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Powassan Virus

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Reporting Obligations

Confirmed and suspected cases shall be reported to the local Health Unit.

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Aetiologic Agent

Powassan Virus (POWV) infection is a tick-borne disease caused by the RNA virus POWV. POWV is a species under the genus Flavivirus, and can be further distinguished by genomic sequencing into two lineages: lineage I (also known as ‘prototype POWV’) and lineage II (also known as ‘deer tick virus’ or DTV)

Powassan virus (POWV, Flaviviridae) is transmitted to humans particularly by deer tick or black legged tick (Ixodes scapularis) and the groundhog tick (I. cookie), or the squirrel tick (I. marxi) in Canada.

Clinical Presentation

Symptoms typically occur after 1 week to 1 month after a bite of an infected blacklegged tick (Ixodes scapularis). Most Powassan virus (POWV) infections are asymptomatic, however infected individuals may show mild to severe symptoms such as fever, headache, nausea, vomiting, asthenia, or myalgia. There may be a transient period of remission after the acute febrile phase, followed by worsening neurological deterioration. Neuroinvasive disease may take the form of meningitis and/or encephalitis syndromes. Symptomatic individuals may have focal neurologic findings such as confusion, loss of coordination, difficulty speaking, paralysis, seizures, or coma.

Approximately 50% of people who survive severe disease have long-term health problems, such as recurring headaches, loss of muscle mass and strength, and memory problems.

Modes of Transmission

A bite by infected tick carrying Powassan Virus. It may take as little as 15 minutes for POWV to be acquired from a blood-feeding tick.

Less commonly, other transmission routes might exist, as there has been one case report of possible transfusion-associated infection so far documented (blood donor).

Incubation Period

Signs and symptoms of POWV illness begin to typically appear from 1 week to 1 month after a bite from an infected tick.

Period of Communicability

There is no evidence of person to person spread.

In the reported case of possible transfusion-associated POWV infection, the asymptomatic infected blood donor reported a tick bite 1 month prior to blood donation.

Risk Factors/Susceptibility

Persons with history of tick bites in an area where ticks carry the POWV

Occupations/activities in tall grass or wooded areas where ticks reside and areas of known endemicity for POWV.

Please refer to PHO’s Infectious Disease Trends in Ontario reporting tool and other reports for the most up-to-date information on infectious disease trends in Ontario.

Diagnosis and Laboratory Testing

Diagnosis is based on laboratory confirmation of an infection or supportive laboratory evidence with clinically compatible signs and symptoms of infection. 

Laboratory Confirmation of Infection:  Serological demonstration of a four-fold or greater increase in POWV neutralizing antibody titres by plaque reduction neutralization testing (PRNT) between acute and convalescent sera taken 2-3 weeks apart;

Supportive Laboratory Evidence of Infection:  Demonstration of POWV antibodies in a single CSF or serum sample by HI or IgM ELISA without confirmation by PRNT.

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Treatment and Case Management

Treatment is under the direction of the attending health care provider.

Patient Information

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Infectious diseases protocol 2023

Appendix 1: Case Definitions and Disease Specific Information: Powassan Virus Infection – Ministry of Health

Public Health Ontario: Powassan Virus

Centers of Disease Control and Prevention: Powassan Virus

Government of Canada: Powassan Virus Disease