First West Nile Virus Positive Mosquito Samples Have Been Identified in Orange County
July 13, 2020
Orange County’s first West Nile Virus (WNV) positive mosquito samples of the season were identified last week in Cypress, Anaheim, and Garden Grove through routine trapping and testing by the Orange County Mosquito and Vector Control District (OCMVCD). The first WNV positive dead bird of the season was also collected in Irvine. St. Louis Encephalitis Virus (SLEV) positive mosquito samples have been identified in Riverside County. One human case of WNV was reported last week from Los Angeles County. No human infections of WNV or SLEV have been identified so far this season in Orange County.
For Providers:
  • Health care providers should consider WNV infection in patients with prolonged fever or acute neurologic illness.
  • Clinical Description: The incubation period is usually 2 to 6 days after a bite from an infected mosquito. Most symptomatic WNV cases have an acute febrile illness. Symptoms also include headache, malaise, arthralgia, or myalgia, and occasionally nausea, vomiting and/or a maculopapular rash. About 1 in 150 WNV-infected persons develops neuroinvasive disease, which can manifest as encephalitis, aseptic meningitis, acute flaccid paralysis, and/or transverse myelitis.
  • Testing for WNV: Testing for WNV should include serum WNV IgM and IgG, with CSF IgM when a lumbar puncture is performed. Because serum WNV IgM may be negative early in the course of disease, repeat serology may be indicated if initial testing was done within 8 days after the onset of illness.
  • Surveillance of West Nile Virus in the community: Report all WNV infections to Orange County Public Health by phone (714-834-8180) or fax (714-564-4050) within 1 working day. For more information on vector surveillance, please visit the OCMVCD web site at
  • Avoiding mosquito bites is the best way to prevent WNV infection:
  • Eliminate standing water which can serve as mosquito breeding sites around the home or workplace.
  • Ensure windows and door screens are in good repair.
  • Use an insect repellent containing active ingredients registered with the U.S. Environmental Protection Agency when outdoors. Active ingredients include DEET, picaridin, and oil of lemon eucalyptus. Natural bug sprays may not be effective against mosquitoes. For more information, visit
Contact Information:
For questions or concerns please contact the Communicable Disease Control Division at 714-834-8180.
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