National case definition: Powassan virus

Date of last revision/review: January 2024

On this page

• National notification
• Type of surveillance
• Case classification
• Laboratory criteria
• Clinical evidence
• Exposure
• ICD code(s)
• Comments

National notification

Confirmed and probable cases of disease should be notified to the Public Health Agency of Canada.

Type of surveillance

Routine case-by-case notification to the federal level.

Case classification

Confirmed case

A case that has confirmatory laboratory results with or without clinical evidence criteria.

Probable case

A case that meets supportive laboratory results and clinical evidence criteria.

Laboratory criteria

Confirmatory laboratory tests include:

• isolation of POW virus (POWV) from blood, cerebrospinal fluid (CSF), brain tissue, or any other biological fluid or tissue; or
• detection of POW-specific nucleic acids in blood, cerebrospinal fluid (CSF), brain tissue or any other biological fluid or tissue; or
• serological detection of POW-specific IgM by enzyme immunoassay (EIA) assay and observation of a significant increase in neutralizing antibody titre by plaque-reduction neutralization tests (PRNTs) between acute and convalescent serum without evidence of other flaviviruses; or
• POW-specific IgM seroconversion by EIA (negative to positive) between acute and convalescent serum and detection of neutralizing antibodies by PRNTs equal to or greater than 20 without evidence of other flaviviruses; or
• significant increase in total antibody titer by hemagglutination inhibition (HI) test between acute and convalescent serum and detection of neutralizing antibodies by PRNTs equal to or greater than 20 without evidence of other flaviviruses; or
• seroconversion of total antibody titre by HI test (negative to positive [equal to or greater than 20]) between acute and convalescent serum and detection of neutralizing antibodies by PRNTs equal to or greater than 20 without evidence of other flaviviruses; or
• detection of POW-specific IgM by an EIA test on CSF (serum is not included) and observation of a neutralizing antibody titre by PRNTs equal to or greater than 20 without evidence of other flaviviruses.

Supportive laboratory tests include:

• serologic detection of POW-specific IgM by EIA assay and observation of a neutralizing antibody titre by PRNTs assay equal to or greater than 20 on a single serum; or
• serological detection of POW-specific IgM by EIA without a significant increase in neutralizing antibody titre by a PRNTs test between serum collected in the acute phase and that collected in the convalescent phase; or
• serological detection of a single HI titre equal to or greater than 20 and detection of neutralizing antibodies by PRNTs.

Clinical evidence

Clinical description:

Most people who become infected with POWV are asymptomatic.

The incubation period ranges from 1 to 5 weeks, although most patients are unaware of when they were exposed.^{Footnote 1} Symptoms in the initial febrile phase of the disease can include fever, sore throat, drowsiness, headache, and disorientation.^{Footnote 2}

This can progress to neuroinvasive disease, which can include fever, vomiting, respiratory distress, loss of coordination, speech difficulties, paralysis, and seizures.^{Footnote 2}

Case fatality of Powassan neuroinvasive disease has been reported as high as 10%, with up to 50% of survivors experiencing long-term neurologic sequelae.^{Footnote 1}

Clinical criteria:

Clinical evidence includes at least one of the symptoms of the initial febrile phase or neuroinvasive disease (see clinical description).

Exposure

Powassan virus disease is a tick-borne illness caused by the flavivirus POWV. There are two lineages of POWV that both cause disease in people, namely: the prototypic lineage, POWV lineage I, and the deer tick virus or POWV lineage II. POWV is transmitted to humans by a small number of species of Ixodes ticks, including the:

• blacklegged tick (Ixodes scapularis), the main vector of deer tick virus
• groundhog tick (Ixodes cookei) and the squirrel tick (Ixodes marxi), the main vectors of POWV lineage I

Consideration should be given to patients who have recently spent time in potential tick habitats, or who have a history of tick bite (although as many patients have no recollection of a tick bite, lack of a tick bite should not preclude consideration of POWV). In addition, travel history to areas endemic for other flaviviruses, such as tick-borne encephalitis , West Nile virus, dengue or Zika virus should be considered due to likelihood of cross-reactivity in laboratory tests. The spatial distribution of deer tick virus in Canada is similar, but more restricted, to the geographical distribution of established blacklegged tick populations that correspond to Lyme disease risk areas.

Updated information regarding the distribution of Lyme disease risk areas

The risk of transfusion-associated transmission remains uncertain and merits vigilence.^{Footnote 3}

ICD code(s)

ICD-10-CA code(s)

• A84.8, Other tick-borne viral encephalitis (includes Louping Ill and Powassan virus disease)

ICD-9-CA code(s)

• 063.8, Other tick-borne viral encephalitis

Comments

• These are definitions for surveillance and epidemiologic purposes only and should not be used for clinical diagnostic purposes.
• Due to serological cross-reactivity between flaviviruses, a single-sample PRNTs antibody titre must be at least four-fold greater than that for other relevant flaviviruses (e.g. tick-borne encephalitis, West Nile virus, dengue, Zika), based on geographic area of exposure, travel and/or vaccination history.
• In immunosuppressed patients, low titres may be observed and alternative tests, such as PCR, should be considered in these cases. Furthermore, in some cases, only a single serum sample may be received as a result of a fatal illness.
• CSF should not be submitted exclusively, as the performance diagnostics of IgM (enzyme-linked immunoassay) ELISA on these samples remains unknown. CSF samples should be submitted in parallel with serum samples.
• Diagnostic testing should be performed by provincial public health laboratories and/or appropriate reference diagnostic centres such as the National Microbiology Laboratory (NML). PRNTs for detecting POWV-specific antibodies should be performed at an appropriate reference centre such as NML.