Powassan virus

Powassan is a flavivirus, the same family as West Nile, Japanese encephalitis, and dengue. It cycles between small mammals and ticks in eastern Canada and the northeastern United States. Canadian human cases are rare (typically a handful per year), but the ones that happen are severe.

The big difference from Lyme: Powassan can transmit within 15 minutes of attachment. There is no 24-hour grace period. Tick checks still help, because a tick you catch crawling hasn’t bitten yet. A tick you remove after a few hours may have already transmitted.

Powassan starts vaguely, with fever, headache, weakness, and sometimes vomiting, 1–4 weeks after the bite. Many infections stay mild. The serious ones progress to the brain and nervous system:

• 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, or 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 possible cause. The response is the same whatever the cause.

For a mild flu-like illness 1–2 weeks after a bite, see a walk-in clinic and tell them about the tick. Most will turn out to be something else, but in Lyme areas Powassan belongs on the list.

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

Prevention works. Repellent, tick checks, and same-day removal of any attached tick are still the best protection. Because Powassan transmits fast, catching a tick while it’s still crawling matters more than usual.

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.