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West Nile Encephalitis in Humans
Questions
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Testing
and Treating
West Nile Virus in Humans UPDATED!
 Questions
about Commercial
Laboratories NEW!
Q.
I think I have symptoms of West Nile virus. What should I do?
A. Contact your health care provider if you have concerns
about your health. If you or your family members develop symptoms
such as high fever, confusion, muscle weakness, and severe headaches,
you should see your doctor immediately.
Q.
How do health care providers test for West Nile virus?
A. Your physician will first take a medical history to assess
your risk for West Nile virus. People who live in or traveled
to areas where West Nile virus activity has been identified are
at risk of getting West Nile encephalitis; persons older than
50 years of age have the highest risk of severe disease. If you
are determined to be at high risk and have symptoms of West Nile
encephalitis, your provider will draw a blood sample and send
it to a commercial or public health laboratory for confirmation.
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 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 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.
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
once a state officially reports and verifies 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-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.
Q.
How is West Nile encephalitis treated?
A. There is no specific treatment for West Nile virus infection.
In more severe cases, intensive supportive therapy is indicated,
often involving hospitalization, intravenous fluids, airway management,
respiratory support (ventilator), prevention of secondary infections
(pneumonia, urinary tract, etc.), and good nursing care.
Questions
about Commercial Laboratories New!
Q.What
role do commercial laboratories play in diagnosing people with
West Nile virus infection?
A.When
a person goes to see a health care provider, and has symptoms of
a West
Nile illness a specimen may be sent to a commercial laboratory
to determine if the person has been infected by West Nile virus.
The tests used in commercial labs check for antibodies to the virus
(the body’s response to infection). The results of the test
will be sent to the doctor and the state health department will be
informed if the results are positive. There is no specific treatment
available for West Nile virus infection, so the diagnosis will not
necessarily change the way the person is being treated but it will
let the doctor know that he/she does not have to investigate another
cause of illness, and it will help the health department know where
the virus is active in order to focus prevention measures.
The state health department may choose to accept the positive results
from the commercial lab, or they may choose to test the sample again
in the state health department laboratory for confirmation of the
infection. The state health department will report the case to CDC.
Q.How
accurate are the tests used in commercial labs?
A.The
tests used in commercial labs are modeled on the tests created
by CDC and used at CDC and in state public health laboratories.
This
is the first year that many of these tests have been widely used
in commercial labs, and laboratories are learning more about the
specific measurements used in each test. Often, a second test will
be done to confirm the infection. State health departments, the FDA
(which licenses and regulates medical tools such as these tests),
the association of Public Health Laboratories and CDC are all engaged
in monitoring new commercial tests, and are committed to working
with industry to make these tests as accurate and useful as possible.
Q.If a test is a “false positive” what does
that mean?
A.A “false
positive” occurs
when an initial tests indicates that a person does have a West
Nile infection, but a later, more
specific tests indicates that the person does not actually have the
infection. While it is important to health department and CDC to
get an accurate idea of where people are being infected in order
to focus prevention and control efforts, the result does not have
a great impact on the individual person. There is no specific treatment
that the person would receive due to West Nile virus infection. The
person may want to work with their physician to see if another cause
of the illness needs to be identified.
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