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What is Heartworm?

Heartworm is a blood parasite that poses severe health risks to pets in Ontario. Heartworms live in the heart and the blood vessels that supply the lungs.

When heartworm burdens are severe they damage the heart, lungs and liver. They are spread from infected pets and wildlife to uninfected animals by mosquitoes. Treatment for heartworm disease is available. It is, however, expensive and can come with its own set of risks. Prevention is the key to protecting your pet. We recommend testing your pet for heartworm annually and placing them on a preventative medication that will target immature heartworms (microfilaria) in the bloodstream.


Heartworms can grow to a length of 15 to 30 centimeters. In severe cases, a pet may be infested with hundreds of worms.

Clinical signs of heartworm disease can include:

  • cough
  • shortness of breath
  • exercise intolerance
  • lethargy
  • reduced appetite
  • weight loss

Severe cases may result in heart failure, which can present as a swollen belly and legs, due to fluid build up.

Heartworm is not spread directly from an infected animal directly to another. The primary vector for heartworm infection is a mosquito.

There is currently no vaccine available to prevent heartworm disease, however, there are oral and topical preventatives available. Consult with your veterinarian for the best prevention plan for your pet.

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