Case Definition for UK-acquired West Nile virus

WNV surveillance concentrates on the period between 1st June and 31st October every year. Clinicians should be aware of the possibility of WNV infection during this period particularly in those aged over 50 years during these months. Microbiologists may wish to consider WNV in patients with otherwise unexplained neurological or other compatible symptoms.

Definition for suspected cases of UK-acquired WNV infection in humans - Indications for considering the diagnosis of WNV infection and requesting a WNV test (adapted from EU case definition*)

WNV Neurological Syndrome: An adult (particularly aged 50 years and over) case of encephalitis or meningoencephalitis or aseptic meningitis or acute flaccid paralysis, defined by the specific criteria below, presenting from 1st June to 31st October and with no travel history outside the UK.

1. Encephalitis or Meningoencephalitis

Any person with suspected viral encephalitis with all the following criteria:

1. Fever >38º and
2. Altered mental state (altered level of consciousness, agitation, lethargy) and/or other evidence of cortical involvement (e.g. focal neurological findings, seizures) and
3. Cerebrospinal fluid (CSF) pleocytosis with predominant lymphocytes and/or elevated protein with a negative Gram stain and culture and
4. No alternative microbiological cause identified

2. Meningitis

Any person with suspected viral (aseptic) meningitis with all the following criteria:

1. Fever >38º and
2. Headache, stiff neck and/or other meningeal signs and
3. CSF pleocytosis with predominant lymphocytes and/or elevated protein with a negative Gram stain and culture and
4. No alternative microbiological cause identified

3. Acute Flaccid Paralysis (AFP)

Any person with suspected AFP (most cases are polio-like) with all the following criteria

1. Fever >38º and
2. Asymmetric limb weakness without sensory loss with diminished deep tendon reflexes and
3. Anterior horn cell disease and
4. May have facial nerve palsy and
5. No alternative microbiological cause identified

*Technical Guidance Document C3/WD159 Procedure for communication to member states and the commission about West Nile virus disease. http://ec.europa.eu/health/ph_threats/com/west_nile/wnv_case_def_en.pdf

Last reviewed: 23 August 2011