Heartworm Prevention for Dogs — Calgary

Heartworm (Dirofilaria immitis) is transmitted through mosquito bites and causes progressive, potentially fatal damage to a dog's heart and lungs over months. While Alberta has historically been considered low-risk, climate change is expanding the mosquito range and increasing case reports across the province — and dogs that travel to BC, Ontario, or the US face significantly higher exposure. Annual testing and year-round prevention are now what AAHA recommends for most of North America.

Key Context

Why This Matters

Preventive

Heartworm treatment is a months-long, high-risk, expensive process involving restricted exercise and repeated melarsomine injections — and it carries meaningful complication risk. Prevention costs $80–150 per year. Treatment costs $1,000–$3,000+, with no guarantee of full recovery. The calculation is straightforward. Beyond heartworm, the broad-spectrum parasiticides used for prevention also control intestinal parasites — roundworms, hookworms, whipworms — making year-round prevention beneficial regardless of your specific heartworm risk level.

Evidence-Based

Key Facts

Source: 2022 AAHA Canine Vaccination Guidelines

Heartworm is transmitted exclusively by mosquitoes — a dog cannot get it from another dog through direct contact.

American Heartworm Society Guidelines

Alberta heartworm risk is increasing with climate warming — cases have been documented in the province, and risk is significantly higher for dogs that travel to endemic regions.

2022 AAHA Canine Vaccination Guidelines

Annual heartworm testing is recommended before starting or restarting preventive medication — giving prevention to an already-infected dog without testing can trigger a dangerous reaction.

2022 AAHA Canine Vaccination Guidelines

Year-round broad-spectrum parasiticides (Interceptor Plus, Sentinel, Revolution Plus, Simparica Trio) prevent heartworm AND control intestinal parasites simultaneously.

2022 AAHA Canine Vaccination Guidelines

Heartworm treatment involves strict exercise restriction for months — an active dog's inability to run and play during treatment is a genuine quality-of-life cost in addition to the financial one.

American Heartworm Society Guidelines

Mosquito activity in Calgary peaks July–August — prevention during these months is the minimum standard; year-round use provides intestinal parasite control year-round.

Alberta Health Services environmental data

Action Steps

What Owners Should Do

Practical steps you can take right now.

1
Have your dog tested for heartworm annually before starting or continuing preventive medication — your vet will do a simple blood test.
2
Start heartworm prevention before mosquito season begins in Calgary (by May at the latest) and continue through October at a minimum.
3
Consider year-round broad-spectrum parasiticide for the intestinal parasite control benefit, even in low heartworm-risk months.
4
If your dog travels to BC, Ontario, or any US state, discuss this travel history with your vet and ensure prevention coverage is in place well before travel.
5
Keep prevention records — some products are monthly, some quarterly (Bravecto) — set reminders so coverage doesn't lapse.
6
Store monthly preventives properly — most require storage below 30°C away from direct sunlight.
7
Ask your vet to recommend a product based on your dog's weight, lifestyle, and whether you also want flea/tick coverage — many combination products address multiple parasites.

Red Flags

Warning Signs to Watch For

Know when something needs attention.

Persistent soft cough and exercise intolerance — hallmark signs of established heartworm infection, though they appear late in the disease course.
Rapid breathing at rest, abdominal swelling — advanced heartworm disease affecting cardiac output.
Sudden collapse after exercise — in severe cases, heartworm can cause cardiac shock.

When to See a Vet

See your vet annually for heartworm testing before starting prevention for the season. If you've lapsed on prevention for more than a month or two, test before restarting. If you notice a persistent cough or exercise intolerance in a dog with uncertain prevention history, see your vet promptly.

From the Pack

The PAWS Perspective

What We See

PAWS dogs are active outdoor dogs — daily 45-minute pack walks through Calgary parks year-round. These are not couch dogs. They're in environments where mosquito exposure happens, and many of them travel with their owners. Year-round prevention is the sensible default for this lifestyle.

How Daycare Connects

We don't track which dogs are on parasite prevention, but we do see the downstream effects of gaps in care — dogs with untreated parasites have changes in coat quality, energy level, and gastrointestinal health that are visible over time. Prevention matters for quality of life, not just disease avoidance.

Eric's Take

"I keep all my own dogs on year-round broad-spectrum prevention. The heartworm risk alone might not justify it in Calgary — but the intestinal parasite coverage absolutely does, given how much time our dogs spend in parks and shared outdoor spaces. It's one of the simplest, most cost-effective things you can do."

— Eric Yeung, Owner, PAWS Dog Daycare

Honest Note

We don't require proof of parasite prevention for daycare enrollment. This is a health decision we encourage but can't enforce. Talk to your vet about what's right for your dog.

Common Questions

Protecting Your Dog Against Heartworm Disease — FAQs

Is heartworm really a risk in Calgary?

Risk is lower here than in endemic regions, but it's not zero — and rising. More importantly, the broad-spectrum parasiticides used for heartworm prevention also control intestinal parasites that are absolutely present in Calgary. Year-round prevention makes sense even in a lower-risk region.

Can I skip heartworm prevention in winter when there are no mosquitoes?

For pure heartworm prevention, winter months have no mosquito transmission risk. But most preventives also control intestinal parasites year-round. Whether you continue through winter is a conversation to have with your vet based on your dog's overall parasite risk profile.

What's the difference between the various preventive products?

Products vary in: what they prevent (heartworm only vs. broad-spectrum), administration frequency (monthly vs. every 3 months), and whether they include flea/tick coverage. Simparica Trio and Revolution Plus are popular combination products. Interceptor Plus covers heartworm and intestinal parasites. Your vet can match the right product to your dog's needs.

How much does heartworm treatment cost if my dog gets infected?

Between $1,000 and $3,000 or more, depending on the severity of infection and the clinic. Treatment involves multiple melarsomine injections, strict exercise restriction for several months, and follow-up testing. It's a long, hard process. Prevention is the obvious alternative.

My dog only goes outdoors in the backyard — do they still need prevention?

Mosquitoes don't respect fence lines. Any dog with outdoor access during mosquito season has heartworm exposure risk. Backyard-only dogs are lower risk than those on wilderness adventures, but they're not at zero risk.

What if I forget to give the monthly prevention on time?

Give it as soon as you remember, then continue on the original schedule. If you've missed more than 8 weeks, talk to your vet about whether a heartworm test is warranted before restarting. Set a recurring phone reminder — these are easy to forget.

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