Do pets in Ontario need tick prevention year-round?

The short answer

Yes. The Canadian Parasitology Expert Panel and OVC’s Worms & Germs Blog now recommend continuous, year-round ectoparasite prevention for dogs and outdoor-access cats in southern Ontario. Blacklegged ticks are active any time the temperature is above ~4°C — which now includes warm winter days. The old “April through November” pattern is out of date.

What changed

Ontario tick guidance used to be straightforward: start prevention in April, stop in November, take the winter off. That advice was reasonable for the climate of the 1990s and early 2000s, when reliably cold winters killed off the active tick population each year.

That climate is gone. Three things have changed in southern Ontario over the past decade:

1. Mild winters with deep snowpack. Blacklegged ticks overwinter in leaf litter. A thick insulating snow blanket — paradoxically — protects them from the lethal cold snaps that historically capped their northern range. Successive mild Halton winters since 2020 have produced near-100% overwinter survival.
2. Tick activity above 4°C. Blacklegged ticks become active whenever soil and air temperatures stabilize above about 4°C. In southern Ontario that now means roughly mid-March through mid-December in most years, with sporadic activity even on warm winter days (we see it during January thaws).
3. A bigger, more dangerous tick population. Halton Region Public Health surveillance found that the proportion of blacklegged ticks carrying Borrelia burgdorferi climbed from 0% in 2018 to 38% in spring 2025. The active season is longer and the per-tick risk is higher.

The Canadian Parasitology Expert Panel updated its guidance in response. Their current recommendation: continuous year-round prevention for dogs in southern Ontario, and for outdoor-access cats. OVC’s Worms & Germs Blog (Drs. Scott Weese and Maureen Anderson) carries the same advice.

Doesn’t year-round prevention mean over-medicating?

This is a fair question. The honest answer:

• Modern oral isoxazolines (NexGard, Bravecto, Simparica, Credelio) have well-characterized safety profiles, including in long-term use.
• The cumulative cost of year-round prevention is meaningful but lower than the cost of a single Lyme disease workup or treatment course.
• The alternative — pausing in November and resuming in April — leaves real exposure windows, particularly during March warm spells when ticks are active but pet parents aren’t yet thinking about them.

For dogs with seizure history or other conditions where isoxazolines might not be ideal, we have alternatives. The “year-round” message is about the prevention plan, not necessarily a single product class.

What about cats?

Cats split into three categories:

• Outdoor or indoor-outdoor cats: year-round cat-safe preventive recommended (NexGard Combo, Revolution Plus, Bravecto Plus, Credelio Cat).
• Strictly indoor cats with a dog in the house: worth a conversation. Dogs bring ticks indoors that can drop off and find the cat. Many of these cats benefit from a topical preventive, particularly during peak tick season.
• Strictly indoor cats in a tick-free home: generally don’t need tick coverage alone, but other parasite considerations may apply.

Critical: never apply permethrin-containing dog products to a cat, and don’t use them on a dog in a multi-pet home.

What about heartworm season vs tick season?

Heartworm and tick prevention used to be on different calendars — heartworm needed only the warm months when mosquitoes were active. Many newer combination products cover both year-round, which has simplified things. We can talk through whether one product covers both, or whether it makes more sense to layer two.

For more on the overlap, see our post on whether heartworm prevention is the same as flea and tick prevention.

What if I forget a dose?

Don’t panic. A single missed monthly dose creates a coverage gap, but doesn’t immediately put your dog at high risk. Resume prevention as soon as you notice. If the gap is more than a couple of weeks during peak season (May–July or October–November), consider doing a 4DX SNAP test 6–8 weeks later to check for exposure during the gap.

For a 12-week product (Bravecto, Bravecto Quantum), set a calendar reminder. We can do this for you at the clinic.

Should I worry about January or February?

Probably not from new tick exposures (ticks are usually quiescent during sustained cold). But ticks that attached in late November or December can still be feeding, and a January thaw can wake up overwintering ticks for a few days. The simple play: don’t pause prevention based on what the calendar says. Pause based on what the climate does — and the climate doesn’t reliably do what it used to.

For the bigger Halton picture, see our 2026 field guide on ticks in Oakville and Halton.

Key takeaways

• Year-round tick prevention is now the standard of care for dogs and outdoor-access cats in southern Ontario.
• Blacklegged ticks are active any time the temperature is above ~4°C — that now includes warm winter days.
• Halton’s blacklegged tick Borrelia-positivity went from 0% in 2018 to 38% in spring 2025.
• Strictly indoor cats in a multi-pet home should still be considered for prevention — dogs bring ticks indoors.
• Never use permethrin-containing dog products in homes with cats.

References

• Canadian Parasitology Expert Panel (CPEP). “Parasite Prevention Recommendations for Dogs in Canada.” Worms & Germs Blog
• Ontario Animal Health Network. “2024 OAHN Public Health Update.” oahn.ca
• NIH/PMC. “Rapid Northward Expansion of the Blacklegged Tick, Ixodes scapularis, in Response to Climate Change.” pmc.ncbi.nlm.nih.gov
• Halton Region Public Health. “Lyme Disease and Other Tick-Borne Diseases.” halton.ca
• Public Health Ontario. “Vector-Borne Disease Tool.” publichealthontario.ca