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Saint Louis Encephalitis Found in Orange County Mosquitoes

August 30, 2016

Photo of of a mosquito

For the first time since 1986, Saint Louis encephalitis virus has been found in adult mosquito samples in Orange County. The mosquito samples were collected in Garden Grove on August 9, 2016 and tested positive for St. Louis encephalitis virus (SLE) on August 17, 2016. SLE is a native arbovirus in the same virus family as West Nile virus (Flaviviridae). SLE is similar to West Nile virus in that both viruses require a bird reservoir to maintain the virus in nature. In 2016, SLE has been found in Los Angeles, Riverside, Kern, San Bernardino, and now Orange County.

The last recorded human infections of SLE in Orange County occurred in 1984. SLE was occasionally found in mosquitoes across the United States until the year 2000. Detections of SLE were reduced significantly after the introduction of West Nile virus around that time. SLE infection results in an immune response and antibody development in birds that is very similar to the immunity resulting from West Nile virus infection. This cross-immunity in birds is believed to be responsible for SLE’s disappearance.

SLE is transmitted to humans by the bite of an infected mosquito. The viruses share the same symptoms: fever, headache, nausea and fatigue. Approximately 80% of those infected will show no symptoms of disease.

Preventative measures to avoid being infected with SLE are the same as they are for West Nile virus and Zika:

  • Eliminate any standing water around your house.
  • Make sure all window and door screens on your house are in good repair.
  • Wear a repellent containing DEET®, Picaridin, Oil of Lemon Eucalyptus, or IR 3535.

Contact the Orange County Mosquito and Vector Control District to report dead birds or neglected pools, call (714) 971-2421 or (949) 654-2421.