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Heartworms in dogs: Myths vs. facts

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How heartworms are spread

All dogs are at risk for potentially deadly heartworm disease. Heartworms live in the heart and blood vessels of the lungs of dogs, cats and other mammals like wolves, foxes and coyotes. Heartworms cannot be spread directly from animal to animal without a mosquito as an intermediary. Heartworms are spread when a mosquito bites an infected dog and picks up tiny larvae called microfilariae from the bloodstream.

Then that mosquito bites another dog infecting it with the heartworm larvae. Over the next several months the heartworm larvae grow and migrate to the heart and lungs. These larvae mature into adults, which can be a foot in length, and they produce microfilariae that circulate in the bloodstream. Now this dog is a reservoir of heartworms and is ready for another mosquito to bite and infect yet another dog.

Untreated, heartworm disease can lead to severe problems with the heart, lungs, liver and kidneys and may result in death.

One mosquito bite is all it takes for your dog to get heartworm disease.

Heartworm life cycle

1. A mosquito bites an infected dog and ingests tiny heartworm larvae along with the animal’s blood. Other carriers of heartworm disease include wolves, foxes, ferrets, coyotes and raccoons.

2. Inside the mosquito, these larvae develop into their infective stage.

3. When the same mosquito bites another dog, the larvae infect the healthy animal.

4. Without a monthly dose of preventive, the larvae continue to develop inside the dog, eventually reaching the heart and lungs.

One million dogs are estimated to be heartworm positive in the United States each year.1

Luckily heartworms in dogs are preventable and heartworm prevention is cheap and easy when compared to the lengthy, stressful and expensive treatment.

Treatment can cost up to $1,000 which makes giving a monthly preventive a bargain in comparison. Treatment requires painful, arsenic-based injections to kill the heartworms present inside the lungs and heart. In addition, this is followed by a 1-3 month period of limited physical activity and possible health complications. Surgery may be required for dogs burdened with large amounts of worms. It is much easier to prevent heartworms than to treat them.

    3 Common Misconceptions About Heartworms in Dogs
  • MYTH 1: Indoor dogs are not at risk
  • FACT: Heartworms are spread by mosquitoes. One mosquito bite is all it takes for a dog to be infected. All dogs are at risk.
  • MYTH 2: Only dogs in heartworm epidemic areas like southeastern states need protection against heartworms
  • FACT: Heartworm disease has been reported in all 50 states. One million dogs are estimated to be heartworm positive in the United States each year.1 The disease continues to spread to new regions. The best way to protect your dog is to administer a heartworm preventive year-round as directed by your veterinarian.
  • MYTH 3: Treatment for heartworm disease is just as easy as preventing it.
  • FACT: Treatment requires arsenic-based injections to kill the heartworms living in a dog’s lungs and heart. In comparison to heartworm prevention, the treatment is expensive (up to $1,000) and can be traumatic and risky. It is much easier to prevent heartworms than to treat them.

Important Safety Information:

HEARTGARD (ivermectin) is well tolerated. All dogs should be tested for heartworm infection before starting a preventive program. Following the use of HEARTGARD, digestive and neurological side effects have rarely been reported. For more information, please visit

Click here to download full prescribing information. | Click for other articles about important parasites

1. Line S. Vet’s advice: Beware the rising risk of heartworms in dogs. Available at
2. Vital statistics for your veterinary practice. In: Landeck E, ed. The Veterinary Fee Reference 6th ed., Lakewood, CO: AAHA Press, 2009;57, 200, 213, 226, 259, 333.
3. Market Dynamics: Share report monthly canine heartworm preventives, vet-dispensed treatment; Data on file at Merial.

®HEARTGARD and the Dog & Hand Logo are registered trademarks of Merial.
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