A dog jumps after a frisbee
A dog jumps after a frisbee

Heartworm Disease in Dogs

Heartworm disease in dogs occurs after heartworms are transmitted via a mosquito bite. Heartworm disease causes lasting damage to the heart, lungs, and arteries, and can affect the dog’s health and quality of life. Read on to learn more about heartworms and what you can do to spot the signs of heartworm disease.

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Image of a mosquito

How Do Dogs Get Heartworms?

Dogs get heartworms through the bite of an infected mosquito. Because heartworms require 2 host animals to complete their life cycle, the mosquito acts as the intermediate host. It first bites an infected animal and ingests the microfilariae, which develop into infective larvae in the mosquito. Then, the infected mosquito feeds on your dog and transfers infective larvae to them, resulting in a heartworm infection. 


Unlike cats, dogs are a natural host for heartworms, which means a heartworm can complete its entire life cycle within them. As a result, your dog could harbor a large number of heartworms and microfilariae at a time.

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A white lab lays on the floor appearing lethargic

Signs of Heartworms in Dogs

In the early stages of heartworm infection, your dog may not show any symptoms. That’s one reason vets recommend heartworm tests during your dog’s annual visits.


Symptoms often do not occur until the disease has progressed in dogs.


These symptoms include:

  • Mild persistent cough
  • Lethargy
  • Exercise intolerance
  • Weight loss
  • Swollen (fluid-filled) belly
  • Difficulty breathing
  • Caval syndrome is a severe form of heartworm disease. Signs include labored breathing, pale gums, and dark, bloody urine
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A vet inspects a dogs heart with a stethoscope

How Heartworms Are Diagnosed in Dogs

The most common method of testing for heartworms is antigen-based testing. Antigen tests detect a substance produced primarily by female heartworms. If the test is positive, it means your dog could be infected. Once a diagnosis is confirmed, your veterinarian will then test your dog to determine the severity of the infection.

These tests include:

  • Chest radiograph
  • EKG
  • Blood pressure
  • Cardiac enzyme evaluation
  • Echocardiogram
  • Blood work
  • Urine testing

How to Treat Heartworms in Dogs

The goal of heartworm treatment is to stabilize your dog, then kill all worms inside their body while keeping side effects to a minimum. Here’s what you can expect if your dog is in treatment: 

Exercise Restrictions

Physical exercise can increase the rate at which heartworms cause further damage in a dog’s heart and lungs.

A dog in a kennel
A dog in a kennel

Disease Stabilization

Your dog’s condition may need to be stabilized with appropriate therapy.

A husky gets an injection from a vet
A husky gets an injection from a vet

Treatment Administration

Once your dog is stable, your vet may recommend a treatment plan that could involve several steps. In a severe, complicated form of heartworm disease (caval syndrome), the heartworms will need to be surgically removed.

A yellow lab wearing a cone gets inspected by a vet
A yellow lab wearing a cone gets inspected by a vet

How to Prevent Heartworms in Dogs

All dogs are at risk for heartworm disease. If your dog isn’t already on a heartworm preventive, ask your vet about starting. For puppies, prevention should begin at 6 to 8 weeks.1 Older dogs should first be tested for heartworms before beginning heartworm preventive. Regardless of age, all dogs should get an annual blood test for heartworm disease.

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A vet is listening to a dog's heartbeat


Heartworm disease is a complex illness that can be deadly for your new pet. In this video, you can learn about common misconceptions surrounding heartworm disease, and the importance of prevention.


  1. 1. Heartworm Basics. American Heartworm Society. https://www.heartwormsociety.org/heartworms-in-dogs. Accessed August 19, 2022.