Wood-Ridge, New Jersey 07075 - Testing and Treating West Nile Virus in Humans

Testing and Treating West Nile Virus
in Humans

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.

Additional Information:

Home
West Nile Virus and Dead Birds
West Nile Virus, Pregnancy and Breast-Feeding
West Nile Virus Is A Risk You Can Do Something
About With A Few Simple Steps.
Cases of West Nile Human Disease
West Nile Virus and Dogs and Cats
West Nile Virus: Information and Guidance for Clinicians
Insect Repellent Use and Safety
Symptoms of West Nile Virus
Testing and Treating West Nile Virus in Humans
Blood Transfusion, Organ Donation and Blood Donation Screening Information
Transmission of the West Nile Virus