There's a disease quietly spreading across Canada that kills thousands of dogs every year. It's transmitted by a pest you encounter every summer. And by the time your dog shows any symptoms, the damage to their heart and lungs may already be severe.
It's called heartworm disease — and if you're not actively preventing it, your dog could be at risk right now.
What Exactly Is Heartworm Disease?
Heartworm disease is caused by a parasitic worm called Dirofilaria immitis. Despite the name, these aren't small worms — adult heartworms can grow up to 30 centimetres long and live inside a dog's heart, lungs, and surrounding blood vessels for 5 to 7 years. A single infected dog can harbour hundreds of worms at once. The damage they cause is progressive and devastating: inflammation of the blood vessels, reduced heart function, lung disease, and in severe cases, complete heart failure.
How Does a Dog Get Heartworm?
Here's the part that surprises most people: heartworm is transmitted by mosquitoes — not by contact with other dogs. When a mosquito bites an infected animal, it picks up microscopic heartworm larvae. When that same mosquito bites your dog, it deposits the larvae into the bloodstream. Over the next 6 months, those larvae migrate through the body and mature into adult worms living in the heart and lungs. Your dog doesn't need to be near another dog. They just need to be bitten by a single infected mosquito — which can happen in your backyard, on a walk, or even through a window screen.
Is Heartworm Really a Problem in Canada?
Many Canadian pet owners assume heartworm is a southern U.S. problem. This is dangerously incorrect. Heartworm has been confirmed in every Canadian province. The highest-risk areas include southern Ontario along the Lake Erie and Lake Ontario shorelines, southern Quebec along the St. Lawrence River valley, British Columbia's Fraser Valley and Lower Mainland, and Manitoba and Saskatchewan where cases are increasing as mosquito ranges expand northward. Climate change is accelerating the problem — warmer temperatures mean longer mosquito seasons and mosquitoes surviving further north than ever before.
The Terrifying Part: There Are No Early Symptoms
This is what makes heartworm so dangerous. In the early stages — the first year or two of infection — most dogs show absolutely no signs of illness. By the time symptoms appear, the worm burden is typically significant: mild persistent cough, fatigue after moderate exercise, decreased appetite and weight loss, swollen belly from fluid accumulation, and difficulty breathing. In severe cases, dogs can experience caval syndrome — a sudden, life-threatening blockage of blood flow through the heart. Without emergency surgery, it is fatal.
Treatment Is Expensive, Risky, and Hard on Your Dog
If your dog tests positive for heartworm, treatment is possible — but it's nothing like a simple course of antibiotics. Treatment involves multiple injections given deep into the back muscles, strict exercise restriction for 2 to 3 months because physical activity can cause dying worms to dislodge and block blood vessels in the lungs, and additional medications to manage inflammation. The total cost of heartworm treatment in Canada typically ranges from $1,500 to $4,000 depending on severity. Compare that to the cost of prevention: less than $15 per month.
Prevention Is Simple, Safe, and Highly Effective
The good news: heartworm is almost entirely preventable with a monthly prescription medication given during mosquito season. Most Canadian veterinarians recommend starting prevention in May or June and continuing through November. In some regions, year-round prevention is advised.
The best time to start heartworm prevention was last spring. The second best time is right now.
What You Should Do Today
- Book a heartworm test with your veterinarian — a simple blood test can detect infection. Dogs should be tested annually before starting prevention.
- Start a prevention protocol — ask your vet for a prescription and fill it at a licensed Canadian pet pharmacy for the best pricing.
- Don't wait for symptoms — by the time your dog is coughing or struggling to breathe, significant damage has already occurred.
Heartworm disease is one of the most preventable serious conditions in veterinary medicine. A single monthly chew is all it takes to keep your dog protected all season long.
Learn about heartworm prevention options for dogs in Canada
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