Heartworms in Dogs: A Serious Disease — But Rare in Quebec

Heartworms in dogs are the parasitic roundworms, also called Dirofilaria immitis. These worms, which can grow to 30 cm (about 12 inches) in dogs, live in the pulmonary arteries and sometimes the heart. They can cause permanent damage to the lungs, heart, and blood vessels and may lead to congestive heart failure, respiratory failure, and even sudden death.

Transmission occurs solely through mosquito bites—mosquitoes feeding on infected animals pick up immature larvae, which develop inside the mosquito (if temperatures are high enough) into infective larvae; when that mosquito bites another dog, the cycle continues.

How common are heartworms in dogs in Quebec?

Heartworm prevalence in Canada overall is extremely low—between 0.15 % and 0.32 % of tested dogs, depending on year and region. In Quebec specifically, prevalence estimates range around 0.27 % as of 2016 (0.24–0.31 %).

Historically southwestern Quebec, including the Hudson and Saint-Lazare area, has been a focus for heartworm. The first documented cases date to 1984, and coyotes and foxes in the region (wild reservoirs) have been shown to carry the parasite, with up to 10 % infection rates reported in 1993.

Despite this, heartworms in dogs remain very uncommon even in southwestern Quebec. In a 2010 Canadian survey of 564 dogs diagnosed with heartworm, only 41 cases (7 %) were in Quebec—and most dogs had not traveled outside Canada [1].

In a 2013–14 lab-based serology study (IDEXX SNAP 4Dx), Quebec dogs had positivity rates below 0.5 %, similar to other provinces—Ontario at about 0.12 %, Quebec around 0.27 % [2].

Why do some vets still recommend blanket prevention?

Traditional recommendations, even in Canada, often advise year‑round preventive medication (or at least seasonally from June 1 to November 1). This stems from:

• Educated caution: mosquito season typically runs June through October,
• Wildlife reservoirs (coyotes, foxes),
• The fact that preventives are highly effective when used correctly,
• Avoiding human risk (rare zoonotic cases reported twice in Quebec)

But guidelines from CPEP (Canadian Parasitology Expert Panel) emphasize that annual testing and risk-based decisions are more appropriate in regions of low prevalence like Quebec [2].

Why Routine Preventive Use May Be Misguided in Quebec

1. Very low prevalence = very low true risk

When the prevalence in your region is well below 1 %, the chance that any given dog is infected is extremely low. In populations not receiving preventive medication, prevalence has been around 0.7 % QA survey in 2010; but with preventive compliance it drops to ~0.15 %.

When prevalence is this low, a positive antigen test result is more likely to be a false positive, and blindly treating based on risk without confirming infection creates unnecessary medical expense and exposure to drug side effects.

2. Risk–benefit shifts in low‑risk regions

Heartworm preventives carry risk and aren’t free. If your dog has never traveled outside Quebec and lives in a very low‑risk region, the odds of preventing a nonexistent infection don’t justify default treatment for many dogs.

Contrastingly, annual testing, tied to evidence of travel or exposure, allows for treatment only when truly necessary. Canadian veterinary guidelines acknowledge that in low prevalence areas, testing less frequently than once a year may be justifiable with informed client consent—though annual testing remains standard if prescribing preventives.

3. Wildlife reservoirs

Yes, coyotes and foxes in southwestern Quebec do carry heartworm and serve as reservoirs, but presence alone doesn’t equate to high transmission rates. Infection still requires the correct mosquito species, sufficient temperatures, and an unprotected dog bite, conditions that mid‑Canada winters rarely sustain long enough [3].

4. Seasonality and climate-driven limits

Heartworm larvae develop in mosquitoes only when mean daily temperatures exceed approximately 14 °C. Canadian transmission seasons (e.g. in Ontario/Quebec) typically run from early June to mid‑October.

Current Canadian expert consensus is that starting prevention on June 1 and continuing through November provides ample buffer, but annual temperature trends show no substantial lengthening of the season at present. So there’s no pressing need to change established preventive timing yet.

The Consequences of Actual Heartworm Infection

If your dog does get infected (despite Quebec’s low prevalence), the risks are serious:

Pathology and progression

• Adult heartworms in dogs live in the lung vessels and heart, damaging arterial walls and reducing blood flow. Over time, the heart and lungs lose function, and congestion builds up [4].
• Early clinical signs are subtle: cough (especially after exercise), fatigue, reduced appetite, weight loss, exercise intolerance. Many dogs show no signs until parasite numbers grow significantly.
• Progressive disease classes:
  • Class I – mild or no symptoms,
  • Class II – mild cough, exercise intolerance,
  • Class III – more severe illness, persistent cough and visible heart/lung changes,
  • Class IV – caval syndrome, where heartworm mass blocks blood flow—life‑threatening and requiring surgery.

Treatment is painful, risky, expensive

• Melarsomine, the standard approved adulticide, is an arsenic‑based drug administered via deep intramuscular injections. It kills adult worms but is harsh on the body and may cause adverse effects.
• Treatment protocols may include injections spaced over one to two months, followed by microfilaricide treatment and strict exercise restriction for weeks to prevent dead worms from causing emboli in the lungs, if the dog gets too active, a worm fragment could block a vessel and lead to respiratory failure or death.
• As described in a real case story from PRCKC, the process involves pain, convalescence, months of forced inactivity, restricted walks, and significant emotional and financial strain [5].
• Even once the interior worms are cleared, long‑term damage to pulmonary vessels and the heart may persist, causing ongoing health issues.

Why the stakes remain high even during treatment

• Even dogs that undergo successful treatment remain susceptible to reinfection. Preventatives remain advisable post‑treatment to avoid new infections [6].
• Dogs with higher worm burdens are at particular risk of embolic complications during treatment.
• The overall cost of treating an infected dog both in veterinary time and emotional toll is far higher than prevention or testing.

Why Annual Testing and Risk Assessment is the Best Strategy

1. Annual veterinary exam + antigen test

Heartworm antigen tests detect proteins released by adult female worms. Because it may take 6–7 months post‑infection to reach detectable levels, testing in spring (7 months after mosquito season) is ideal to find late infections

Testing is standard practice when prescribing preventives, and in low‑risk areas, veterinarians may recommend testing only if the dog travels or other risk factors exist. If a dog lives entirely in Quebec year‑round, testing once a year is reasonable; testing less often may be offered under veterinary supervision and with owner consent.

2. Prevent only as needed

If the test result is positive, treatment can proceed. If negative and the dog has never traveled outside Quebec and owner compliance is high many vets will advise skipping preventives during the low‑risk season.

Seasonal preventive use (June through November) may be suggested for dogs with some travel history or uncertain exposure, but it’s better targeted than routine year‑round authorization in low‑risk areas.

3. Good compliance matters most

In 85–90 % of the confirmed cases of heartworms in dogs in Canada, the dogs had not been on preventive during the prior year – meaning risk‑based targeting would identify nearly all infected dogs without overmedicating the entire population.

For the typical Quebec dog, using medications only when indicated (after negative test or as recommended) protects the dog from unnecessary drug exposure while still safeguarding health.

What we recommend

Heartworms in dogs are uncommon in Quebec, and we want to emphasize that preventive measures if used indiscriminately may give false reassurance, making treatment unreliable when dogs truly develop the disease. We recommend:

• Annual full veterinary check‑up,
• Heartworm antigen testing in spring,
• Preventive medication only if necessary, based on test results or travel history,
• Recognizing the seriousness of the disease and the risky nature of treatment if the dog becomes infected.

They cite authoritative sources including veterinary literature and Canadian parasitology guidelines and often reference the Canadian Parasitology Expert Panel and peer‑reviewed studies on prevalence.

Take away message

• Heartworm disease is severe, difficult, and painful to treat and can leave lasting damage even after recovery.
• In Quebec, heartworms in dogs are very rare, with prevalence well below 0.5 %.
• Blanket preventive use (especially year‑round) in low‑risk areas offers minimal benefit and may lead to unnecessary drug exposure.
• Annual veterinary check‑ups with antigen testing in the spring are the safest and most prudent approach.
• If a test is positive or travel or exposure risk is high then treat only when necessary.
• Preventives should be applied responsibly based on evidence, not assumption.

Final Thoughts

While the saying “it’s better to prevent than to treat” holds true, in heartworm’s case, in Quebec, the real and responsible approach is test first, prevent only when needed.

By doing so, you reduce unnecessary drug use, avoid false reassurances, and keep treatment trusted and effective if and when your dog truly needs it.

Also check our other posts on vaccination, tick control and Doberman health in general.