How Do You Know If Your Dog Has Heartworms?

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Most dog owners have heard of heartworms, but many owners may not be fully aware of the threats heartworms can pose to their dogs. Here’s how to recognize the signs and symptoms of a heartworm infection in dogs.

A sad looking Australian Shepard lying on the wood floor behind a couch.

Heartworms, which are transmitted by mosquitoes and live and grow in your dog’s heart and lungs, can only be diagnosed when they are adults, which typically means after they have been inside your dog for at least four to five months. Knowing how to recognize the symptoms of heartworm earlier can help you get the necessary treatment and care your dog needs faster.

What Are the First Signs of Heartworm in Dogs?

A dog being held in its owner’s arms while a vet is drawing blood from its leg.

A blood test administered by a veterinarian is the most common way to uncover a heartworm infection. However, you may be able to observe several physical signs of heartworms, including:

  • Cough
  • Lethargy or inactivity
  • Decreased appetite resulting in weight loss
  • Heart failure
  • Blood in urine
  • Collapsing or fainting


One of the most noticeable symptoms of heartworms in dogs is a cough. You may hear your dog coughing from time to time, especially after exercise. This can also indicate kennel cough, which is similar to bronchitis for dogs. Either way, a visit to the vet will help identify which illness your dog might have.

Lethargy or inactivity

Any time your dog’s activity level or stamina changes, take them to the vet. Because heartworms get into your dog’s heart, it can be difficult for your pet to complete even the easiest activities.

Decreased appetite resulting in weight loss

As the heartworms mature and migrate into the heart and lungs, the dog may lose their appetite. Losing weight may also be a symptom of heartworm disease.

Heart failure

In most cases, a veterinarian is more likely than a pet owner to recognize the signs of heart failure. However, you may notice erratic heartbeats, or your dog’s stomach may get bigger because their abdomen is filling up with fluid.

Other signs of heartworms in dogs

Some dogs will have blood in their urine, and some may even collapse because their hearts are almost completely blocked by the worms.

Remember: Dogs tend to show more obvious symptoms as the heartworms mature, meaning that when you do find out that your dog has heartworms, the infection is probably already serious.

What Should You Do If Your Dog Has Heartworms?

Your veterinarian will administer a series of treatments over the course of several months, and you’ll have to keep your dog calm and limit activity while the worms die off.

Because heartworm treatment can be a difficult and long process for your dog, preventing heartworm infections is the most effective form of treatment. This is why regularly administering a heartworm prevention treatment for your dog is the best way to keep them protected from this serious infection.

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