Powassan virus is a rare tick-borne disease that may cause severe illness in children and adults bitten by a tick infected with the virus. Sometimes, the disease doesn’t cause symptoms. Other times, people experience life-threatening complications that affect the brain or nervous system.  

The first case was identified in Powassan, Ontario, in 1958—hence the name Powassan virus. Ticks (of the genus Ixodes) found in the Great Lakes regions in the United States. and Canada, as well as in the Northeastern U.S., may be infected by Powassan virus.  

Powassan virus is very rare, but the number of people who have contracted the disease has risen significantly over the past decade. The Centers for Disease Control and Prevention (CDC) reported a total of 53 cases of Powassan virus in the U.S between 2011 and 2015, compared to 141 cases between 2016 and 2020. Of these, only a small minority of cases lead to death. Cases occur predominantly from the late spring to mid-fall, when ticks are most active.  

No treatments are available to cure Powassan virus, although doctors may treat symptoms of the illness, especially when the virus causes neurological symptoms and requires hospitalization.

Powassan virus is a virus that may transmit to humans through tick bites and causes an illness that ranges from asymptomatic and mild to severe. Symptoms may appear one to five weeks after a person is bitten, although some people don’t experience any symptoms or illness.  

The virus may be spread by deer ticks, groundhog ticks, or Asian longhorned ticks when they attach to people who are spending time outside.  

When a person contracts Powassan virus, there’s no predictable course of illness. Some people don’t experience notable symptoms and may not know they have the condition. Others may experience mild illness. Still others may develop inflammation of the brain (encephalitis) or spinal cord (myelitis). These advanced cases are fatal in about 10% of cases.

A tick bite from an infected deer tick or groundhog tick is the most common cause of acquiring Powassan virus in the U.S.  

During their life cycle, these ticks feed on blood from animals. During the first (larva) and second (nymph) stages of development, the ticks attach themselves to small mammals or birds. During the third stage of development—adulthood—these ticks seek out larger mammals, including humans.  

If, during its first or second stage of development, a tick consumes blood from an animal infected by Powassan virus, it may acquire the virus and then pass it along to the person to whom it attaches during its third stage of development.  

People may contract Powassan virus from an infected tick very quickly after the tick bites them: The virus may pass from tick to human in as little as 15 minutes. By comparison, ticks that spread Lyme disease typically need to be attached to a human host for 36 to 48 hours before they can transmit Lyme disease.  

It may also be possible to contract Powassan virus by receiving a blood transfusion from an infected person, but this is very uncommon. The virus cannot be spread from person to person in any other manner, such as being coughed or sneezed on by someone with Powassan virus.

Some people who have Powassan virus don’t experience any symptoms. Other people may have symptoms such as:

• Fever
• Chills
• Fatigue
• Headache
• Sore throat
• Nausea
• Vomiting
• Rash on the trunk of the body
• Muscle weakness affecting one or both sides of the body
• Neck stiffness
• Limb weakness or paralysis affecting one or both sides of the body
• Dizziness
• Confusion
• Depression
• Poor coordination or loss of coordination
• Trouble speaking or breathing
• Blurry vision
• Double vision
• Photosensitivity
• Abnormal eye movements (nystagmus)
• Seizures
• Memory loss

Because Powassan virus is so uncommon, it may be difficult to diagnose. Doctors may diagnose the illness by asking about a person’s medical history, performing a physical exam, ruling out other conditions, and obtaining diagnostic tests.  

When being evaluated, it’s important for patients to communicate if they spend a lot of time outdoors and whether they noticed any ticks on them after spending time outside.  

During a physical exam, doctors should perform a neurological exam to check for the possibility of neurologic problems, such as headache, photosensitivity, loss of balance, muscle weakness, memory loss, confusion, or difficulty speaking. Doctors may also check for a subtle rash on the trunk of the body, and they may ask about fever, nausea, headache, and other flu-like symptoms.  

A CT scan or MRI may be used to look for neurological changes or rule out other conditions. Blood tests may provide clues of a viral infection (such as mild decreases in peripheral white blood cell or platelet counts), be used to rule out other conditions, and be used to check for the presence of the antibodies specific to Powassan virus. Additionally, a lumbar puncture may be performed to obtain a sample of cerebrospinal fluid, which may contain antibodies particular to Powassan virus. Testing for the direct presence of Powassan virus in the blood or cerebrospinal fluid can sometimes be done, but it is less reliable than antibody testing.  

If a person recovers a tick discovered on their body, some facilities may be able to have the tick tested for the presence of Powassan virus.

There are no treatments for Powassan virus at this time, but the following treatments are available to help address symptoms:

• Rest and liquids may help to ease flu-like symptoms
• Hospitalization is generally necessary for people with symptoms of encephalitis or meningitis; patients may receive intravenous fluids and respiratory support, which may help to reduce swelling in the brain
• Anti-seizure medications may help to control seizures
• High-dose corticosteroids or intravenous immunoglobulin may help to control symptoms in severe cases of encephalitis or meningitis, but more research is needed to verify their effectiveness

About 10% of people who contract Powassan virus die from the disease. About half of people who experience neurological symptoms and recover from Powassan virus report lingering, long-term effects,such as muscle weakness, partial paralysis, headaches, or personality changes. Other people may recover fully.

“The section of Infectious Diseases at Yale is home to internationally recognized experts in vector-borne infectious diseases and provides high-level expertise in the early diagnosis, treatment, and management of arthropod-borne infections, including Powassan virus,” says Yale Medicine infectious diseases specialist Marwan Azar, MD. “Together with specialists in Virology and Neurology, Yale infectious diseases doctors leverage expertise and experience to provide state-of-the-art care for patients with suspected or confirmed Powassan virus infection. Yale’s multidisciplinary approach incorporates broad institutional knowledge and the latest medical innovations to provide top-notch medical care for every patient.”