Samples from cases of suspected WNV infection

Indications for considering the diagnosis of WNV infection and requesting a WNV test can be found here for UK-acquired cases, or here for Travel-associated cases. Appropriate serum and/ or cerebrospinal fluid samples (as described below) from suspect cases should be sent to the Special Pathogens Reference Unit (SPRU) at HPA Porton

Details of samples from cases of suspected WNV infection to be sent to SPRU

1. Paired serum or whole blood specimens. The acute phase specimen 0 to 8 days after onset and the convalescent phase sample 14 to 21 days after onset.
By the eighth day of illness, a large majority of infected persons will have detectable IgM antibody to West Nile Virus. In most cases this will still be detectable up to 2 months post illness and can be detected in some cases 12 months post infection. By 3 weeks post-infection serum IgG to West Nile Virus is detectable.
2. CSF, ideally acute phase (< 8 days of onset).
As early as the first few days following infection, anti-capture ELISA can detect IgM to West Nile Virus. Virus may also be isolated or be detected by reverse transcriptase polymerase chain reaction (RT-PCR), in acute phase CSF within 8 days of onset. CSF samples collected later in the disease can also be useful for diagnosis.

Definitions of probable and confirmed WNV infection

Probable case (positive for one of the following criteria):
1. Single serum specimen: A positive WNV IgM test
2. CSF: detection of WNV IgM
Confirmed case (positive for one or more of the following criteria*):
1. Serum or CSF:  isolation of WNV
2. Serum or CSF: detection of WNV genomic RNA sequences by RT-PCR
3. Serum or CSF: detection of neutralising WNV antibodies with significant titre
4. Paired Serum specimens: A fourfold rise in WNV specific antibody titre
*isolation of the virus or detection of nucleic acid is unusual and the most likely confirmatory testing is through detection of neutralising antibodies or a rising titre

Last reviewed: 23 August 2011