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National Surveillance for West Nile Virus (WNV)Section A: Case DefinitionsThe current Case Definitions were drafted with available information at the time of writing. Case Definitions and Diagnostic Test Criteria are subject to change as new information becomes available. 1) West Nile Virus Neurological Syndrome (WNNS): Clinical Criteria: History of exposure in an area where WN virus (WNV) activity is occurring1
Note: Other emerging clinical syndromes, identified during 2002 included, but were not limited to the following: myelopathy, rhabdomyolysis (acute destruction of skeletal muscle cells), peripheral neuropathy; polyradiculoneuropathy; optic neuritis; and acute demyelinating encephalomyelitis (ADEM). Ophthalmologic conditions including chorioretinitis and vitritis were also reported. Facial weakness was also reported. Myocarditis, pancreatitis and fulminant hepatitis have not been identified in North America, but were reported in outbreaks of West Nile virus in South Africa. “Aseptic” meningitis without encephalitis or acute flaccid paralysis occurring in August and September when WNV is circulating may be due to non- polio enteroviruses circulating at the same time. This should be considered in the differential diagnosis. [Sejvar J et al. JAMA (2003) Vol.290 (4) p. 511-515, Sejvar, J. et al. Emerg Infect Dis (2003) Vol 9 (7) p.788-93 and Burton, JM et al Can. J. Neurol. Sci. (2004) Vol.31 (2) p.185-193] Suspect WNNS Case: Clinical criteria IN THE ABSENCE OF OR PENDING diagnostic test criteria (see below) AND IN THE ABSENCE of any other obvious cause. Probable WNNS Case: Clinical criteria AND AT LEAST ONE of the probable case diagnostic test criteria (see below). Confirmed WNNS Case: Clinical criteria AND AT LEAST ONE of the confirmed case diagnostic test criteria (see below). 2) West Nile Virus Non-Neurological Syndrome (WN Non-NS): History of exposure in an area where WN virus (WNV) activity is occurring1
Suspect WN Non-NS Case: Clinical criteria IN THE ABSENCE OF OR PENDING diagnostic test criteria (see below) AND IN THE ABSENCE of any other obvious cause. Probable WN Non-NS Case: Clinical criteria AND AT LEAST ONE of the probable case diagnostic test criteria (see below) Confirmed WN Non-NS Case: Clinical criteria AND AT LEAST ONE of the confirmed case diagnostic test criteria (see below) 3) West Nile Virus Asymptomatic Infection (WNAI)6: Probable WNAI Case: Probable case diagnostic test criteria (see below) IN THE ABSENCE of clinical criteria Confirmed WNAI Case: Confirmed case diagnostic test criteria (see below) IN THE ABSENCE of clinical criteria Section B: West Nile Virus Diagnostic Test Criteria:Probable Case Diagnostic Test Criteria:AT LEAST ONE of the following:
Note: Immunocompromised individuals may not be able to mount an immune response necessary for a serological diagnosis. West Nile virus diagnostic test criteria for these individuals should be discussed with a medical microbiologist. Confirmed Case Diagnostic Test Criteria: It is currently recommended that health jurisdictions/authorities use the Confirmed Case Diagnostic Test Criteria to confirm index cases (locally acquired) in their area each year; for subsequent cases, health jurisdictions/authorities could use the Probable Case Diagnostic Test Criteria to classify cases in their area as “confirmed”, for the purposes of surveillance. Throughout the remainder of the transmission season health jurisdictions/authorities may wish to document PRNT antibody titres to West Nile virus in a proportion of cases, to be determined by that health jurisdiction/authority, in order to rule-out the possibility of concurrent activity by other flaviviruses. [For further information on diagnostic testing algorithms for West Nile virus, see the section entitled Laboratory Specimen Diagnostic Testing Algorithm in Appendix 4 of the National Guidelines for Response to West Nile virus.] AT LEAST ONE of the following:
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 West Nile virus 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 West Nile virus 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. The NAT that will be used by Blood Operators in Canada is designed to detect all viruses in the Japanese encephalitis (JE) serocomplex. The JE serocomplex includes WN virus and 9 other viruses, although from this group only WN virus and St Louis encephalitis virus are currently endemic to parts of North America. Blood Operators in Canada perform a supplementary WN virus-specific NAT following any positive donor screen test result. 7 Both CDC and commercial IgM / IgG ELISAs are now available for front line serological testing. Refer to appropriate assay procedures and kit inserts for the interpretation of test results. 8 Early in infection the immune system generates antibodies that bind relatively weakly to viral antigen (low avidity). As the infection proceeds, an increasing percentage of newly generated IgG antibody displays higher binding affinity to virus antigen and thus avidity also rises (Note: avidity is usually measured based upon the ability of IgG to dissociate from antigen preparations after incubation with a solution of urea). As long as high avidity IgG is not yet detected in the serum it can be assumed that the individual was exposed to the viral agent during a recent exposure. With respect to West Nile virus infection it has not been precisely determined when (i.e. post-exposure) high avidity antibodies reach levels in serum that can be accurately detected by serological assays (there may be significant variation depending on the individual). However, it has been shown that greater than 95% of sera collected from individuals exposed to WNV 6-8 months previously will have IgG antibodies that bind strongly to viral antigen and will give high avidity scores using both IFA and ELISA testing formats. Note: Avidity testing will not replace confirmatory neutralization testing, non-WNV flavivirus IgG antibody (e.g. dengue, SLE, etc.) may bind to the antigen preparations used in avidity assays. |
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Last Updated: 2005-07-28 | ![]() |