West Nile Virus

Other names: WNV

West Nile Virus is an infection which is usually transmitted to humans by mosquitoes; in some rare cases, the infection may be spread from human to human through blood and organ donation. Most people who are infected by the West Nile Virus do not suffer any symptoms; however, around 20 percent of patients do experience symptoms. Most cases of the virus are very mild and patients usually recover quickly; however, around 1 in 150 people fall very ill with the virus. In severe cases, symptoms include convulsions, stiffness, severe headaches, muscular paralysis, comas and high temperatures; patients with these symptoms may have conditions known as encephalitis or meningitis.

Why is the test used?

The test is used to determine whether a patient has been infected with the West Nile Virus. If a patient has severe symptoms the test may be used to enable doctors to diagnose the cause of the symptoms, as some conditions, including meningitis, cause similar symptoms to West Nile Virus infections.

The test is used to detect the presence of antibodies to WNV; a nucleic acid test may also be carried out if the blood is to be used as donor blood.

The test is usually ordered when a patient displays symptoms which may be associated with a West Nile viral infection; symptoms include headaches, chills, fever, nausea, skin rashes and muscular weakness. The test may also be carried out if a patient has travelled to a country where the West Nile Virus is prevalent.

How is the test carried out?

The test is carried out by collecting and analysing a sample of the patient’s blood; a needle is inserted into the vein and the blood is drawn out into a syringe. Once the doctor or nurse has a sufficient sample, the blood will be bottled, labelled with the patient’s name and sent off to the laboratory for analysis.

A sample of cerebral spinal fluid may also be collected.

What do the test results mean?

If the test detects IgM antibodies, this usually indicates that the patient has an active West Nile viral infection; if the antibodies are present in the cerebral spinal fluid, this means that the infection is present in the central nervous system.

If both IgG and IgM antibodies are present in the sample, this usually indicates that the patient was infected at least three weeks before the test. If only IgG antibodies are detectable, this usually indicates that the patient has been exposed to the virus in the past but does not have an active infection.

If the results of the nucleic acid test are positive this indicates that the virus is present in the patient’s blood; if the result is negative this usually indicates that the patient has not got the virus in their blood.

Specific Blood Tests

Blood Test Guide

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