Nile Virus Home > Questions
and Answers about Laboratory Practice
and Answers about Laboratory Practice (FAQ)
How are human cases of WNV diagnosed?
A. West Nile virus (WNV) infection
can be suspected in a person based on clinical symptoms and patient
history. Laboratory testing is required for a confirmed diagnosis.
most commonly used WNV laboratory test measures antibodies that
that are produced very early in the infected person. these antibodies,
called IgM antibodies, can be measured in blood or cerebrospinal
fluid (CSF), which is the fluid surrounding the brain and spinal
cord. this blood test may not be positive when symptoms first
occur; however, the test is positive in most infected people within
8 days of onset of symptoms.
test for WNV IgM-antibody is used by CDC, state and local public
health labs and increasingly at private laboratories. When testing
is conducted at private laboratories, the health department or
CDC will often confirm results in their own laboratories before
officially reporting WNV cases.
some instances, health departments may conduct or request additional
testing from CDC before officially reporting a case to CDC's ArboNET
Surveillance System. the state or CDC reference laboratory may
repeat the initial IgM-antibody testing.
state may also perform or ask CDC to perform an additional, different
test on a specimen. this latter test (plaque reduction neutralization
test [PRNT]) is usually performed when:
state finds its initial case(s) of human WNV illness,
results are not definitive due to equivocal laboratory testing
results or insufficient specimens,
patient might have been exposed to other closely related viruses
(like St. Louis encephalitis virus) which may result in a "false"
positive laboratory test for WNV.
additional tests require growth of the virus and may take a week
or longer (plus shipping time) to conduct. the results from the
PRNT are often needed before CDC considers a human WNV infection
How does CDC decide when to report a case of WNV?
A. CDC reports a case of WNV
once a state officially reports and verifies that case to CDC.
timing of the official report to CDC, relative to onset of symptoms
in a person, is variable and depends on when an individual first
seeks medical care and the extent of the laboratory testing, as
described above, that the state determines is necessary before
any given time, in addition to the official case count reported
by CDC, there may be additional suspect cases under investigation
or in various stages of testing, including supplemental or confirmatory
How many of the human WNV cases are being confirmed by the CDC
A. When WNV was first found in
the United States in 1999, the CDC reference laboratory confirmed
all human cases of WNV. through a comprehensive, CDC-sponsored
laboratory training program, most states are now able to perform
the initial blood tests to identify IgM-antibody in the blood
or CSF of suspect human WNV infections, and many state laboratories
are also able to perform the more involved PRNT. the CDC reference
lab is called upon for confirmatory testing by fewer and fewer
states; although the increased activity of WNV still requires
that many tests be performed at the CDC reference laboratory.