Powassan virus

Powassan virus is a flavivirus — the same family as West Nile virus, Japanese encephalitis, and dengue. It cycles between small mammals and ticks in eastern Canada and the northeastern United States. Canadian human cases are rare (a handful per year, typically), but the severity of the disease means it’s worth knowing about.

The critical difference from Lyme: Powassan can transmit within 15 minutes of attachment. There is no 24-hour grace period. Same-day tick checks still help — finding a tick before it embeds at all is what matters — but a tick you remove after a few hours may have already transmitted.

Powassan starts vaguely — fever, headache, weakness, sometimes vomiting — one to four weeks after the bite. Many infections stay mild. The serious cases progress to neurological involvement:

• Confusion or disorientation
• Severe, unrelenting headache
• Seizures
• Difficulty speaking or moving
• Stiff neck
• Loss of coordination

Any of these signs in the month after a tick bite is a same-day emergency.

Emergency room — call 911. Severe headache, confusion, seizures, neck stiffness, sudden weakness, or vision changes within a month of a tick bite are signs of encephalitis or meningitis. Powassan is one cause; the response is the same regardless.

For a mild flu-like illness one to two weeks after a bite, see a walk-in clinic and tell them about the tick. Most cases will turn out to be something else, but Powassan should be on the radar in established Lyme areas.

There is no specific antiviral for Powassan. Severe cases are managed in hospital with supportive care — fluids, breathing support, anticonvulsants, intracranial pressure management. Recovery from severe Powassan can take months and is often incomplete.

The flip side: prevention works. Tick checks, repellent, and same-day removal of any attached tick remain the best protection — Powassan’s short transmission window makes early detection of crawling ticks especially valuable.

Confirmed human cases are concentrated in Ontario and Quebec, with sporadic reports from the Maritimes. The virus has been detected in ticks across most of the blacklegged tick’s Canadian range, but human cases remain in single digits per year nationally.

Public Health Agency of Canada (Powassan virus disease); Ontario Ministry of Health.