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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Acute Communicable Disease Control
Saint Louis Encephalitis Virus (SLEV) 

Saint Louis encephalitis is a mosquito-borne disease caused by the Saint Louis encephalitis virus (SLEV) and spread by the bite of an infected mosquito. Most people infected with SLEV will have few to no symptoms. The most common symptoms are mild, flu-like symptoms including fever and headache from 5-15 days after being infected. Severe cases can affect the central nervous system resulting in meningitis and/or encephalitis and can result in death and long-term disability.

SLEV is in the same virus family as West Nile virus (Flaviviridae). Saint Louis encephalitis virus, like West Nile virus (WNV), maintains itself in nature by cycling between mosquitoes and birds. Humans are infected when bitten by an infected mosquito. Neither virus is spread directly from person-to-person or from birds to humans. SLEV was commonly detected in mosquitoes prior to the introduction of WNV into California in 2003, but seemingly disappeared thereafter. This is believed to be the result of cross-immunity caused by West Nile virus in birds. However, the virus reappeared in California in 2015 and has continued to be detected each year since then.

Human SLEV and WNV infections are reportable in California. Reporting of cases guides the Los Angeles County Department of Public Health and local vector control 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 and clinical cases of SLEV and WNV infection to the patientís local public health department within one working day.

All cases of acute encephalitis and meningitis regardless of etiology are also 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 or (213) 482-5508. 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.

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Additional Resources


Public Health has made reasonable efforts to provide accurate translation. However, no computerized translation is perfect and is not intended to replace traditional translation methods. If questions arise concerning the accuracy of the information, please refer to the English edition of the website, which is the official version.
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