West Nile virus - Advice for Health Professionals

West Nile virus within Europe, Summer 2011

Following the outbreak in 2010, public health authorities in Greece have confirmed new cases of West Nile virus (WNV) on the Greek mainland. More details on the West Nile in Europe 2011 page. Cases have also been reported in the Russian Federation, Albania and Romania; countries which have a history of West Nile virus infections.

West Nile virus infection

The incubation period is usually 3 to 15 days. Most (80%) of those infected with WNV have no symptoms at all, and 20% have a mild influenza-like illness (fever, headache, body ache), sometimes with swollen lymph glands or a rash on the chest, stomach and back. However, a small proportion (less than 1%, about one in 150 people infected) develop more severe disease; usually an encephalitis, meningitis or meningo-encephalitis. Patients may suffer headaches, fever, stiff neck, sore eyes, disorientation, muscle weakness, convulsions and coma. Numbness and paralysis may also occur. People over the age of 50 are more likely to develop serious symptoms of WNV.

Imported cases of WNV have been very rarely reported in England and Wales.

Making a diagnosis

A diagnosis of WNV infection should be considered in patients presenting with neurological syndromes as listed below, or with other compatible symptoms, especially in those who have a recent history of travel to southern Europe or North America.

Samples should be sent to the Special Pathogens Reference Unit at HPA Porton:

1. Paired serum or whole blood specimens. (Acute phase specimen 0 to 8 days after onset, and convalescent phase 14 to 21 days after onset)
By the eighth day of illness, a large majority of infected persons will have detectable IgM antibody to WNV. In most cases this will still be detectable up to 2 months post illness and can be detected in some cases more than 12 months post infection. By 3 weeks post-infection serum IgG to WNV is detectable.
2. CSF, ideally acute phase (< 8 days of onset)
As early as the first few days following infection, IgM to WNV can be detected. 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.

Please use the Suspected West Nile case report form (PDF, 21 KB) to inform HPA Colindale of suspected cases. Do not send samples to Colindale.

WNV Neurological Syndrome:
1. Encephalitis or Meningoencephalitis	1. Fever >38º and
Any person with suspected viral encephalitis with all the following criteria	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	1. Fever >38º and
Any person with suspected viral (aseptic) meningitis with all the following criteria	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)	1. Fever >38º and
Any person with suspected AFP (most cases are polio-like) with all the following criteria	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

Last reviewed: 23 August 2011