Lyme disease

Lyme disease is a bacterial infection. A blacklegged tick picks up the bacterium from a small mammal (often a mouse), then passes it on the next time it feeds — sometimes that’s you. Most Canadian Lyme cases come from the same handful of regions where blacklegged ticks are established: southern Ontario, southern Quebec, Nova Scotia, New Brunswick, and pockets of southeastern Manitoba, the lower mainland of British Columbia, and Prince Edward Island.

Most tick bites do not cause Lyme. A bite has to be from a blacklegged tick, the tick has to be carrying the bacterium, and it has to stay attached long enough to transmit. Even in high-prevalence areas, the chance any one bite leads to Lyme is much smaller than most people assume.

The 24-hour rule: an infected blacklegged tick generally needs to be attached for 24 hours or more before it transmits the Lyme bacterium. Some studies suggest transmission can occasionally happen earlier; almost all confirmed cases come from ticks attached well past the 24-hour mark.

This is why same-day tick checks matter so much. A tick you find and remove in the evening after a hike almost certainly hasn’t transmitted yet. A tick you find engorged on day three has had time.

Early Lyme symptoms usually appear three to thirty days after the bite. The classic sign is a rash at the bite site that slowly expands — this is called erythema migrans. The textbook version is a bullseye (red ring around a clear centre), but in Canada most cases produce a solid red oval, not a true bullseye. Either pattern, if it’s expanding over days, is the signal.

Other early signs:

• Fever and chills
• Fatigue that feels disproportionate
• Headache
• Muscle and joint aches
• Swollen lymph nodes near the bite

Later signs, if untreated: arthritis (most often in the knees), facial palsy on one side, numbness or weakness in the arms or legs, neurological symptoms, and irregular heartbeat.

Today (walk-in or family doctor): any expanding rash near a known or suspected tick bite; flu-like symptoms within a month of a bite; or simply a confirmed blacklegged-tick bite in a high-risk area (your doctor may not wait for symptoms).

Emergency room: facial weakness on one side, severe headache with neck stiffness, irregular or very slow heartbeat, seizures, or sudden vision changes within a month of a bite. These are signs Lyme has progressed.

Early Lyme is treated with a course of oral antibiotics, typically doxycycline for adults and older children, or amoxicillin or cefuroxime for younger children and pregnant patients. A standard course is two to three weeks. Most people recover fully when treatment starts early.

Diagnosis in the very early stages is often clinical — your doctor looks at the rash and your exposure history, and treats. Blood tests for Lyme look for antibodies, which take a few weeks to build up; testing too early can return a false negative. If you have an expanding rash after a tick bite in a Lyme area, your doctor will likely treat without waiting for a positive test.

Lyme disease is reported in every province, but the risk is heavily concentrated. The highest rates are in:

• Nova Scotia — the highest per-capita rate in Canada
• Southern New Brunswick
• Southern and eastern Ontario, particularly the Kingston-to-Ottawa corridor and the Bruce Peninsula
• Southern Quebec, particularly the Estrie and Montérégie regions
• Southeastern Manitoba and the Whiteshell
• British Columbia’s south coast and Lower Mainland (western blacklegged tick)

The Public Health Agency of Canada (PHAC) publishes annual case counts and risk maps. The 2024 report logged 5,809 confirmed cases nationally — a rate of 14.1 per 100,000. See the 2024 numbers broken down by province and month.

Public Health Agency of Canada (Lyme disease overview and annual surveillance reports); Canadian Lyme Disease Foundation; provincial public-health units.