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Powassan virus encephalitis
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Powassan virus encephalitis

Contributors: Neil Mendoza MD, Raquel Ramos Garcia MD, Zaw Min MD, FACP, Paritosh Prasad MD
Other Resources UpToDate PubMed


Powassan virus (POWV) is a flavivirus spread by ticks that causes encephalitis. Cases have been reported in the United States (primarily the northeastern states and Great Lakes region), Canada, and Russia. The vectors responsible for POWV include 4 tick species: Ixodes cookei (American castor bean tick), Ixodes marxi (squirrel tick), Ixodes spinipalpis, and Dermacentor andersoni (Rocky Mountain wood tick). Multiple mammals act as reservoirs for the virus. Transmission of infection usually occurs from June to September.

Asymptomatic infection is thought to be common. The incubation period is 8-34 days. Few patients recall a tick bite. Symptomatic patients present with fever, headache, nonspecific upper respiratory symptoms, gastrointestinal complaints (including vomiting), and weakness. If infection progresses to meningoencephalitis, then confusion, seizures, and hemiplegia may ensue.

Typical cerebrospinal fluid (CSF) analysis findings are similar to those seen in tick-borne encephalitis (TBE): lymphocytic pleocytosis (usually <500 WBC/mm3) and normal to high protein level. MRI of the brain may reveal abnormalities in the parietal and temporal lobes. Significant morbidity with residual neurological impairment occurs in most cases of meningoencephalitis following infection.

Diagnosis is made by measuring CSF or serum for POWV-specific IgM antibodies or detecting a fourfold rise of virus-specific IgG in paired acute and convalescent sera. These tests are not commercially available but can be requested through state health department laboratories and the Centers for Disease Control and Prevention (CDC). The case fatality rate is estimated at 5%-10%. Half of all survivors have permanent neurologic symptoms (recurring headaches, memory problems, and muscle wasting).

Deer tick virus (DTV), also known as Powassan virus lineage II, has also been reported as a cause of encephalitis. It is a Flavivirus antigenically related to POWV, with 84% of its RNA sequence being identical to POWV. The main vector identified to date is Ixodes scapularis. Although the prevalence of adult deer ticks with the virus is high in the north central and northeastern United States, the first case of DTV encephalitis in humans was reported only in 2009.

A 2013 study suggests that some cases of POWV encephalitis reported in patients from the Lower Hudson Valley, NY, between 2004 and 2012 may have in fact been the result of DTV infection.


A84.81 – Powassan virus disease

416707008 – Powassan encephalitis virus infection

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Differential Diagnosis & Pitfalls

  • Other tick-borne viral encephalitides – May be difficult to distinguish clinically.
  • Bacterial meningitis – Difficult to clinically distinguish from the meningeal form of Powassan encephalitis. CSF analysis in the early phase of POWV infection may be difficult to distinguish from bacterial meningitis if polymorphonuclear cells are predominant. Antimicrobial therapy must be given until this diagnosis is excluded.
  • Lyme disease – Coinfection with DTV and Lyme is possible, since DTV has been detected in I. scapularis ticks, the main vector of Lyme. Serology may help distinguishing these entities, and treatment of Lyme disease may be initiated while awaiting diagnostic confirmation if clinical suspicion is high.
  • Anaplasmosis – Coinfection with DTV is possible, since ticks infected with both DTV and Anaplasma phagocytophilum have been identified. Laboratory findings on biochemistry, peripheral smear, and serology may aid in the differential. Empiric treatment with doxycycline is indicated if there is a high index of clinical suspicion.
  • Poliomyelitis – In endemic areas, presents with ascending paralysis that progresses over days to months. Ataxia is absent.
  • Tick-borne relapsing fever, Borrelia spp. (also Borrelia miyamotoi infection) – High fever (>39°C [102.2°F]) in relapsing episodes that last 1-3 days. This may also present with neurological manifestations including cranial nerve palsies and meningitis but is usually associated with splenomegaly and hepatomegaly.
  • Herpes simplex virus (HSV) encephalitis – Brain MRI usually shows abnormal findings in the temporal lobes. HSV polymerase chain reaction (PCR) in CSF can be diagnostic. Effective antiviral therapy for HSV is available.
  • Leptospiral meningitis (see leptospirosis) – May cause aseptic meningitis. Leptospirosis PCR in CSF is available. Antimicrobial therapy is available.
  • Tuberculous meningitis (see tuberculosis) – Diagnosis is important because treatment must not be delayed. Mycobacterium tuberculosis complex PCR in CSF and CSF acid-fast bacillus (AFB) culture may help in diagnosis.

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Last Reviewed:06/26/2017
Last Updated:08/19/2018
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Powassan virus encephalitis
A medical illustration showing key findings of Powassan virus encephalitis : Seizures, Fever, Headache, Vomiting, Ataxia, Delirium, Tick bite, Asthenia
Copyright © 2022 VisualDx®. All rights reserved.