A mosquito sample collected in Stanislaus County has tested positive for St. Louis Encephalitis virus, according to the area’s two mosquito abatement districts. 

Like West Nile virus, most people who become infected with SLEV will never feel sick. Most people who do feel sick will have mild flu-like symptoms; a small number of people will have severe disease with headache, confusion, disorientation, and dizziness. Seizures, paralysis, coma, and sometimes death may occur. Severe disease is more likely in people who are older and those with weakened immune systems. 

SLEV is related to the West Nile virus and is transmitted via the bite of Culex mosquitoes, the same mosquitoes that transmit WNV. 

“The discovery of positive mosquito pools is a reminder that we need to continuously prevent mosquito breeding in our community. There is no specific treatment for SLEV or WNV, so it is very important that people protect themselves and their families from mosquito bites,” advises Dr. Julie Vaishampayan, Stanislaus County Public Health Officer. “We ask everyone to use mosquito repellents to protect themselves and their loved ones from mosquito-borne viruses, especially when they are outdoors.” 

Statewide, there have been 26 mosquito samples that have tested positive for SLEV this year. They are from six counties including Fresno, Imperial, Madera, Riverside, Stanislaus, and Tulare. There have been no reports of SLEV in people this year; during 2020, there were four reports of people with SLEV in California; one of these was from Stanislaus County. 

Merced County has not reported any mosquito samples positive for SLEV this year, so far. In previous years the county has seen this virus in mosquitoes. After a 40-plus year absence it reappeared in 2017.

WNV detection in mosquitoes is increasing; a total of 133 mosquito samples have tested positive this year in Stanislaus County. WNV has also been confirmed in two dead birds. In Merced County there has been six mosquito samples test positive for WNV and confirmed in one dead bird.

The MADs will continue with their surveillance programs identifying mosquito breeding sources and mosquito-borne disease activity. They will treat according to their surveillance results. The districts anticipate more SLEV and WNV detections in the coming months and would like to remind residents they can help by taking the following precautions: 

• Dump or drain standing water. These are places mosquitoes like to lay their eggs. 

• Defend yourself against mosquitoes by using repellents containing DEET, Picaridin, or Oil of 

Lemon Eucalyptus. 

• Avoid being outdoors at dusk and dawn. These are the times when WNV carrying mosquitoes 

are generally most active. 

• Report Neglected Swimming pools to your local mosquito abatement district. 

• Use tight-fitting door and window screens to keep mosquitoes from entering your home. 

For additional information or to request service, residents should contact their local District: Stanislaus County residents: 

• North of the Tuolumne River contact: 

Eastside Mosquito Abatement District at (209) 522-4098 (www.eastsidemosquito.com). 

• South of the Tuolumne River contact: 

Turlock Mosquito Abatement District at (209) 634-1234 (www.turlockmosquito.org) or the Merced County Mosquito Abatement District at (209) 722-1527 or visit them at www.mcmosquito.org