St. Louis encephalitis (SLE) is one of the most common mosquito-borne viruses transmitted to humans in the United States. Culex tarsalis is the most common vector of SLE in Butte County. SLE virus is a flavivirus that was first detected in St. Louis, Missouri in 1933. SLE virus is diagnosed based on symptoms, physical findings, laboratory testing, and the possibility of exposure to infected mosquitoes. There is no specific treatment for SLE. Treatment is based on symptoms, not the virus. SLE was detected in Butte County most recently in the mid 1990’s with the detection of positive infected mosquitoes and sentinel chickens. The District still actively tests for this virus in mosquitoes and sentinel chickens.