West Nile Virus Testing

Update

To:
Hospital Infection Control Coordinators and Emergency
Department Medical Directors;
Infectious Disease Physicians; Neurologists;
Obstetricians/Gynecologists;
Hospital Laboratory Directors

From:
Francie Wise, MPH
Director of Communicable Disease Control
Contra Costa Public Health

Date: July 15, 2008

In 2007, California reported 380 cases of West Nile Virus in humans with positive laboratory confirmation and 21 deaths. In 2008 in Contra Costa, we already have detected West Nile Virus in mosquitoes and birds. We expect the emergence of human cases of WNV throughout the summer and fall.

Contra Costa Public Health Laboratory (PHL) and the California Department of Public Health (CDPH) Viral and Rickettsial Disease Laboratory (VRDL) are providing testing services to assist in your evaluation of West Nile cases. Testing will be available for cases of encephalitis, adults with aseptic meningitis (18 years or older), and acute flaccid paralysis/atypical Guillain_Barre' syndrome.

Accompanying this memo, and on our website are:

West Nile Virus (WNV) Infection Case Report (109k PDF, 1p.)
West Nile Virus specimen submission form; this form includes criteria for testing and infomation on specimen collection. Initial testing will be performed at our local Contra Costa Public Health Laboratory by IFA (Immunofluorescent antibody) methods.

West Nile Virus Specimen Submittal Form (113k PDF, 1p.)

The IFA is a screening test. As with any screening test there are important limitations to the test. The IFA test can detect and identify IgM and IgG antibody to West Nile Virus. However, it may also detect antibodies to other closely related flavivirus infections (e.g. patients with previous dengue infection or a history of receiving yellow fever vaccine may also test positive). The IFA test will be positive for IgM in recent flavivirus infections, but in West Nile Virus infections the IgM positivity may last for an extended period of time (over one year). Additional testing may need to be performed at the State VRDL to confirm the initial positive results.

Because of these cross-reactive testing results, a complete West Nile Virus clinical case history form is needed for proper interpretation of results. All specimens that have borderline or positive results will be forwarded immediately to CDHS for further testing.

A negative IFA test means that there is no detectable antibody for West Nile Virus. A false negative may occur if the specimen is taken too early in the course of disease (approximately 75% of patients will have detectable IgM in serum or CSF by day four after onset, 90% test positive by day eight after onset). If West Nile Virus is highly suspected and the acute serum is negative, please submit a second sample collected 3 to 5 days after the acute serum.

The Contra Costa Communicable Disease Unit must be notified of the possible case, prior to submission of specimens: Call 925-313-6740 or fax the West Nile Case History form to 925-313-6465. Specimens may be submitted Monday-Friday (refrigerate Saturday-Sunday), and submit Monday.

Please check the following websites for further information about WNV, including information for clinicians:

For questions about testing for WNV please call:

Contra Costa Public Health Laboratory
Rick Alexander 925-370-5775

California Department of Health Services VRDL
510-307-8606

Content provided by the Public Health Division of Contra Costa Health Services.