More Than a Summer Nuisance: What to know about Heartworm Season in Canada

Heartworm disease is a serious and potentially life-threatening condition in cats and dogs caused by parasitic worms that live in the heart, lungs, and blood vessels of infected animals. It is spread through mosquito bites, making it a seasonal concern in many parts of Canada.

While often associated with warmer climates, heartworm is present in pets across Canada. According to the Canadian Animal Health Institute, thousands of cases are reported each year, with higher prevalence in regions where mosquitoes are more populous.

Dogs are most commonly affected, with symptoms such as coughing, fatigue, reduced exercise tolerance, and in more advanced cases, difficulty breathing or weight loss.

Cats can also contract heartworm, though signs are often more subtle and may include coughing, vomiting, lethargy, or sudden behavioural changes. In some cases, symptoms can appear suddenly and be more severe despite a lower number of worms.

Because symptoms can be difficult to detect early and vary between species, prevention and routine testing remain especially important.

The “June to November” Rule

In Canada, heartworm transmission typically occurs between June and November. This window reflects the time when mosquitoes are active and capable of carrying and transmitting heartworm larvae.

However, risk levels vary by region. Higher-risk areas include:

• Southern Ontario

• Southern Quebec

• Manitoba

• The Okanagan Valley in British Columbia

Even within these regions, local conditions such as rainfall and temperature can influence mosquito activity year to year, or even day to day.

Why Temperature Matters: The 14°C Rule

Heartworm larvae require consistent temperatures of about 14°C or higher to mature inside mosquitoes. Below this threshold, development stops.

This is important because:

• Warmer temperatures allow larvae to reach the infectious stage

• Extended warm periods can lengthen the transmission season

• Urban environments may retain heat, allowing mosquitoes to remain active longer

In cities, sidewalks, roadways, and dense buildings can create warmer microclimates, which may extend mosquito activity beyond typical seasonal expectations.

Because of this, some pets may be at risk more often than expected, especially those who travel or move between regions.

The Importance of Spring Check-Ups

A regular veterinary visit in the spring is an important step in heartworm prevention, and a good idea for your pet’s overall health. Heartworm testing is routine for a vet, and easily done through a simple blood test. Screening for heartworm helps:

• Detect existing infections early

• Screen for other vector-borne diseases

• Ensure it is safe to begin or continue preventative heartworm medication

Annual testing is especially important for dogs, even if they are already receiving heartworm prevention.

Prioritize Preventative Care

Heartworm is much easier to prevent than to treat. Preventative medications work by eliminating immature larvae before they develop into adult worms. Veterinarians commonly prescribe options such as:

• Revolution Heartworm & Flea Control for Dogs , Revolution Heartworm & Flea Control for Cats, and Revolution Plus for Cats & Kittens are applied topically providing protection against heartworm, fleas, and other parasites for a full month.

• Advantage Multi, a topical solution that protects against multiple parasites

• Interceptor Plus Flavoured Tablets for Dogs, an oral preventative that targets heartworm and intestinal parasites

These medications are typically given monthly and may offer multi-parasite protection depending on the product. When giving a prescription, your veterinarian will advise you of the proper medication usage and schedule.

Pets Drug Mart works alongside veterinarians to ensure accurate prescription fulfillment and convenient delivery.

Protecting Your Pet Year-Round

While heartworm transmission is seasonal, prevention plans may extend beyond peak months depending on your pet’s lifestyle, travel habits, and regional risk.

Staying consistent with medication, scheduling annual testing, and monitoring seasonal changes can help reduce the risk of infection.

Take Action This Season

Heartworm prevention starts with awareness and early action. Speak with your veterinarian about the right protection plan for your pet, and don’t delay starting preventative care.

Once prescribed, you can ask your veterinarian to email or fax the script to Pets Drug Mart - if you have a paper copy, you can mail it to us directly. Get full details about your options in our FAQ (Where do I send prescriptions?) or Contact Us for individual advice. 

Protecting your pet from heartworm is one of the most important steps you can take for their long-term health.