What Causes Heartworms in Dogs and How to Prevent It

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Keep heartworms from affecting your dog.

You’ve likely heard about the dangers of heartworms in dogs. Here’s some good news: You can take at-home measures to decrease the chances of this problematic parasite causing a positive test. Here’s everything you need to know to help prevent heartworm disease in dogs.

What Causes Heartworms in Dogs?

Heartworm disease in dogs is caused by large, string-like worms that can reach up to a foot in length. Heartworms can live in the pulmonary vessels near your dog’s heart for several years, where they can cause severe damage to your dog’s heart and blood vessels.

How Do Dogs Get Heartworms?

Dogs get heartworms from pesky mosquitoes. But preventing your dog from getting mosquito bites is nearly impossible, no matter where you and your dog live. One of the keys to preventing heartworms in dogs is to reduce the number of dogs testing positive for heartworm, to reduce spread.

When a mosquito bites a heartworm-positive dog, it becomes infected with microfilariae, which are heartworms very early in their life cycle. The microfilariae develop into larvae inside the mosquito and as the mosquito bites new dogs, it passes the larvae to the dogs. Once a dog is infected, the larvae grow into adult heartworms and the cycle continues.

How to Prevent Heartworms in Dogs

Preventing heartworms in dogs requires a multifaced approach: using a heartworm preventive product year-round, yearly testing and taking steps to repel mosquitos. Let’s take a closer look.

What Heartworm Disease Preventives are Available for Dogs?

A heartworm preventive product will kill the larval stages of heartworms before they become adult worms. Testing and heartworm preventive products go hand in hand. Talk to your vet for more information but this guidance is a good place to start:

  • For puppies: According to the American Heartworm Society, puppies under 7 months of age can start heartworm prevention without being tested. Why? After a dog is infected it takes at least six months for a test to show positive. You should have your dog tested after 6 months of age, at 1 years old and thereon after yearly.
  • For dogs over 7 months of age who have not been on a preventive: Test first before starting a heartworm preventive. Why? Heartworm preventives don’t kill adult heartworms. Also, giving a dog heartworm preventive if microfilariae are in the dog’s bloodstream can be dangerous.1 Test again six months later, and again six months later (one year from when you began starting the heartworm preventive). Your vet will then test your dog annually, so it’s important to go to every check-up.
  • If you miss a dose: Read the preventive packaging and talk to your vet. Some heartworm preventives such as Interceptor® Plus allow you to create medication dosing and refill reminders. After signing up, you will receive a text or email reminder when it’s time to give your dog the monthly dose.

What Kind of Behaviors Can Lead to Potential Heartworm Infections in Dogs?

Every dog is at risk for heartworm infections, perhaps more so if your dog spends time outside or if you live in hot, humid climates. However, mosquitoes can be found indoors too. According to the American Heartworm Society, heartworm disease has been diagnosed in every state in the U.S.2 How much time your dog spends outside in different climates plays as part, but because mosquitoes are so ubiquitous, giving a monthly heartworm preventive all year long, in every climate, is vital. In addition to taking steps to controlling mosquitoes in your yard and your home, you can also consider using a product to repel mosquitoes on your dog for added protection.

Tips for Remembering Heartworm Prevention in Dogs

It can be easy to remember heartworm preventives immediately following a vet appointment or during peak mosquito season. But as the months go buy, it can also be easy to forget. Here are some tips to keep heartworm prevention top of mind:

  • Give the preventive on the same day every month
  • Put a reminder on your calendar, or set up monthly text or email reminders
  • Don’t delay or skip annual check-ups at the vet, even if your dog seems healthy and happy.
  • Make a habit to renew your preventive prescription at each annual check-up (or better yet — set up automatic renewals)
  • Get in the habit of undertaking seasonal mosquito control tactics each year

What are the Symptoms of Heartworm Disease in Dogs?

In the early stages of heartworm infection, a dog may show no signs of disease. Symptoms can include fatigue, not wanting to exercise, decreased appetite and weight loss, and a mild persistent cough. If heartworm disease persists with no treatment, a dog’s belly may swell due to excess fluid. Heart failure is a possibility.

If a large number of heartworms are present, something called caval syndrome can occur — this is where one or more heartworms block blood flow to the heart. It can be life threatening. Signs of caval syndrome happen quickly and include bloody or dark urine, pale gums and labored breathing. Prompt surgery is necessary.

How Do I Know if My Dog Has Heartworm Disease?

Heartworm is a dangerous disease in dogs and only gets worse the longer it persists. If your dog is exhibiting symptoms of heartworm disease, you haven’t been giving your dog heartworm preventives or it’s been more than a year since you’ve had your dog tested, call your vet. The earlier you catch heartworm disease, the better. The test, a quick blood sample, is simple. Treatment, however, can be complex.

Treating your dog for heartworm disease can be a long and expensive process. Testing for and preventing heartworm disease in your dog is not. Monthly heartworm prevention treatment is one of the easiest, and most loving, things you can do to keep your dog — and other dogs — safe.


  1. “Keep the Worms Out of Your Pet’s Heart! The Facts about Heartworm Disease” (August 22, 2019). U.S. Food & Drug Administration. https://www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease
  2. “Heartworm Prevention for Dogs” (2022). American Heartworm Society. https://www.heartwormsociety.org/pet-owner-resources/heartworm-prevention-for-dogs

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