CaliToday (19/8/2025): California is experiencing a significant and concerning surge in West Nile virus (WNV) infections, with health officials reporting a sharp increase in cases across multiple counties. The spike in the mosquito-borne illness has prompted public health authorities to issue heightened warnings, urging residents to take stringent precautions to protect themselves from mosquito bites.
The California Department of Public Health (CDPH) has confirmed a substantial uptick in both human cases and the detection of the virus in mosquito populations, indicating a period of high transmission. This year's outbreak is on track to be one of the most severe in recent years, raising alarms among healthcare providers and vector control agencies statewide. Major metropolitan areas, as well as agricultural regions in the Central Valley, have been identified as hotspots for the virus's activity.
Understanding the Threat
West Nile virus is the most common mosquito-borne disease in the continental United States and is primarily transmitted to people through the bite of an infected mosquito. While approximately 80% of those infected will not show any symptoms, about 1 in 5 people will develop a fever accompanied by other symptoms like headache, body aches, joint pains, vomiting, diarrhea, or a rash.
The primary concern for public health officials is the potential for severe neuroinvasive disease, which occurs in less than 1% of infected individuals (about 1 in 150). This severe form of the illness can lead to serious neurological conditions such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes surrounding the brain and spinal cord). Symptoms of severe WNV can include high fever, severe headache, neck stiffness, disorientation, coma, tremors, seizures, or paralysis. Recovery from severe illness can take weeks or months, and some neurological effects may be permanent. Individuals over 60 and those with certain medical conditions, such as cancer, diabetes, or hypertension, are at the greatest risk for severe illness.
Factors Contributing to the Surge
State and local health experts point to a combination of environmental factors contributing to the current surge. A winter with significant rainfall followed by a period of sustained high temperatures has created ideal breeding conditions for Culex mosquitoes, the primary carriers of the virus. Stagnant water sources—from puddles and ponds to unmaintained swimming pools and even small containers in backyards—have become prolific breeding grounds, leading to a boom in the mosquito population.
"We are seeing the perfect storm of conditions for West Nile virus transmission," said a spokesperson for a local vector control district. "The mosquito population is thriving, and the virus is actively circulating between birds, the primary hosts, and the mosquitoes that bite them. This significantly increases the risk for human infection."
Official Warnings and Prevention Measures
In response to the rising case numbers, public health departments across California have intensified their surveillance and control efforts, including setting more mosquito traps and conducting targeted larvicide treatments.
Officials are strongly urging all residents to take an active role in protecting themselves and their communities by following the "Three D's" of prevention:
- DEET: Use insect repellents containing DEET, picaridin, IR3535, or oil of lemon eucalyptus on exposed skin and clothing.
- Dawn and Dusk: Limit outdoor activities during the early morning and evening hours when mosquitoes are most active.
- Drain: Eliminate all sources of standing water on your property, including in flowerpots, old tires, buckets, and birdbaths, to prevent mosquitoes from breeding.
Californians are also advised to ensure that their window and door screens are in good condition to prevent mosquitoes from entering their homes. Any dead birds should be reported to the state's WNV hotline, as they are a key indicator of the virus's presence in a specific area.