Preventing Heartworm - Dog

Protecting Your Pets from Heartworm 

| April 1, 2022

As temperatures rise, so does the risk of heartworm.  

Testing for heartworm disease and using heartworm prevention is an important aspect of preventative care for your pet. Here’s a brief explanation of what heartworm is, how to test for it, and how to treat it to help lower the risk of your pet contracting this potentially deadly disease.  

What are heartworms?

Heartworms are parasites that can have fatal results for your pet if left untreated. They are transmitted by mosquitoes and spread to the bloodstream. They can travel to your dog’s heart, lungs, and associated blood vessels where they grow and reproduce and can cause a blockage in the arteries. Heartworms can grow as big as 15-30cm in length and in severe cases, a dog can be infested with hundreds of them. 

Early in the disease, infected dogs may show few or no symptoms. Left untreated, it will progress, and the animal may develop a cough or have difficulty exercising. More serious signs include congestive heart failure, anemia, weakness, and collapse. If you notice any of these signs or symptoms, contact your vet right away. 

How do we prevent heartworm infections?


Vets recommend having your pets tested for heartworm year-round, but it is an especially good idea to have your pet tested before heartworm season begins (during the spring). Toronto Humane Society is currently offering heartworm preventatives and heartworm testing for dogs through our Public Veterinary Service. We’re able to offer accessible veterinary care options like this thanks to a grant from PetSmart Charities of Canada.

Even if you’ve used preventative medication in the past, testing is still important as no medication can be 100% guaranteed. Talk to your veterinarian about what options there are for heartworm testing your pet. 


There are treatments available for pets infected with heartworm disease – both medical and surgical. However, treatment can be expensive, hard on the animal, and carries the risk of complications. The best and safest way to protect your dog from heartworm is by giving them preventative medication. 

Your veterinarian can recommend a monthly medication that you can administer at home, usually from June to November.  

Cats can be at risk too

Although heartworm disease is much more likely to affect dogs, it can still cause problems in cats. If you’re a cat owner, you can give preventative medication to cats too, especially if your cat spends time outdoors.  Ask your vet about your cat and heartworm preventatives.