Symptoms of Heartworm in Dogs

In its earliest stages, dogs may show little to no signs of heartworm disease, making it easy to overlook…until symptoms become more severe.

Recognising the symptoms of heartworm in dogs early can make a critical difference in how well your dog recovers.

Whether you’re watching for subtle changes or trying to understand heartworm symptoms in more advanced cases, this guide outlines what to look for, what the signs mean, and how to take action.

Signs of heartworm in dogs

The severity of heartworm disease can vary depending on factors such as the number of worms and the dog’s size, age, and overall health.

Here’s what to watch for — from the first warning signs to the symptoms of late-stage heartworms in dogs.

Early symptoms of heartworms in dogs

These initial signs may be mild and easy to miss. However, they may still indicate the early presence of heartworms.

- Mild cough

A persistent, dry cough is often one of the first signs of heartworms.

- Lethargy

You might notice your dog sleeping more than usual or appearing tired even after light activity.

- Weight loss

Dogs with heartworm infections may gradually lose weight, even without changes in diet.

- Exercise intolerance

What used to be an enjoyable walk may now leave your dog out of breath or unwilling to continue.

Advanced symptoms of heartworms in dogs

As the disease progresses, the symptoms of late-stage heartworms in dogs become more pronounced and serious. The symptoms are associated with heart failure.

- Difficulty breathing

Dogs may begin to wheeze, pant heavily, or show laboured breathing, even at rest.

- Swollen abdomen

Fluid buildup caused by heart failure can cause the belly to appear distended or bloated.

- Fainting or collapse

If heartworms block blood flow or disrupt heart rhythm, dogs may suddenly collapse.

- Abnormal heart sounds

A vet may detect changes in heart rhythm or sounds, such as murmurs or irregular beats. These symptoms of heartworms in dogs reflect the widespread impact heartworms can have on the circulatory and respiratory systems.

Heartworm treatment for dogs

Treating heartworm disease is possible, but it involves the use of injectable medications prescribed by a veterinarian. Treatment can involve significant costs and comes with risks, making prevention all the more important.

Using a vet-recommended heartworm prevention product such as NexGard SPECTRA^® for Dogs is recommended to protect your dog. NexGard SPECTRA provides the most complete protection against fleas, ticks, mites, lice, heartworm and intestinal worms, all in one tasty, monthly chew.

FAQ

How can you tell if your dog has heartworms?
Your veterinarian can perform blood tests to assess whether your dog is infected. Typical signs include coughing, lethargy, weight loss, and difficulty breathing.
What is the first stage of heartworm?
In early infection, dogs may have no visible symptoms. The first signs often include a mild cough or, fatigue/breathlessness after exercise.

What are the symptoms of late-stage heartworms in dogs?
Severe breathing problems, fainting, abnormal heart sounds, and a fluid-filled abdomen are all symptoms of late-stage heartworms in dogs.
Can a dog survive heartworm disease?
Heartworm disease can be fatal without veterinary treatment and the prognosis depends on the severity of the disease.