Why are there so many ticks this year?

The short answer

You’re not imagining it. Blacklegged ticks in Halton Region jumped from 0% Borrelia-positive in 2018 to 38% in spring 2025, the highest single-season rate ever recorded locally. Mild winters, expanding deer and mouse populations, migratory birds, and a longer activity window have all aligned. 2026 is on track to be the worst year yet.

What the data says

Halton Region Public Health does a biannual tick drag — pulling a white cloth through forest and meadow to count and test the ticks that latch on. The trend is steep:

• 2018: 0 of 46 blacklegged ticks tested positive for Borrelia burgdorferi (the bacterium that causes Lyme disease).
• 2023: 24 of 143 positive (17%).
• 2024: 30 of 112 positive (27%).
• 2025: 41 of 106 positive (≈39%) — and spring 2025 alone returned 23 of 61 (38%).

Reported human Lyme cases in Halton tell the same story: 3 cases in 2010, 50 cases in 2023, 42 in 2024. Halton’s commissioner of health Dr. Deepika Lobo has publicly attributed the trend to climate change.

The entire region is now classified as an “established” blacklegged tick risk area by Public Health Ontario. As of 2025, 2025 also produced Halton’s first surveillance detections of Anaplasma phagocytophilum, a second tick-borne pathogen carried by the same blacklegged ticks.

Five reasons ticks are surging

1. Mild winters with deep snowpack. Blacklegged ticks overwinter in leaf litter, and 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. Expanding deer and white-footed mouse populations. Mice are the primary reservoir for Borrelia burgdorferi; they infect the larval and nymphal ticks. Deer don’t transmit Borrelia but are the reproductive host adult females need to lay eggs. Both are now abundant in Halton’s wooded ravines and suburban edges.
3. Migratory birds. Researchers estimate that migratory birds carry millions of Ixodes scapularis into Canada each spring, with single birds capable of moving ticks 1,000 km in days. Lake Ontario’s north shore corridor is squarely in the flyway.
4. A longer activity season. Ticks are active any time the temperature is consistently above ~4°C. In southern Ontario that now means roughly mid-March through mid-December, with sporadic activity even on warm winter days.
5. Habitat encroachment. Leaf-litter, brushy edges, and unmowed grass — common in Oakville’s urban forests, ravines, and yards backing onto greenspace — are ideal tick microhabitat.

What it means for Oakville pet parents

The practical implication is that you cannot rely on “tick season” as a window of risk. The Canadian Parasitology Expert Panel and OVC’s Worms & Germs Blog now recommend year-round ectoparasite prevention for dogs and outdoor-access cats in southern Ontario — not the older “April-to-November” pattern.

Local hotspots match the worst-tick habitat: Bronte Creek Provincial Park (where Ontario Parks now posts formal tick warnings), Iroquois Shoreline Woods, the Sixteen Mile Creek and Lions Valley corridor, and all the Conservation Halton parks. Suburban yards backing onto greenspace are also high-exposure — most Halton tick bites happen in someone’s own yard, not on a hike.

If you have a dog and a cat in your home and no tick prevention in place, you’re in the highest-exposure household tier for 2026. Your dog is the most likely member of the family to get bitten and to bring ticks indoors. The bigger danger, though, is your cat — many of the products that protect dogs (permethrin in K9 Advantix II, certain flea collars, DIY tick tubes) are severely toxic to cats.

For the full breakdown of what’s changed and what to do, see our 2026 field guide on ticks in Oakville and Halton.

What to do this week

• Book a vet visit for any pet not currently on a tick preventive. We’ll match the product to your pet’s lifestyle, and run a baseline 4DX test on dogs.
• If you find a tick, save it and submit it to eTick.ca. Pet ticks count for surveillance.
• Do a daily tick check on your dog. Nymph-stage ticks are about the size of a poppy seed and easy to miss.
• For where ticks are landing on dogs and cats, see the safest way to protect your dog from ticks and the worst month for ticks.

Key takeaways

• Halton’s blacklegged tick Borrelia-positivity went from 0% in 2018 to 38% in spring 2025.
• Reported human Lyme cases in Halton went from 3 in 2010 to 50 in 2023.
• Mild winters, deer and mouse populations, migratory birds, and a longer warm season are all driving the surge.
• All of Halton is now an “established” blacklegged tick risk area.
• Year-round prevention is now the standard of care for dogs and outdoor-access cats in southern Ontario.

References

• Halton Region Public Health. “Lyme Disease and Other Tick-Borne Diseases.” halton.ca
• Halton Hills Today. “Halton sees sharp increase in ticks carrying Lyme disease.” haltonhillstoday.ca
• Public Health Ontario. “Vector-Borne Disease Tool.” publichealthontario.ca
• NIH/PMC. “Rapid Northward Expansion of the Blacklegged Tick, Ixodes scapularis, in Response to Climate Change.” pmc.ncbi.nlm.nih.gov
• Ontario Animal Health Network. “2024 OAHN Public Health Update.” oahn.ca