Clinical Information

Powassan virus (POWV) is an emerging tick-borne virus, harbored by Ixodes species ticks, which are the same ticks that transmit Lyme disease (Borrelia spp.), Babesia spp., and Anaplasma phagocytophilum, among other pathogens. POWV is a member of the Flavivirus genus, which includes other arthropod-borne viruses (arboviruses) such as West Nile virus (WNV) and St. Louis encephalitis virus. Two lineages of POWV have been identified, sharing approximately 94% amino acid sequence identity, including lineage 1, which is the prototypical POWV lineage transmitted by Ixodes marxi and Ixodes cookei, and lineage 2, which includes deer tick virus and is transmitted by Ixodes scapularis. POWV is maintained in the environment in groundhogs, skunks, squirrels, and white footed mice. Unlike other tick-borne pathogens, following tick attachment to a host, POWV can be transmitted in as little as 15 minutes.

Following infection, the incubation period can last anywhere from 4 to 14 days, after which approximately 66% of patients will remain asymptomatic. Symptomatic patients may present with a nonspecific influenza-like illness, including high fever, fatigue, malaise, and myalgia. Approximately 30% of symptomatic patients will progress to develop neurologic manifestations, most commonly encephalitis. While some patients may recover, over 50% of individuals will have persistent neurologic sequelae. POWV has been associated with an overall mortality rate of 10%. Although there is no targeted antiviral therapy and treatment is entirely supportive care, diagnosis is important for a number of reasons, including the ability to discontinue empiric antibiotics and to provide prognostic information for patients and families.

While limited data exist for POWV real-time reverse transcription polymerase chain reaction (RT-PCR) detection, data from testing for the related flavivirus, WNV, suggests that blood may be a more sensitive source than serum for detecting viral RNA. Similarly, data from WNV and other flaviviruses suggest that viral RNA may be detected in urine for a longer period of time than in blood or serum. POWV RNA may be detected from cerebrospinal fluid in cases of neuroinvasive disease.

Powassan infections are often diagnosed based on a patient's symptoms and exposure risk in conjunction with molecular and serologic testing. The use of RT-PCR can provide a rapid laboratory confirmation of POWV RNA early in infection, particularly during the first 7 days of illness when serologic testing is typically negative. After 7 days, POWV RT-PCR is less sensitive and serologic testing is the preferred diagnostic method. It is common for RT-PCR and serology to be used together for diagnosis of early infection, as they are complementary methods.