The Orange County Mosquito and Vector Control District (OCMVCD) has confirmed that mosquitoes collected in the cities of Anaheim and Garden Grove have tested positive for West Nile virus (WNV). These are the first samples to test positive in the county this year. A dead bird recovered in Garden Grove has also tested positive for the virus.
The positive mosquitoes were collected in areas bordered by Euclid Ave., W. Broadway, Interstate 5, and W. Ball Rd. in Anaheim; Magnolia St., W. Cerritos Ave., Gilbert St., and W. Katella Rd. in Anaheim; Haster St., Lampson Ave., Lewis St., and State Route 22 in Garden Grove; and Gilbert St., Lampson Ave., Brookhurst St., and Garden Grove Blvd. in Garden Grove.
West Nile virus is primarily transmitted to humans through the bite of infected mosquitoes, which acquire the virus when feeding on infected birds. At present, Orange County has not reported any human cases of West Nile virus.
“Multiple mosquito samples and a dead bird testing positive for West Nile virus indicate that the virus is circulating in the community,” said OCMVCD Director of Scientific and Technical Services Amber Semrow. “Anaheim and Garden Grove are historically high-risk areas for West Nile virus activity. Conditions across the region are favorable for sustained virus activity during the warm summer months.”
OCMVCD staff will continue to conduct surveillance, inspections, and control measures for mosquitoes in the area to prevent additional mosquito breeding. OCMVCD will also post WNV advisory signs alerting residents of WNV activity in the affected areas.
“Everyone in Orange County should do their part to prevent mosquitoes. Dumping and draining standing water is the most effective and environmentally friendly way to rid mosquitoes from our communities,” said OCMVCD Public Information Officer Brian Brannon.
To prevent mosquito bites, take action and follow these tips:
- Dump and drain containers holding water at least once a week
- Clean and scrub bird baths and pet water bowls
- Wear repellent containing DEET, Picaridin, IR3535, or oil of lemon eucalyptus
- Close all unscreened doors and windows to prevent mosquitoes from entering your home or space; repair broken or damaged screens
- Wear light-colored, loose-fitting, long-sleeved shirts, and long pants while outside at dawn and dusk
For more information about prevention, visit www.ocvector.org.
West Nile Virus in California (2020–2025)
Human Cases by Year
- 2020: 231 cases, 11 deaths
- 2021: 125 cases, 13 deaths
- 2022: 207 cases, 15 deaths
- 2023: 26 cases, 2 deaths (notably low due to drought and vector control efforts)
- 2024: 48 cases, 4 deaths (provisional data)
- 2025 (as of July): 3 confirmed cases, 1 death (early in the season)
Trends and Observations
- Peak Years: 2020 and 2022 saw the highest case counts and fatalities.
- Low Activity: 2023 had unusually low WNV activity, attributed to environmental conditions and aggressive mosquito control.
- 2025 Outlook: Early signs suggest a potential rebound in activity, with positive mosquito pools and bird cases already reported in multiple counties.
Geographic Distribution
- Most human cases occurred in Central Valley and Southern California, including counties like Los Angeles, Riverside, Kern, and Fresno.
- San Diego and Orange Counties have had consistent mosquito and bird activity, though fewer human cases.
Public Health Response
- The Vector-Borne Disease Section (VBDS) and local vector control agencies conduct:
- Mosquito and bird surveillance
- Public education campaigns
- Emergency pesticide spraying when needed
- Sentinel chickens and dead bird reporting remain key early warning tools.