The Canine Ledger

Heartworm: Year-Round or Seasonal, and Why the US and Canada Disagree

US experts advise heartworm prevention all year. Canadian experts advise six months. What each says, why they differ, and what to ask your vet.

Written by thecanineledger

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If your dog has a cough, tires quickly on walks, is losing weight or has a swollen belly, book a vet visit soon. If it suddenly collapses, has pale gums or passes red-brown urine, go now. These can have many causes, and heartworm is one.

Our note: These signs come from the Companion Animal Parasite Council’s lists. The urgency levels are ours.

In the United States, the American Heartworm Society tells vets to prescribe heartworm prevention every month of the year. In Canada, a university parasitology group says that for dogs that don’t travel, it “means six treatments per year from June 1st to November 1st” (Companion Animal Parasite Education Program, University of Saskatchewan, 2019).

While both groups are reading the same parasite, they’re answering different questions. One is about where heartworm is spread. The other is about how often owners forget a dose. This page explains both, and what to ask your vet.

1. What heartworm is

A mosquito-borne worm. Heartworm is “a mosquito-borne parasite that primarily infects domestic and wild canids” (McGill et al., Canadian Veterinary Journal 2019, abstract). Canids are the dog family. The adult worms live in the arteries of the lungs and, in heavy infections, the heart.

Tests lag behind infection. The Canadian group explains that the standard blood tests detect adult female worms “about 6-7 months after exposure”. A negative test in early summer says little about the mosquito season that has just started.

Signs. The Companion Animal Parasite Council (CAPC), a US body, lists them: “Clinical signs include cough, dyspnea, weight loss, ascites, jugular venous distension, exercise intolerance, and arrhythmias” (CAPC heartworm guidelines). Dyspnea is laboured breathing. Ascites is fluid in the belly.

2. The US position: every month, all year

The American Heartworm Society (AHS). Its 2024 guidelines recommend vets “prescribe year-round administration of preventive drugs approved by the US Food and Drug Administration (FDA) to prevent heartworm infection and enhance compliance, the latter being particularly important in light of the documented presence of resistant subpopulations” (AHS canine guidelines, revised 2024).

CAPC agrees. “All dogs, regardless of their lifestyle, should be on year-round heartworm prevention.”

The main reason is missed doses, not winter mosquitoes. The AHS concedes that “continuous, year-round transmission may not occur in all regions”. Its argument is that a year-round habit “is a partial safeguard in the event of inadvertent delay or omission of regularly scheduled doses.” It also says that “owner compliance/adherence remains the biggest factor in the “failure” of preventives”.

Disclosure. The AHS guidelines thank sponsors on their second page. All six logos are animal health companies.

3. The Canadian position: match the mosquito season

Heartworm is rare in Canada. The Canadian group says “Heartworm prevalence is, overall, low in Canada”, with local spread only in parts of southern British Columbia, Manitoba, Ontario, Quebec and New Brunswick. Its supplementary page cites a national positive rate of “0.42% (485/115,636 dogs tested)”.

The season is short. “Transmission seasons vary across Canada, but generally range from May to October.” In southern Ontario, based on 30 years of climate data, it “conservatively lasts from the first week of June to the second week in October”.

So prevention follows the season. Dogs in or travelling to affected areas “should receive a heartworm preventive starting one month after transmission begins and finishing within one month following the end of the transmission season.”

Travel matters. Nationally, “at least one third of all cases appear to be travel associated.” A dog that goes south for the winter isn’t covered by the June-to-November plan.

What about climate change? The group says that “At the moment, there is no evidence that the transmission season has lengthened.” That page is dated 2019. The 2019 Canadian study found prevalence fell nationally until the early 2000s, then rose among tested dogs in Manitoba and Quebec from 2007 to 2016.

Our note: The Canadian pages are dated May 2019. We couldn’t find a newer version.

4. Where they agree

Prevention works. The Canadian study estimated that preventive drugs were associated with a “93.0%” reduction in infections in Canadian dogs from 1977 to 2010.

Resistance is real. The Canadian group notes reports of drug resistance “in various parts of the USA – most notably the Mississippi Delta region – as well as in one dog imported from Louisiana into Ontario”.

No “slow kill”. Both groups warn against using monthly preventives alone to treat an existing infection. The Canadian group “strongly advises against use of the “slow kill” method”, because it favours resistance. The AHS says such a method “is not recommended.”

5. Where they differ on testing

The US view. CAPC says “All dogs, including those on heartworm prevention, should be tested annually”. The AHS calls annual testing “an integral part of ensuring that prophylaxis is achieved and maintained.”

The Canadian view. The group notes that annual testing is required for licensed use of preventives in Canada. But it argues that “the problem with annual testing in low risk areas is that little is gained by carrying out the test.” Its alternative is a yearly risk check: has the dog travelled, was it on preventives, and were doses missed? “If any of these issues are a concern, the dog should be tested.”

Our note: We present the Canadian risk-based testing idea as an option to discuss, not a recommendation. It conflicts with the licensing requirement it mentions.

6. What this means for your dog

In the southern US, where the season is long and resistance has been reported, the year-round advice has the strongest case.

In most of Canada, a seasonal plan has expert support, as long as your dog doesn’t travel and you don’t miss doses.

In between, the honest answer depends on where you live, where your dog travels, and how reliably you give a monthly dose. If you often forget, year-round may protect your dog better, wherever you are.

Outside North America, this page doesn’t apply. Heartworm risk in Europe and elsewhere is set by local guidance we haven’t covered here.

Ask your vet:

  • Is heartworm spread locally here, and when is the season?
  • Does my dog’s travel change the plan?
  • Does my dog need a test before starting, and how often after that?

Our note: Section 6 is our summary, not either group’s advice.

→ Preventive Care Without the Upsell: What Your Dog Actually Needs · → Deworming: How Often Depends on Your Dog

7. What we could not verify

Commonly saidWhat we found
“Every dog needs heartworm pills all year”It depends where you live. US groups say yes. A Canadian group says six months for dogs that don’t travel.
“Mosquitoes spread heartworm all winter”Not everywhere. The AHS itself says year-round spread may not happen in all regions.
“Climate change has made the Canadian season longer”No evidence yet, according to the Canadian group’s 2019 page. We found no newer data.
“A negative test means my dog is clear”Not straight away. Tests detect infections about 6 to 7 months after exposure.
“Monthly preventives can treat an infection slowly”Not recommended. Both groups warn it can breed resistance.
“Heartworm is a problem in the UK”Not checked. This page covers North America only.

Sources

Always confirm with your vet before acting on anything on this page.