How does Powassan encephalitis spread?
Powassan virus spreads to people through the bite of an infected tick. Three types of tick carry it: the groundhog tick, the squirrel tick, and the blacklegged or deer tick. The groundhog tick and the squirrel tick rarely bite people. The blacklegged, or deer tick, which feeds mainly on white-footed mice and deer, often does.
Ticks become infected when they feed on groundhogs, squirrels, mice or other rodents that have the virus in their blood. People are considered “dead-end” hosts: you cannot pass Powassan virus to another person, and a tick cannot catch it from you. In rare instances the virus has been passed on by blood transfusion, and people with a confirmed infection should not donate blood for 120 days (4 months) after their illness.
Ticks can attach to any part of the body, but are often found in hard-to-see areas such as the groin, armpits and scalp.
Powassan virus can pass from a tick very quickly, within hours of it attaching. This is unlike Lyme disease, where a tick usually has to be attached for a day or more. It is why removing a tick promptly matters.
Where is Powassan encephalitis found?
Powassan virus infections have been identified in the United States, Canada and Russia. Most cases are reported in the northeastern United States and the Great Lakes region. Cases have also been reported in eastern Canada, and Powassan virus is present in parts of eastern Russia, though far fewer cases have been recorded there. Most cases occur from late spring through to mid-autumn, when ticks are most active.
Powassan is uncommon, but the number of reported cases has been rising in recent years. The Centers for Disease Control and Prevention (CDC) has recorded at least 400 cases of Powassan virus disease in the United States between 2004 and 2024. Canada has reported far fewer: 22 cases as of 2018. Because Powassan is difficult to diagnose, the true number is likely to be higher than the number reported.
Anyone living in or visiting an area where Powassan virus activity has been identified is at risk of infection. People who work or take part in recreational activities outdoors in endemic areas are at increased risk.
Can you get Powassan encephalitis in the UK?
Human cases of Powassan virus have been reported only in the United States, Canada and Russia. There is no evidence of Powassan virus in the UK, and no UK cases have been reported.
UK ticks can carry other viruses, and these are not Powassan virus. Tick-borne encephalitis virus was first detected in UK ticks in 2019, and infected ticks have since been found in Thetford Forest in Norfolk and Suffolk, on the Hampshire and Dorset border, and in parts of the North Yorkshire Moors. As of April 2026, six UK-acquired cases of tick-borne encephalitis had been reported, the first in 2019. Several are recorded as “TBE complex” cases, because antibody testing could not separate tick-borne encephalitis virus from louping ill virus. Louping ill virus is a related tick-borne virus present in areas of the UK. It causes disease in sheep, and human infections can occur but are very rare: there have been none confirmed in the UK for over 25 years, and reports have been in people whose work brings them into contact with sheep.
The UK Health Security Agency assesses the probability of tick-borne encephalitis infection in the general UK population as very low. Tick-borne encephalitis is a different illness from Powassan encephalitis, and Encephalitis International has a separate factsheet on it: Tick-borne encephalitis
What are the symptoms of Powassan encephalitis?
Many people infected with Powassan virus have no symptoms at all. Where symptoms do develop, they usually start between one and five weeks after the tick bite.
Initial symptoms can include:
- fever
- headache
- vomiting
- weakness
Powassan virus can cause severe illness, including encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain). Symptoms of severe illness include:
- confusion
- loss of coordination
- difficulty speaking
- seizures
- weakness or paralysis (the inability to move your arms and/or legs)
Anyone can develop severe illness if infected with Powassan virus, but some people are at higher risk: older adults, children including infants, and people with a weakened immune system.
How is Powassan encephalitis diagnosed?
It is diagnosed by first reviewing the medical and travel histories. This is followed by a blood test for antibodies, specifically immunoglobulin M, which the body has produced to defend against the infection. A lumbar puncture (spinal tap) may also be conducted to test the cerebrospinal fluid for immunoglobulin M. The immunoglobulin M antibodies can become detectable within a few days to about 1 week after infection and are useful for early detection.
The PRNT (plaque reduction neutralization test), performed on both blood and cerebrospinal fluid samples, is more specific than immunoglobulin M for confirmation. The test can distinguish the Powassan virus from other similar viruses. However, a positive result may take time to develop. The plaque reduction neutralization test typically becomes positive 1-3 weeks after infection and is used for confirmation rather than early diagnosis.
If neurological symptoms are present, brain imaging may be performed, typically using CT (computed tomography) or MRI (magnetic resonance imaging).
How is Powassan encephalitis treated?
Unfortunately, there are currently no specific antivirals, so treatment is largely supportive. Supportive treatment focuses on relieving symptoms and supporting the body while it recovers. It may include pain control for headaches, medicine for nausea and vomiting, and fluids. People with severe disease often need to be hospitalised for support with breathing, staying hydrated, or reducing swelling in the brain, and are monitored closely for raised pressure inside the skull and for seizures.
Some patients have been given steroids, sometimes with apparent benefit and sometimes without. There is too little evidence to say whether they help. Intravenous immunoglobulin (IVIG) has been used in some cases, based on reports of success in West Nile virus, but its effectiveness is unclear.
What is the outlook after Powassan encephalitis?
About half of those who survive Powassan neuroinvasive disease are left with long-lasting neurological problems, such as headaches, muscle weakness, focal paralysis (the inability to move your arms and/or legs) or cognitive difficulties.
What is the mortality rate of Powassan virus?
Around 1 in 10 people who develop Powassan neuroinvasive disease (encephalitis or meningitis) die.
That figure is the risk to people who become severely ill, not the risk to everyone who is infected: many people infected with Powassan virus have no symptoms at all and never develop neuroinvasive disease. Older adults, children, and people who are immunocompromised are at higher risk of severe illness, and it is severe illness that carries this risk of death.
Encephalitis International supports people affected by encephalitis and their families. If you would like to talk to someone, you can contact our support team.
Can Powassan encephalitis be prevented?
There are currently no vaccines, but research is ongoing. Unfortunately, tick-borne encephalitis virus vaccines do not provide cross-protection against Powassan encephalitis as they do for other tick-borne flaviviruses.
Precautions should be taken to avoid tick bites by wearing long trousers and sleeves and using an insect repellent that is effective against ticks. If a tick bites and attaches, this should be removed as soon as possible, using tweezers. Pinch as close to the skin attachment as possible and pull steadily and straight back.
By Prav Prathapan, Encephalitis International, and reviewed by Molly Wilson-Murphy, MD, Boston Children’s Hospital, MA, USA.
FS087V1 Powassan encephalitis
Date created: May 2026 / Last updated: May 2026 / Review date: May 2029
Disclaimer: We try to ensure that the information is easy to understand, accurate and up to date as possible. If you would like more information on the source material and references the author used to write this document, please contact Encephalitis International. None of the authors of the above document has declared any conflict of interest, which may arise from being named as an author of this document.
