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Questions
and Answers
Understanding
the Numbers Posted for West Nile Virus Cases
Q. Why
do the media, my state health department, and the CDC sometimes
report different statistics on the number of human West Nile virus
cases?
A. The
CDC human case count, as reported in our ArboNET Surveillance
System, is based on the number of West Nile virus cases that have
been officially reported by each state health department to CDC.
Before a state makes its report to CDC, follow-up laboratory testing
is often conducted. CDC believes it is important to report the
most accurate information possible, so our numbers may be lower
than those reported in the media until official case reports are
received from the states.
As
West Nile virus has become more familiar in the US, many private
labs are now able to do early testing on suspected human cases
of disease. Physicians often send samples to private labs in order
to get quick preliminary results to know if they need to look
for another source of illness that may need treatment. Some states
and often the media may incorporate these early test results in
their total case count.
Q.
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.
The
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.
A
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.
In
some instances, health departments may conduct or request additional
testing before officially reporting a case to CDC's Arbonet Surveillance
System. The state or the CDC reference laboratory may repeat the
initial IgM-antibody testing.
A
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:
- the
state finds its initial case(s) of human WNV illness
- IgM
results are not definitive due to equivocal laboratory testing
results or insufficient specimens
- the
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
These
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
confirmed.
Q.
How does CDC decide when to report a case of WNV?
A. CDC
reports a case of WNV infection once a state officially reports
that case to CDC.
The
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
reporting.
At
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
laboratory testing.
Q.
How many of the human WNV cases are being confirmed by the CDC
laboratories?
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 antibodies 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 require that many tests be performed at
the CDC reference laboratory.
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