West Nile virus
causes an infection that can lead to inflammation of the brain (encephalitis), the spinal cord (myelitis), or the
tissues surrounding it and the spinal cord (meningitis). If
your doctor suspects a mosquito-borne illness, he or she will take a medical
history to assess your risk of West Nile virus infection. People who live in or
travel to areas where the virus has been found are at risk of West Nile virus
If you have symptoms that West Nile is affecting the
brain and spinal cord, your blood will be tested for
antibodies to the virus, a sign that you have been
infected. The initial blood test screens for
immunoglobulin M (IgM) and
immunoglobulin G (IgG) antibodies to the virus. If the
initial test shows West Nile virus infection, you may have a follow-up test 2
weeks later to see if antibodies are increasing. For more information on the
immunoglobulins test, see the topic
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False-positive results, which show that you have the virus when in fact you do not, can
occur if you have been infected with a similar virus, such as the St. Louis
encephalitis virus. But this does not affect treatment, because the treatment
for all forms of encephalitis is similar. A false-positive result may also
occur if you have recently received certain vaccinations, such as those for
yellow fever and Japanese encephalitis.
If you have symptoms of
meningitis or encephalitis, a
lumbar puncture, also called a spinal tap, may be done
to look for antibodies and signs of infection in the cerebral spinal fluid,
which surrounds the brain and spinal cord. If antibodies are found, another
test will be done to look at the virus's genetic material.
You may have a
magnetic resonance imaging (MRI) test to identify
encephalitis or inflammation of the brain and the membranes surrounding the
brain and the spinal cord (meningoencephalitis).
Donated blood may
be screened with a West Nile virus blood test called Procleix. This blood test
RNA in donated blood and
plasma. Screening donated blood for West Nile virus
helps to keep it safe so that people who are receiving the blood do not get the