As fluid continues to fill the lungs, your dog’s chest may seem to protrude. The ribs will also have a bulging appearance as a result of weight loss. This can also be caused by fluid buildup in response to the parasite’s presence. Once the heartworms have fully grown in your pet’s heart and lungs, your dog will exhibit very visible heartworm symptoms, which are unfortunately accompanied by long- term implications for the dog’s health.

Late-stage symptoms of heartworm disease

Just like in the early stage of heartworm disease, you will notice your pet’s lack of appetite, dry cough, and lethargy. In the late stage, however, these symptoms become heightened, and more complications with your pet will start to emerge. Consistent signs of late-stage heartworm disease include:
  • Abnormal sounds within the dog’s lungs
  • An enlarged liver
  • Heart murmur

Treating heartworms before they spread

Identifying the problem is the first step toward recovery. Once the disease has been identified, the next step is killing all adult and immature worms while keeping the side effects of the treatment to a minimum. As we mentioned before, treatment for heartworm disease can be very expensive. The process is long and difficult and can be painful for your dog. In the end, there is no guarantee that your dog will be fully cured of the disease. That’s why prevention is the best medicine. If your pet is free and clear of heartworms, keeping it that way is the best course of action. While heartworm disease is dangerous, it can also be completely avoided, saving you time, money and stress. Talk to your vet about heartworm preventives like HEARTGARD® Plus (ivermectin/pyrantel), a beef-flavored chewable you can administer to your dog once a month.


HEARTGARD Plus (ivermectin/pyrantel) is well tolerated. All dogs should be tested for heartworm infection before starting a preventive program. Following the use of HEARTGARD Plus, digestive and neurological side effects have rarely been reported. For more information, contact your veterinarian or click here for full prescribing information.

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