Los Angeles County West Vector & Vector-Borne Disease Control District

St. Louis Encephalitis

    St. Louis Encephalitis (SLE) is a central nervous system infection in man.  In the U.S., SLE virus is cycled primarily between Culex spp. mosquitoes and passeriform and columbiform birds.   Infected mosquitoes and birds apparently suffer no adverse effects.  Culex tarsalis is the primary vector mosquito of SLE in the western United States.

    An SLE infection can be inapparent, acute, or fatal. The majority of SLE cases are inapparent infections which are mild or subclinical.  Symptoms of infection appear 7 to 21 days after a bite from an infected mosquito.  SLE has three separate syndroms:
      febrile headache
      aseptic meningitis

    The original epidemic of SLE occurred in St. Louis in 1933.  The epidemic caused over 1,000 clinical cases and at least 200 deaths.  In the 1950's major outbreaks occurred in California's Central Valley followed by large epidemics that occurred in 1962 in Tampa Bay, FL; in 1964 in Houston, TX, the Delaware River Valley, Philadelphia and New Jersey; in 1974 in Memphis, TN; and in 1975, the largest epidemic of SLE occurred in the central part  of the U.S. 

     Investigations of outbreaks from the 1950's to the present by the Centers for Disease Control and Arbovirology Laboratory have established that SLE is the principal arboviral problem in the United States.  SLE is endemic in Los Angeles County as evidenced by outbreaks in 1983 and 1984 and by serological data obtained from wild birds and sentinel chickens from 1985 to present.

SLE was the leading cause of reported viral encephalitis in the U.S. from 1971 to 1984.


    SLE has been isolated from other mosquito species, mites, and a tick.  The significance of most of these isolations is unknown; however, these species may serve as secondary vectors in some areas and may contribute to enzootic and/or epidemic virus transmission.

    The best approach for vector control and implementation measures for preventing or interrupting SLE outbreaks is the establishment of continuous mosquito abatement programs.   Assessing vector population levels and production areas and performing routine control measures at minimizing mosquito  production are fundamental to such programs.   There are numerous examples of St. Louis encephalitis that were stopped or truncated by the application of control procedures.




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