Powassan Encephalitis

Powassan (POW) virus is a flavivirus and currently the only well documented tick-borne transmitted arbovirus occurring in the United States and Canada. A Powassan-like virus was isolated from the deer tick, Ixodes scapularis. Its relationship to POW and its ability to cause human disease has not been fully elucidated. POW's range in the United States is primarily in the upper tier States. In addition to isolations from man, the virus has been recovered from ticks (Ixodes marxiIxodes cookei and Dermacentor andersoni) and from the tissues of a skunk (Spiligale putorius). It is a rare cause of acute viral encephalitis. POW virus was first isolated from the brain of a 5-year-old child who died in Powassan, Ontario in 1958. Patients who recover may have residual neurological problems.

Powassan virus is transmitted by Ixodes cookei among small mammals in eastern Canada and the United States, where it has been responsible for 39 deaths in the U.S. since 2008. Other ticks may transmit the virus in a wider geographic area, and there is some concern that Ixodes scapularis (also called I. dammini), a competent vector in the laboratory, may become involved as it becomes more prominent in the United States.

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Powassan virus neuroinvasive disease cases reported by state, 2004–2013

pow-by-state-2004-2013


Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention

Downloadable version of above map

Data table: From 2004 through 2013, Powassan virus neuroinvasive disease cases have been reported in Maine (2), Massachusetts (1), Minnesota (20), New Hampshire (1), New Jersey (1), New York (17), Pennsylvania (1), Virginia (1) and Wisconsin (13).


For further information we refer you to:

CDC's Tick-borne diseases reference manual

Powassan virus on the Wisconsin Ticks and Tick-borne Diseases website - Department of Entomology, University of Wisconsin-Madison





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