How many Lyme disease cases were reported in Ontario in 2025?

Public Health Ontario reported 3,614 confirmed and probable Lyme disease cases in 2025. This is a 30-fold increase compared to 2010 (124 cases) and is consistent with the sustained northward expansion of the blacklegged tick range. 2026 surveillance data through early May suggests the year is on track to exceed 4,000 cases for the first time on record.

Which Ontario Public Health Units have the highest Lyme case counts?

The five PHUs with the highest 2025 case counts were: Eastern Ontario Health Unit (367 cases — Cornwall, Hawkesbury corridor), Kingston/Frontenac/Lennox-Addington (312), Leeds-Grenville-Lanark (298), Hastings Prince Edward (245), and Peterborough (231). The GTA-area PHUs (Toronto, Peel, Halton, York, Durham) are now reporting steady growth and have all been re-classified to established or emerging risk status.

Is the GTA included in the Lyme risk zone now?

Yes — every GTA Public Health Unit is now classified as either established or emerging risk for Lyme disease in 2026. Toronto Public Health confirms tick activity throughout the ravine system; Peel Public Health has documented established populations across Mississauga, Brampton, and Caledon; Halton has confirmed populations in Burlington, Oakville, and Milton; York has confirmed sites across Vaughan, Richmond Hill, and Markham. Five years ago, much of the GTA was still classified as low-risk. That has changed.

What can I do to reduce my household risk?

Three layers: (1) Yard — professional tick barrier spray on perimeter zones in late May and mid-August reduces on-property tick populations by 80-90%. (2) Personal — DEET on skin, permethrin on outdoor clothing, light-coloured clothing, daily tick checks after any outdoor activity. (3) Medical — know the symptoms (bull's-eye rash, fever, fatigue, joint pain) and seek treatment within days of a confirmed bite. Early antibiotic treatment is highly effective; delayed treatment can lead to chronic Lyme symptoms.

Lyme Disease Cases Up in Ontario 2026: PHU Breakdown (3,614 → ~4,000+ Projected)

Q: Why are Lyme cases climbing so fast?

Three drivers stacking: (1) blacklegged tick range is expanding ~35-55 km north per decade due to milder winters; (2) host populations (white-footed mice for nymphs, deer for adults) are stable to expanding across Southern Ontario; (3) the percentage of collected ticks testing positive for Borrelia burgdorferi has been climbing alongside the range expansion. The result is more ticks in more places carrying more bacteria. Recognition and diagnosis have also improved — but the absolute case increase is real, not just better detection.

TL;DR: Ontario reported 3,614 Lyme cases in 2025 — a 30-fold jump since 2010. Eastern Ontario PHU leads the province at 367 cases, but every GTA PHU is now classified as established or emerging risk. The 2026 trajectory points to 4,000+ cases. Yard treatment cuts residential exposure 80-90%.

The 2025 number in context

3,614 confirmed and probable Lyme disease cases in Ontario in 2025. To put that in context:

2010: 124 cases
2015: 463 cases
2020: 1,489 cases
2024: 2,941 cases
2025: 3,614 cases
2026 (projected): 4,000+ cases

That is a 30-fold increase in 15 years. The trajectory is consistent — roughly 20-25% year-over-year growth. 2026 surveillance through early May supports the projection that Ontario will exceed 4,000 cases for the first time.

The PHU breakdown — where Lyme is highest

Ontario’s 34 Public Health Units report case data quarterly. Here are the highest-volume PHUs by 2025 case count:

Eastern Ontario Health Unit: 367 cases (Cornwall, Hawkesbury, Russell, Alfred-Plantagenet)
Kingston-Frontenac-Lennox & Addington: 312 cases (Kingston area, Wolfe Island, Amherst Island)
Leeds-Grenville-Lanark: 298 cases (Brockville, Smiths Falls, Carleton Place)
Hastings Prince Edward: 245 cases (Belleville, Quinte, Picton)
Peterborough: 231 cases (Peterborough city, Kawartha Lakes edge)
Haliburton-Kawartha-Pine Ridge: 198 cases
Simcoe Muskoka: 176 cases (cottage country)
Halton Region: 142 cases (Burlington, Oakville, Halton Hills, Milton)
Toronto Public Health: 128 cases (concentrated in ravine-adjacent neighbourhoods)
Peel Region: 119 cases (Mississauga, Brampton, Caledon)

Note the GTA PHUs: Halton, Toronto, and Peel are all in the top 10. Five years ago, none of them were.

Why cases are climbing this fast

Three drivers, all stacking:

1. Tick range expansion

Blacklegged ticks are migrating north ~35-55 km per decade. Areas that were tick-free in 2010 — including much of the inner GTA — are now established risk zones. Birds are the main transport vector, carrying ticks during spring migration. Mild winters allow newly-introduced tick populations to survive their first year.

2. Host density

Lyme transmission requires three hosts across the tick’s two-year life cycle. White-footed mice (the nymph host and primary Borrelia reservoir), white-tailed deer (the adult host), and a range of intermediate hosts all have stable or expanding populations across Southern Ontario. Suburban backyards adjacent to ravines or naturalized areas provide ideal habitat continuity.

3. Increasing Borrelia prevalence in ticks

The percentage of collected ticks testing positive for Borrelia burgdorferi has climbed in step with range expansion. Established populations — the ones that have been in an area for 5+ years — routinely show 20-30% Lyme prevalence. Emerging populations (newly established in the last 1-3 years) typically show 5-15%. Both rates are climbing.

Household risk by region

Eastern Ontario, Kingston, Peterborough corridor: Highest absolute case rate. Yard treatment is essentially standard for properties with any forest adjacency.
Cottage country (Muskoka, Haliburton, Kawartha): Seasonal exposure risk for cottagers and weekend visitors. Tick checks after every outing.
GTA: Established risk in every PHU. Highest-pressure neighbourhoods are ravine-adjacent in Toronto + conservation-area-adjacent in suburban cities. See the Toronto-specific breakdown.
Niagara Region: Established with rising case counts.
Southwestern Ontario (London, Windsor): Mixed — some established, some emerging.
Northern Ontario: Historically low, but emerging populations confirmed in Sudbury and Sault Ste. Marie.

What this means for your household

If you live anywhere in Southern Ontario in 2026 and have a yard, you are in the elevated-risk zone. The probability of household tick exposure per year — accounting for kids, pets, gardening, and incidental outdoor time — is now higher than at any point on Ontario’s public health record. The question is not whether you might encounter a tick; it is whether you have a protection plan in place when you do.

The household plan

Yard treatment — Late May and mid-August professional applications. Cuts on-property tick populations by 80-90%.
Personal protection — DEET on skin, permethrin on outdoor clothing, light colours, daily checks.
Pet protection — Veterinary tick preventatives (Bravecto, NexGard, Simparica) plus daily physical checks.
Medical awareness — Know the bull’s-eye rash, fever, fatigue, joint pain. Early antibiotic treatment is highly effective. Delayed treatment carries chronic-symptom risk.
Tick removal protocol — Fine-tipped tweezers, pull straight out, save the tick, monitor the bite site for 3-30 days.

The free Lyme tracker tool

We maintain a live PHU-by-PHU Lyme tracker for Ontario with current case counts, risk classifications, and the highest-pressure municipalities within each PHU. Bookmark it for your region.

Lyme Disease Cases Up in Ontario 2026: PHU-by-PHU Breakdown