The short answer
June 1 for prevention, with a heartworm test first in spring. The last dose should be given November 1. That window covers the period when mosquitoes are active enough to transmit heartworm in southern Ontario.
Why June 1
Heartworm transmission requires mosquitoes, and the larvae inside the mosquito need sustained warmth (above 14 degrees Celsius) to develop to the infective stage. In the Oakville and Halton Region area, those conditions reliably start in late May to early June.
Monthly heartworm preventives work retroactively. They kill larvae that entered your dog’s body during the previous 30 days. So a June 1 dose covers any exposure from early May onward, and a November 1 dose catches any late-season transmission from October.
The test comes first
Before starting prevention each spring, your dog needs a blood test for heartworm. This is usually done in April or May. The test detects adult heartworm proteins in the blood.
Why test if your dog was on prevention last year? Because no preventive is 100% effective if a dose was late, missed, or spit out without you noticing. Missing even one month creates a risk window. Giving preventive medication to a dog with an active heartworm infection can cause dangerous complications in some cases.
The test also takes about 7 months to detect an infection, so a spring test picks up anything from the previous summer that the prevention might have missed.
The full annual schedule
April or May: heartworm blood test at your vet visit.
June 1: first monthly preventive dose.
July through October: monthly doses on the same date each month.
November 1: last dose of the season.
Some vets recommend year-round dosing, especially if you’re using a combination product like Simparica Trio that also covers ticks and intestinal parasites. There’s no harm in year-round prevention, and it eliminates the risk of forgetting the start date.
Puppies
Puppies can start heartworm prevention as young as 6 to 8 weeks old, depending on the product. If your puppy was born in spring or early summer, start prevention right away rather than waiting until the next June.
Key takeaways
- Start heartworm prevention June 1 in southern Ontario. Last dose November 1.
- Get a heartworm blood test every spring before starting prevention.
- Monthly preventives work retroactively, killing larvae from the previous 30 days.
- Year-round prevention is fine and simplifies things, especially with combo products.
- Puppies can start as young as 6 to 8 weeks.
References
- Canadian Parasitology Expert Panel. “Ontario Parasite Data.” usask.ca
- Ontario SPCA. “Commonly Asked Questions About Heartworm in Dogs.” ontariospca.ca
- American Heartworm Society. “Heartworm Basics.” heartwormsociety.org
- Worms & Germs Blog. “Seasonal Canine Heartworm Prevention in Canada.” wormsandgermsblog.com