Early and Intense West Nile Virus Activity;

New West Nile Virus Diagnostics

July 29, 2026

Key Messages:

  • Multiple detections of West Nile Virus (WNV)-positive mosquitoes in Orange County and a much higher-than-usual number of human WNV cases in California to date signal that this WNV season is starting earlier and may be more intense than usual.
  • California Department of Health (CDPH) is offering a new WNV diagnostic test with improved sensitivity and specificity for WNV through their Viral and Rickettsial Disease Laboratory (VRDL).
  • The new assay requires whole blood, a specimen type not required for WNV testing in the past.


Multiple detections of West Nile Virus in mosquitoes have recently occurred in Orange County.

  • The Orange County Mosquito and Vector Control District has identified several WNV‑positive mosquito pools this season, with the first detections reported on June 1, 2026. This is a significant shift from 2025, when mosquito pools did not test positive for WNV until late July.
  • A July 1, 2026 CDC press release reported 38 human WNV cases nationwide in June, far above the 10‑year June average of 10 cases.


Together, these findings signal that Orange County is likely facing an earlier—and potentially more intense—West Nile virus season in 2026, placing residents at increased risk for infection.

 

Recently, the California Department of Public Health (CDPH), Viral and Rickettsial Disease Laboratory (VRDL) completed validation for a diagnostic whole blood real-time RT- PCR assay for WNV. 

  • This assay has improved sensitivity and specificity compared to serology. 
  • The new whole blood PCR assay will replace WNV PCR testing on CSF, which VRDL has discontinued.

 

To date, no human cases of West Nile virus (WNV) have been detected in Orange County. However, as of July 24, five human cases have been confirmed in California—one each in Kings, Los Angeles, Sacramento, San Diego, and Tulare counties.


Given the increased detection of WNV in local mosquito pools, as well as state and national indicators of rising WNV activity, healthcare providers should counsel patients on preventing mosquito‑borne illness. Providers are also encouraged to consider WNV in their differential diagnosis for patients with clinically compatible illness and to make use of newly available, improved diagnostic testing offered by CDPH.

Provider Recommendations and Resources
  • 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 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 people develop neuroinvasive diseases, which can manifest as encephalitis, aseptic meningitis, acute flaccid paralysis, and/or transverse myelitis.

 

  • Testing for WNV:
  • Whole blood for RT-PCR – this newly available test from CDPH’s VRDL can detect virus without the need for confirmatory serology testing. This assay is replacing the now discontinued CSF PCR for WNV which has historically had low probability of detecting WNV.
  • Whole blood in an EDTA lavender-topped tube (5 ml) should be collected, stored, and transported in refrigerated conditions and shipped within 14 days of collection. OC Public Health Laboratory (PHL) can facilitate the send-out to VRDL.
  • Serology - 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.
  • Other testing for WNV such as PCR testing on serum, CSF, or tissue as well as viral culture have been shown to have low sensitivity for detecting the virus.


  • 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 mosquito surveillance and control activities, please visit the Orange County Mosquito and Vector Control District website at http://www.ocvector.org.

 

  • Prevention: 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 https://www.cdc.gov/mosquitoes/prevention/index.html.


Additional Resources

 

For questions or concerns please contact the Communicable Disease Control Division at 714-834-8180.
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