Powassan is a virus from tick bites that can cause encephalitis. It is incredibly rare - at most a dozen cases are diagnosed in the US each year. The disease though is very serious when it occurs. There's concern it may become more common.

What does it cause?

Most people who become infected with the Powassan virus - which is often called the POW virus - do not become sick. In fact, some people in affected areas have antibodies indicating past infection.

(possibly as many as 1-4% in endemic areas, as seen in a small study in 1960 in Ontario).

The few who do become sick, however, can face severe disease.  If symptoms develop, they usually do 1 week to 1 month after the bite. These symptoms can include: fever, headache, confusion, weakness, vomiting, problems speaking or walking, paralysis, facial droop or other nerve damage, and potentially seizures. 

The virus can infect the brain and central nervous system causing encephalitis (brain inflammation) or meningitis (inflammation of the covering of the brain and spinal cord). This illness can be quite severe. About 10% who develop encephalitis die. Of those who live, about half will have neurologic problems, such as headaches, muscles loss, or memory issues.

How do you treat POW?

There are no specific medications. There's no vaccine. Those who become very ill will need to be hospitalized. Some will need supportive care to breathe and IV hydration.

A healthcare provider should be consulted if there's any concern for POW virus.

How do you get it?

It comes from a bite from an infected tick. These are black-legged ticks that will have fed on a rodent with the virus before biting and infecting someone. Normally, the tick will simply bite and spread the virus to another rodent, rather than a person.

Transmission occurs with the tick Ixodes scapularis that bites white-footed mice. This tick is also responsible for spreading Lyme disease, Babesia, and Anaplasmosis. Patients may actually be co-infected with one of these other infections as well. It is though POW can be transmitted faster through a tick bite than Lyme or Anaplasmosis - maybe less than 12 -24 hours.

Transmission can also occur with other ticks: Ixodes cookei with woodchucks and Ixodes marxi with squirrels, but these ticks do not often bite people. There are actually 38 different mammals that can spread the infection.

People do not have high enough blood levels of the virus for the infection to spread from an infected person to a tick. It has not been noted to naturally affect dogs and cats.

All tick stages can spread the virus.

What sort of virus is it?

It's a Flavivirus, like West Nile and St Louis encephalitis - and also Dengue, Yellow Fever, and Tick-borne encephalitis. 

There are different types of POW virus. In the US, there are 2 main strains.

The first type often called lineage 1 POW, is associated with the ticks that don't often bite humans (Ixodes cookei and marxi). Lineage 2 POW virus is associated with Ixodes scapularis, like Lyme disease, and is sometimes called the Deer tick virus (DTV).

Is it rare?

Yes, very. Since 2004, no more than 12 cases have been identified each year in the US; some years there have been only 1 case.

Where is it found?

US, Canada, and Russia (or a similar virus in Siberia)

It is largely found in the Great Lakes region and the Northeast. Over 10 years (2004-13), most cases have been in Wisconsin (13), Minnesota (20), and New York (17). However, because it is spreading through deer ticks now, there is concern the area of its spread may grow.

Some areas have higher rates of the virus in ticks or rodents. A recent study points to a high prevalence in Wisconsin.

It usually happens in late spring to mid-fall, when ticks are biting.

Why does it matter?

POW is an emerging infection. The disease spreads through the deer tick (or Ixodes Scapularis which spreads Lyme Disease).  It is worried that this deer tick will make POW become more common. Lyme Disease has been spreading through this tick into a wider geographic area than before. There's the concern that POW will also spread further. There has been slight increase in cases in the last few years, but only from 1-4 cases to at most 12 cases identified a year. This may simply represent better recognition of the disease. More testing for POW virus has occurred as West Nile Virus has spread.

How is it diagnosed?

Diagnosis is hard. The disease is rare, but it is also likely under-diagnosed.

Antibody testing on blood or CSF(cerebrospinal fluid from a lumbar puncture) can be performed. This testing would need to be requested and occur through state health department laboratories and the CDC. It would not be a standard test performed. Initial antibody testing (for IgM) should be confirmed by testing later on for confirmation (for later development of confirmatory IgG antibodies). It is hard to find any means of culturing the virus. Even most PCR positive samples were from postmortem brain tissue or CSF.

Other testing may point towards POW viral infection, but may not clearly identify it. MRI can show some unusual findings but these are not the most specific (ischemia or demyelinating disease in the parietal or temporal lobes). A test for seizures may show findings similar to another viral brain infection (an EEG may look like HSV encephalitis). A lumbar puncture test for meningitis may show an alarming number of immune cells (white cells), as occurs in meningitis, but will not necessarily identify the cause.

There is no specific treatment and care is supportive, so specific care is not delayed. 

Likely, there are cases which are not diagnosed.

How to avoid ticks:

Avoid areas with high grasses, bushes, and wooded areas

When in these potential tick areas:

  • Use insect repellent with DEET
  • Wear clothing with long sleeves and pants; pull socks over pant legs
  • Use permethrin on clothing

Immediately after visiting tick areas:

  • Do a full-body tick check – especially through bathing/showering
  • Check pets and any gear for ticks

Why is it called POW?

It was named after Powassan, Ontario in 1958 where it was identified.

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