The short answer
Mosquito bites. That’s the only way. A mosquito bites an infected animal, picks up heartworm larvae, and passes them to your dog on its next blood meal. Dogs cannot spread heartworm directly to other dogs.
The lifecycle in detail
The parasite behind heartworm disease is Dirofilaria immitis, a roundworm that lives in the heart and pulmonary arteries of infected animals. Here’s how the cycle works:
A mosquito bites a dog (or fox, coyote, or wolf) that already has heartworm. It picks up microscopic larvae called microfilariae during the blood meal.
Those larvae develop inside the mosquito over 10 to 14 days, but only if temperatures stay above 14 degrees Celsius. This temperature requirement is why heartworm transmission in Ontario is seasonal, not year-round.
Once the larvae reach the infective stage, the mosquito bites another dog. The larvae enter through the bite wound and begin migrating through the dog’s body.
Over the next 6 to 7 months, the larvae travel through tissue, eventually reaching the heart and pulmonary arteries where they mature into adults. Adult heartworms can be 25 to 30 centimetres long. They mate, produce new microfilariae, and the cycle is ready to repeat whenever a mosquito bites that dog.
Adults can live 5 to 7 years in a dog’s heart.
Why this matters in Oakville
Over 30 species of mosquitoes can transmit heartworm. Southern Ontario has plenty of mosquito habitat, especially around creeks, ravines, and the Lake Ontario shoreline. Bronte Creek, Sixteen Mile Creek, and the network of stormwater ponds throughout Oakville all support mosquito populations through the summer months.
Wildlife reservoirs keep the parasite circulating even if every pet dog were on prevention. Coyotes, which are well-established in Oakville, can carry heartworm and serve as a source for local mosquitoes.
What prevention actually does
Monthly heartworm preventives don’t stop mosquitoes from biting or stop larvae from entering your dog. They kill the larvae that have entered your dog’s body over the previous 30 days, before the larvae can mature and reach the heart. That’s why timing and consistency matter so much.
Key takeaways
- Heartworm spreads only through mosquito bites. No other transmission route exists.
- Larvae need warm temperatures (above 14 degrees C) to develop inside the mosquito.
- It takes 6 to 7 months for larvae to mature into adults in a dog’s heart.
- Adult heartworms live 5 to 7 years and can reach 30 cm in length.
- Wildlife like coyotes keep the parasite circulating in Ontario, making prevention necessary even if local pet infection rates seem low.
References
- American Heartworm Society. “Heartworms in Dogs.” heartwormsociety.org
- VCA Animal Hospitals. “Heartworm Disease in Dogs.” vcahospitals.com
- Cornell University College of Veterinary Medicine. “Heartworm Disease.” vet.cornell.edu
- FDA. “Keep the Worms Out of Your Pet’s Heart.” fda.gov