Acute Communicable
Disease Control


Contact Information
County of Los Angeles
Department of Public Health
Acute Communicable Disease Control
313 N. Figueroa Street, #212
Los Angeles, CA 90012
Phone: (213) 240-7941
Fax: (213) 482-4856

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(Back to West Nile Virus Main Page)
West Nile Virus Information for Medical Professionals

The Los Angeles County (LAC) Department of Public Health (DPH) reminds residents and clinicians that endemic West Nile virus (WNV) continues to affect county residents each year. In 2016, 153 cases of locally transmitted WNV were documented, 108 of which were neuro-invasive disease including 5 deaths. Although this is a decrease from 300 cases in 2015, it marked the 5th consecutive year that LAC DPH has documented high case counts. In 2016, as in other recent years, the WNV season continued through late November and cases occurred throughout LAC, with the highest risk among residents of the San Fernando Valley region.

Who Should be Tested for WNV
WNV activity began early this year with the confirmation of WNV illness in March in a San Gabriel Valley resident and the detection of WNV positive mosquito pools, sentinel chickens, and dead birds across the county. The Acute Communicable Disease Control Program (ACDC) recommends that medical providers order WNV testing for all patients with aseptic meningitis, encephalitis, or acute flaccid paralysis during the WNV season, late spring through November in California. Also consider testing patients who are experiencing a nonspecific illness compatible with WNV fever (an acute infection characterized by headache, fever, muscle pain, and/or rash lasting three days or longer).

Testing Information
Serum serologic (IgM) testing is the preferred diagnostic approach for suspect cases of WNV fever and neuroinvasive WNV infection. Specimens positive for acute WNV infection in commercial laboratories do not require confirmation by the LAC Public Health Laboratory (PHL) to meet the WNV case definition. Excellent correlation has been shown between tests performed at most commercial labs and subsequent confirmation at LAC PHL and the California Department of Public Health (CDPH).

From May through November, the LAC PHL is available for initial tests and confirmation of ambiguous results on serum specimens at no charge to the submitter. A A standard laboratory submittal form must be completed and accompany the specimen(s). The PHL accepts serum specimens for WNV testing on patients hospitalized or evaluated in an emergency department with aseptic meningitis, encephalitis, or acute flaccid paralysis syndrome (atypical Guillain-Barré syndrome); outpatients with possible WNV fever may also be tested. Prior approval from ACDC physicians is not required before WNV testing. Although LAC PHL no longer tests cerebrospinal fluid (CSF) for WNV infection, CSF testing is available at CDPH. CSF specimens sent to LAC PHL will be forwarded to CDPH for testing.

Saint Louis Encephalitis Testing
In addition to WNV, Saint Louis encephalitis virus (SLEV) has recently re-emerged in LAC and California. Positive mosquito pools and sentinel chickens were found positive for SLEV in 2016, the first detection since 2003 in LAC. Last year, California also confirmed its first human case of SLEV since 1997. Like WNV infection, SLEV infection is usually mild or asymptomatic but can cause severe neurologic illness, especially in the elderly. SLEV are transmitted by the same mosquitoes as WNV. Providers should consider SLEV infection in their differential diagnosis if WNV testing is negative and there is no other likely etiology. Serum serologic testing for SLEV is offered at commercial laboratories and at LAC PHL.

LAC DPH requests your participation in the reporting of human WNV and SLEV infections. Reporting of WNV and SLEV cases guides the DPH and the LAC mosquito abatement districts to target mosquito abatement services, surveillance activities and health education. The California Code of Regulations, section 2500, require providers to report all positive acute laboratory findings of WNV and SLEV to the patient’s local public health department within one working day. Clinical cases of WNV and SLEV infection and asymptomatic WNV positive blood donors are also reportable. For all suspect cases, medical records will be requested and reviewed.

We remind clinicians that all cases of acute encephalitis and meningitis (including those pending definitive diagnosis or suspected to be of viral, bacterial, fungal, or parasitic etiologies) also are reportable within one working day. A standard Confidential Morbidity Report (CMR) can be used to file a report; the CMR may be faxed to the DPH Morbidity Unit at (888) 397-3778. You may also report cases by telephone during normal business hours from 8:00 a.m. to 5:00 p.m. to (888) 397-3993. For cases among residents of the cities of Long Beach or Pasadena, please contact their local health departments.

Information for Medical Providers and Infection Control Practitioners

Additional Resources

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