How Does a Doctor Diagnose West Nile Virus?

Ask the experts

I was at a barbecue last weekend and got totally bit up by mosquitoes. This week, I've started to feel fatigued, like I might be getting the flu. Then I read in the newspaper that county health officials have detected West Nile Virus in our area. Could I have West Nile? How does a doctor diagnose West Nile virus?

Doctor's response

The diagnosis of West Nile virus infection is confirmed with a blood or cerebrospinal fluid (CSF) test to detect WNV-specific IgM antibodies. A CSF test requires a lumbar puncture (spinal tap) to obtain a specimen. IgM antibodies represent recent infection and are usually detectable during active or recent infection within three to eight days after infection, but a negative test within eight days should still be repeated if WNV infection is truly suspected. Unfortunately, WNV IgM antibodies can persist for three months or more, so the test may be positive from a prior infection, or a positive test may be due to cross-reactivity with antibodies to other flaviviruses. Therefore, a positive WNV IgM antibody must be confirmed by much more specialized testing by CDC.

WNV-specific IgG antibodies appear soon after the IgM antibodies, and remain present for life, so testing for these antibodies is not helpful for diagnosis of new infection. However, it can help sort out past infection from new infection when a person lives in an area where WNV is active or has been exposed. For example, a positive IgG with a negative IgM suggests no current or active WNV infection. This may help decide whether other causes of illness could be considered.

For more information, read our full medical article on West Nile virus.

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Reviewed on 9/27/2018
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