West Nile virus activity in California has reached its highest level in five years, with health officials warning that August and September represent peak transmission periods. As of August 14, the California Department of Public Health reported 30 human cases of West Nile virus across 13 counties, including 3 fatalities, already exceeding the typical five-year average for cases reported by this time of year.

Why Is West Nile Virus Activity Spiking This Year?

The surge in West Nile virus cases reflects elevated activity across multiple surveillance indicators. Dead birds testing positive for the virus have reached 24% of samples examined, compared to the five-year average of 13%. Mosquito surveillance shows 8 out of every 1,000 mosquitoes testing positive for West Nile virus, compared to the historical average of 5 out of 1,000. Signs of West Nile virus activity have been detected in 32 California counties through monitoring of mosquitoes, dead birds, sentinel chickens, and horses.

Several factors influence West Nile virus transmission patterns, including climate conditions, the population and types of birds and mosquitoes in an area, and the level of immunity to the virus that birds have developed. The virus spreads to humans and animals exclusively through the bite of an infected mosquito.

Who Is Most at Risk for Serious Illness?

While most people infected with West Nile virus experience mild or no symptoms, certain groups face significantly higher risks. People aged 60 and older, those with diabetes, hypertension, or chronic kidney disease, and individuals taking immunosuppressive medications are at elevated risk for serious complications. The Centers for Disease Control and Prevention recently issued a health alert noting that patients on certain immunosuppressive medications face much higher risk of experiencing long-term complications or death from West Nile virus.

Serious neurological complications can develop in some cases, including encephalitis and meningitis, which can be potentially fatal. Dr. Erica Pan, Director and State Health Officer at the California Department of Public Health, emphasized the severity of the threat.

"West Nile virus can be fatal, particularly for the elderly and immunocompromised. August and September are peak periods of West Nile virus transmission in the state, so we urge everyone to take precautions to protect themselves against mosquito bites," said Dr. Erica Pan.

Dr. Erica Pan, Director and State Health Officer, California Department of Public Health

How to Protect Yourself From West Nile Virus

Health officials recommend a straightforward three-part prevention strategy to reduce mosquito bites and lower infection risk:

• DEET Application: Apply an EPA-registered insect repellent containing active ingredients such as DEET, picaridin, oil of lemon eucalyptus, or IR3535 according to label instructions. When used as directed, DEET is safe for pregnant or breastfeeding women and children.
• Dawn and Dusk Precautions: Mosquitoes that transmit West Nile virus typically bite during early morning and evening hours. Wear loose-fitting long-sleeved shirts and pants during these times, and ensure doors and windows have tight-fitting screens to prevent mosquitoes from entering your home. Repair or replace any screens with tears or holes.
• Drain Standing Water: Eliminate all sources of standing water on your property, including flowerpots, old car tires, buckets, and other containers where mosquitoes lay eggs. If you identify a swimming pool that is not being properly maintained, contact your local mosquito and vector control agency.

People taking medications that suppress their immune system should discuss their specific risk for serious West Nile virus illness with their doctor and take extra care to follow recommended safety steps.

Reporting dead birds can help public health officials track where the virus is spreading. Californians can report dead birds through the West Nile virus website or by calling the toll-free number 1-877-WNV-BIRD (968-2473). Over 24% of dead birds identified through surveillance have tested positive for the virus, making bird monitoring a critical early warning system for community transmission.

As West Nile virus activity continues through the fall months, maintaining vigilance about mosquito prevention and staying informed about local transmission levels remains essential for protecting individual and community health.