West Nile Virus

Case Definition

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

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

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

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

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

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

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

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

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

history of exposure to an alternative mode of transmission²

AND

AT LEAST TWO of the following⁴:

Confirmed case diagnostic test criteria IN THE ABSENCE of clinical criteria

Probable case diagnostic test criteria IN THE ABSENCE of clinical criteria

AT LEAST ONE of the following:

a significant (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
isolation of WN virus from, or demonstration of WN virus-specific genomic sequences in, tissue, blood, CSF or other body fluids
demonstration of WN virus antigen in tissue
demonstration of flavivirus antibodies in a single serum or CSF sample using a WN virus IgM EIA, confirmed by the detection of WN virus specific antibodies using a PRN (acute or convalescent specimen)
a significant (e.g., fourfold or greater) change in flavivirus haemagglutination inhibition (HI) titres in paired acute and convalescent sera or demonstration of a seroconversion using a WN virus IgG EIA
the detection of WN specific antibodies using a PRN (acute or convalescent serum sample)

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)
a significant (e.g., fourfold or greater) change in flavivirus HI titres in paired acute and convalescent sera or demonstration of a seroconversion using a WN virus IgG EIA
a titre of > 1:320 in a single WN virus HI test or an elevated titre in a WN virus IgG EIA, with a confirmatory PRN result (Note: a confirmatory PRN or other kind of neutralization assay is not required in a health jurisdiction/authority where cases have already been confirmed in the current year.)
demonstration of Japanese encephalitis (JE) serocomplex-specific genomic sequences in blood by NAT screening on donor blood, by Blood Operators in Canada

Notes:

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.
Alternative modes of transmission, identified to date, include: laboratory-acquired; in utero; receipt of blood components; organ/tissue transplant; and, possibly via breast milk.
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.
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.
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.