August 25, 2023

Contact:  Luz Maria Robles, Public Information Officer
Office:  916-405-2082 |  Cell:  916-416-6337

Mosquitoes in Yolo County Test Positive for St. Louis Encephalitis
Elk Grove, Ca— The Sacramento-Yolo Mosquito & Vector Control District announced today that a mosquito sample has tested positive for St. Louis encephalitis virus (SLEV) in Yolo County. Laboratory staff detected the virus while performing routine testing of mosquito samples collected from an area north of Woodland. SLEV is transmitted to humans by the bite of an infected mosquito just like West Nile virus (WNV). Mosquitoes infected with SLEV are more commonly found in the Central Valley and some areas of southern California, however in recent years, the virus has been spreading. This year SLEV has been found in neighboring San Joaquin and Stanislaus counties.  “The finding of St. Louis encephalitis along with a record amount of West Nile virus activity this season highlights the importance of mosquito control programs to protect residents from these and other mosquito transmitted diseases that pose a threat,” said Gary Goodman District Manager. As a result of the SLEV finding the District will continue closely monitoring the area by setting additional traps and enhancing surveillance and control methods.
In northern California, the mosquito species that carry SLEV, Culex pipiens and Culex tarsalis, are the same mosquitoes that are able to transmit WNV. The viruses also share the same symptoms. According to the Centers for Disease Control and Prevention, most people infected will not become ill. Initial symptoms include headache, fever, nausea, vomiting and fatigue from 5 to 15 days after being infected. Severe neuroinvasive disease often involving encephalitis, an inflammation of the brain occurs most commonly in older adults, those with a previous health condition or a weakened immune system. Since there is no human vaccine or cure for SLEV or WNV, residents must be proactive and take measures against mosquito bites. “This is yet another reminder of the importance of wearing an effective insect repellent when spending time outdoors, especially at dawn or dusk,” said Goodman.
Mosquito control is a shared responsibility and residents must take an active role in reducing the threat of mosquitoes in their neighborhoods and backyards by taking these steps:

  • Eliminate stagnant water that may produce mosquitoes from common sources such as plant saucers, buckets, wheel barrows, children’s toys, tires and other containers
  • When outdoors, apply an insect repellent that contains DEET, Picaridin, IR3535 or oil of lemon eucalyptus
  • Ensure swimming pools, spas, fountains and ponds are properly maintained
  • Change water in pet dishes, bird baths and other small containers every 2-3 days
  • Request mosquitofish for your ornamental ponds and fountains
  • Report neglected swimming pools and other mosquito breeding sources

For more information about SLEV visit:  or
For current West Nile virus information, to report a mosquito breeding source or request our services, please call us at 1-800-429-1022 or please visit

Practice the District D’s of Mosquito Prevention: 

DRAIN standing water that may produce mosquitoes. 
DAWN and DUSK are times to avoid being outdoors.   
DRESS appropriately be wearing long sleeves and pants when outside. 
DEFEND yourself by using an effective insect repellent.  Make sure to follow label directions! 
DOOR and window screens should be in good working condition.
DISTRICT personnel are also available to address any mosquito problems. Call them at 1-800-429-1022 or visit