Powassan (POW) virus is an RNA virus that belongs to the genus Flavivirus. It is related to West Nile, St. Louis encephalitis, and Tick-borne encephalitis viruses.
Humans become infected with POW virus from the bite of an infected tick. Humans do not develop high enough concentrations of POW virus in their bloodstreams to infect feeding ticks. Humans are therefore considered to be “dead-end” hosts of the virus.
POW virus is maintained in a cycle between ticks and small-to-medium-sized rodents. In North America, three main enzootic cycles occur: Ixodes cookei and woodchucks, Ixodes marxi and squirrels, and Ixodes scapularis and white-footed mice. Ixodes cookei and Ixodes marxi rarely bite humans. Ixodes scapularis often bite humans and is the primary vector of Lyme disease.
There are two types of POW virus in the United States. The first type, often called lineage 1 POW virus, appears to be associated with Ixodes cookei or Ixodes marxi ticks. The other type, lineage 2 POW virus is sometimes called Deer tick virus, and is associated with Ixodes scapularis ticks. Both lineages have been linked to human disease.