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Epidemiology
Powassan Virus (POWV) is the only known tick-borne virus to be endemic in North America. POWV human illnesses have been reported in the United States, Canada and Russia. POWV has different genetic variations including deer tick virus (DTV) which is transmitted by the black legged deer tick, Ixodes scapularis. It has two distinct lineages. POWV lineage I is transmitted by the Ixodes cookei which is endemic in the Great Lakes region of the United States. POWV lineage II is transmitted by Ixodes scapularis which is endemic in the Northeast United States. Humans can become infected in 15-30 minutes after tick attachment.

Ixodes ticks have three life stages that require a host: larva, nymph and adult. Each stage requires a blood meal to progress to the next life stage. The nymph stage frequently bites humans and is the stage in which I. scapularis is most likely to infection a human host with a pathogen. The most common reservoir (or host) for I. scapularis are white-footed mouse and white-tail deer. The most common reservoirs for I. cookei are skunks, woodchucks and squirrels. Humans are incidental hosts which means the ticks do not need to feed on humans to survive, humans are merely the host they find at the time for their next blood meal.

In the US, the highest incidence of POWV is in Minnesota and Wisconsin, with Massachusetts and New York also having higher incidence than other states in the Great Lakes or Northeast region. POWV was included in the list of nationally notifiable diseases to the U.S. Centers for Disease Control and Prevention (CDC) in 2002. Between 2009 and 2018, 133 cases of neuroinvasive POWV and 12 cases of non-neuroinvasive POWV were reported to the CDC. Since its discovery in 1958, there have only been 150 reported human illnesses caused by POWV. The incidence rate of POWV in the United States was 1 case per year from 1958-2005, and has risen to an average of 10 cases per year since then.

Currently, POWV is detected with IgM antibody capture ELISA of an IgM immunofluorescence antibody (IFA) assay, plaque reduction neutralization test (PRNT), detection of virus-specific nucleic acids, isolation in culture, or a >4-fold increase in antibody titers from paired acute and convalescent sera. These specific tests for POWV can only be done at a state lab or the CDC. Diagnostic criteria as set by the CDC are: resides in an endemic area, reported tick exposure, and presented with fever, altered mental status, seizures and focal neurological deficits and blood, tissue or cerebrospinal fluid (CSF) are positive on Powassan IgM or Powassan PRNT tests.