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West Nile Virus

Case Definition

West Nile Neurological Syndrome (WNNS)

Confirmed Case

Clinical illness AND at least one of the confirmed case diagnostic test criteria

Probable Case

Clinical illness AND at least one of the probable case diagnostic test criteria

Suspect Case

Clinical illness in the absence or pending diagnostic test criteria AND in the absence of any other obvious cause

Clinical Criteria

History of exposure in an area where West Nile virus (WNv) activity is occurring1

OR

history of exposure to an alternative mode of transmission2

AND

onset of fever

AND

recent onset of at least one of the following:

  • encephalitis (acute signs of central or peripheral neurologic dysfunction), or
  • viral meningitis (pleocytosis and signs of infection e.g., headache, nuchal rigidity), or
  • acute flaccid paralysis (e.g., poliomyelitis-like syndrome or Guillain-Barré-like syndrome)3, or
  • movement disorders (e.g., tremor, myoclonus), or
  • Parkinsonism or Parkinsonian-like conditions (e.g., cogwheel rigidity, bradykinesia,
  • postural instability), or
  • other neurological syndromes

West Nile Virus Non-Neurological Syndrome (WN Non-NS)

Confirmed Case

clinical criteria AND at least one of the confirmed case diagnostic test criteria

Probable Case

clinical criteria AND at least one of the probable case diagnostic test criteria

Suspect Case

clinical criteria in the absence of or pending diagnostic test criteria and in the absence of any other obvious cause

Clinical Criteria

History of exposure in an area where WN virus (WNv) activity is occurring1

OR

history of exposure to an alternative mode of transmission2

AND

AT LEAST TWO of the following4:

  • fever,
  • myalgia5,
  • arthalgia,
  • headache,
  • fatigue,
  • lymphadenopathy,
  • maculopapular rash

West Nile Virus Asymptomatic Infection (WNAI)

Confirmed Case

Confirmed case diagnostic test criteria IN THE ABSENCE of clinical criteria

Probable Case

Probable case diagnostic test criteria IN THE ABSENCE of clinical criteria

Confirmed Case Diagnostic Test Criteria:

AT LEAST ONE of the following:

  • a signifi cant (e.g. fourfold or greater) change in WN virus neutralizing antibody titres (using a PRN or other kind of neutralization assay) in paired acute and convalescent sera, or CSF
  • OR
  • isolation of WN virus from, or demonstration of WN virus-specific genomic sequences in, tissue, blood, CSF or other body fluids
  • OR
  • demonstration of WN virus antigen in tissue
  • OR
  • demonstration of fl avivirus antibodies in a single serum or CSF sample using a WN virus IgM EIA (see section 8.0), confirmed by the detection of WN virus specifi c antibodies using a PRN (acute or convalescent specimen)
  • OR
  • a signifi cant (e.g. fourfold or greater) change in fl avivirus haemagglutination inhibition (HI) titres in paired acute and convalescent sera or demonstration of a seroconversion using a WN virus IgG EIA
  • AND
  • the detection of WN specific antibodies using a PRN (acute or convalescent serum sample)

Probable Case Diagnostic Test Criteria

AT LEAST ONE of the following:

  • detection of flavivirus antibodies in a single serum or CSF sample using a WN virus IgM EIA without confirmatory neutralization serology (e.g. PRN)
  • OR
  • a significant (e.g. fourfold or greater) change in fl avivirus HI titres in paired acute and convalescent sera or demonstration of a seroconversion using a WN virus IgG EIA
  • OR
  • a titre of > 1:320 in a single WN virus HI test or an elevated titre in a WN virus IgG EIA, with a confi rmatory PRN result (Note: a confi rmatory PRN or other kind of neutralization assay is not required in a health jurisdiction/authority where cases have already been confi rmed in the current year.)
  • OR
  • demonstration of Japanese encephalitis (JE) serocomplex-specifi c genomic sequences in blood by NAT screening on donor blood, by Blood Operators in Canada

Notes:

  1. History of exposure when and where West Nile virus transmission is present, or could be present, or history of travel to an area with confirmed WNV activity in birds, horses, other mammals, sentinel chickens, mosquitoes, or humans.
  2. Alternative modes of transmission, identified to date, include: laboratory-acquired; in utero; receipt of blood components; organ/tissue transplant; and, possibly via breast milk.
  3. A person with WNV-associated acute flaccid paralysis may present with or without fever or mental status changes. Altered mental status could range from confusion to coma with or without additional signs of brain dysfunction (e.g. paralysis, cranial nerve palsies, sensory deficits, abnormal reflexes, generalized convulsions and abnormal movements). Acute flaccid paralysis with respiratory failure is also a problem.
  4. It is possible that other clinical signs and symptoms could be identified that have not been listed and may accompany probable case or confirmed case diagnostic test criteria. For example, gastrointestinal (GI) symptoms were seen in many WNV patients in Canada and the USA in 2003 and 2004.
  5. Muscle weakness may be a presenting feature of WNV illness. For the purpose of WNV Non-Neurological Syndrome classification, muscle weakness or myalgia (muscle aches and pains) is characterized by a mild, transient, unlikely prolonged symptoms that are not associated with motor neuropathy.
  6. This category could include asymptomatic blood donors whose blood is screened using a Nucleic Acid Amplification Test (NAT), by Blood Operators (i.e. Canadian Blood Services or Hema-Quebec) and is subsequently brought to the attention of public health officials.
SOURCE: West Nile Virus ( )
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