Powassan Virus

Case Definition

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Confirmed case

Laboratory confirmation of infection by one of the following methods with or without clinical evidence of illness1:
  • ​Isolation of Powassan​ (POW) virus from blood, cerebrospinal fluid (CSF), brain tissue, or any other biological fluid or tissue.
  • Detection of POW-specific RNA in blood, CSF, brain tissue or any other biological fluid or tissue by amplification of a specific target Nucleic Acid Amplification Test (NAAT). 
  • Serological detection of POW-specific IgM by enzyme immunoassay (EIA) AND observation of a fourfold increase in neutralizing antibody titre by plaque-reduction neutralization tests (PRNTs) between acute and convalescent serum without evidence of other flaviviruses.
  • POW-specific IgM seroconversion by EIA (negative to positive) between acute and convalescent serum AND detection of neutralizing antibodies by PRNTs ≥ 20 without evidence of other flaviviruses.
  • Fourfold increase in total antibody titre by hemagglutination inhibition (HI) test between acute and convalescent serum AND detection of neutralizing antibodies by PRNTs ≥ 20 without evidence of other flaviviruses.
  • Seroconversion of total antibody titre by HI test (negative to positive [≥ 20]) between acute and convalescent serum AND detection of neutralizing antibodies by PRNTs ≥ 20 without evidence of other flaviviruses.
  • Detection of POW-specific IgM by an EIA test on CSF (serum is not included) AND observation of a neutralizing antibody titre by PRNTs ≥ 20 without evidence of other flaviviruses.

Probable case

Laboratory evidence of infection in the form of one of the following:
  • ​Serologic detection of POW-specific IgM by EIA assay AND observation of a neutralizing antibody titre by PRNTs assay ≥ 20 on a single serum.
  • 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.
  • Serological detection of a single HI titre ≥ 20 AND detection of neutralizing antibodies by PRNTs.
AND 

Clinical evidence of illness1

1Symptoms in the initial febrile phase of the disease can include fever, sore throat, drowsiness, headache, and disorientation. This can progress to neuroinvasive disease, which can include fever, vomiting, respiratory distress, loss of coordination, speech difficulties, paralysis, and seizures. Clinical evidence includes at least one of the symptoms of the initial febrile phase or neuroinvasive disease.