California is experiencing its highest level of West Nile virus activity in five years. As of November 6, the virus has seriously sickened 528 people and caused 32 deaths, according to the California Department of Public Health. Southern California has been hardest hit, with Los Angeles County accounting for more than half of all reported human cases.
A new CalMatters report examines how this year’s warm, wet summer and fall created favorable conditions for Culex mosquitoes and virus transmission—and why experts are concerned about the season ahead. Remnants of Hurricane Marie brought heavy rain to the Los Angeles region in September, flushing larvicide from storm drains and allowing mosquito populations to surge. Temperatures then climbed well above the threshold that accelerates West Nile virus replication in mosquitoes.
“We’re all scared of next year,” said Paula Macedo, President of the Mosquito and Vector Control Association of California. “With the amount of activity that we already saw this year and having the forecast of being an El Niño year, we are all bracing ourselves.”
Federal forecasters currently give a strong El Niño a greater than 90 percent chance this winter, increasing the odds of heavy precipitation. If floodwaters linger into spring as temperatures rise, mosquito populations could expand significantly. Extreme weather events—both flooding and drought—have historically aligned to drive West Nile outbreaks in California.
There is no vaccine or specific treatment for West Nile virus. Most people who become infected never develop symptoms, while about 1 in 5 experience fever, headache, and body aches. Approximately 1 in 150 develop severe neurological illness that can lead to long-term complications or death. Adults over 60 and people with certain medical conditions are at higher risk.
Prevention remains the most effective protection. MVCAC and local mosquito and vector control districts recommend the following steps:
- Dump and drain standing water at least once a week (plant saucers, birdbaths, toys, buckets, clogged gutters, and other containers).
- Use EPA-registered insect repellent containing DEET, picaridin, or oil of lemon eucalyptus.
- Wear long-sleeved shirts and long pants when outdoors at dawn and dusk.
- Report dead birds—especially crows, ravens, and California scrub-jays—to the California Department of Public Health at westnile.ca.gov/report.
Local mosquito and vector control districts monitor mosquito populations, test for West Nile virus, and apply targeted control measures throughout the year. However, districts cannot access every property or treat every potential breeding source. Public cooperation is essential to reducing risk in neighborhoods and communities.
Read the full CalMatters article: California climate, El Niño conditions fuel West Nile Virus fears
For the latest West Nile virus surveillance data, visit westnile.ca.gov. To learn more about local mosquito control efforts, contact your local mosquito and vector control district.