Heartworm Season: Prevention for Ontario and BC Coastal Areas

Understanding the Heartworm Threat

Heartworm (Dirofilaria immitis) is a serious and potentially fatal disease caused by parasitic worms that live in the heart, lungs, and blood vessels of infected pets. While many people associate heartworm with warmer, tropical climates, the disease is a very real and growing threat in several regions of Canada, specifically Southern Ontario, the Maritimes, and the southern coastal regions of British Columbia.

The Mosquito Connection

Heartworm cannot be passed directly from dog to dog. It requires an intermediary host: the mosquito. When a mosquito bites an infected animal, it picks up microscopic larvae. After about two weeks of warm weather (above 14°C), these larvae mature and are injected into the next animal the mosquito bites. This is why "Heartworm Season" in Canada is directly tied to mosquito activity.

The Canadian Prevention Schedule

In most parts of Canada, mosquito season begins in late May and lasts until the first hard, killing frost in October or November. Veterinarians typically recommend starting preventative medication (pills or topical treatments) in June and continuing through November. This "buffer month" ensures that any late-season bites are treated before the larvae can mature into adult worms.

The "Rescue Dog" Factor

If you have recently adopted a dog from the Southern United States or a tropical region, they must be tested for heartworm immediately upon arrival in Canada. Adult heartworms can take up to 6 months to show up on a standard blood test, so a "clear" test in a puppy may need to be repeated as they age.

Symptoms: The Silent Killer

In the early stages, most dogs show no symptoms at all. As the disease progresses, you may notice a persistent cough, reluctance to exercise, fatigue after moderate activity, and weight loss. By the time severe symptoms appear, the treatment is incredibly invasive and expensive, often requiring weeks of strict crate rest and painful injections.