How worried should you be about West Nile virus?
California has seen a sharp rise in West Nile virus activity in 2025, with over 250 infected dead birds and increased mosquito pools and human cases compared to last year, prompting public health warnings.
Dr. Justin Remais of UC Berkeley has tracked West Nile virus in California since 2014, noting that climate shifts may worsen transmission, particularly in coastal areas. As of mid-July 2025, the state has recorded over 250 dead birds testing positive for the virus, a fivefold increase from last year. Los Angeles County alone reported 67 positive mosquito pools, compared to just 6 in 2025, while Southern and Central California have seen five human cases, up from three in 2025. Public health officials attribute the surge to rising temperatures and sustained warm conditions, which favor mosquito breeding and virus transmission.
West Nile virus, the leading cause of mosquito-borne disease in the contiguous U.S., is primarily spread through infected mosquito bites, particularly from the Culex species. Most infected individuals show no symptoms, but severe cases can lead to neurological illnesses like encephalitis or meningitis, with less than 1% of infections resulting in such outcomes. The virus, first identified in the U.S. in 1999, has been present in Los Angeles since 2003. Transmission is seasonal, peaking in summer and early fall, with higher risk in regions like the Central Valley and Southern California due to favorable environmental conditions.
Research by UC Berkeley shows that West Nile virus transmission in Southern California is strongly linked to temperatures between 70 and 73 degrees Fahrenheit, where the likelihood of infected mosquitoes increases sharply. As climate change pushes coastal temperatures into this favorable range more consistently, the risk of transmission is expected to rise. The virus cycles between mosquitoes and birds, particularly corvids like crows and jays, before potentially infecting humans. While direct transmission from birds to humans does not occur, dead birds can signal early virus activity and guide local mosquito control efforts.
Public health guidance emphasizes preventing mosquito bites to reduce infection risk. The CDC recommends eliminating standing water, using EPA-registered repellents, and wearing protective clothing during peak mosquito hours. Severe cases require immediate medical attention, though no human vaccines or specific treatments exist. Most mild infections resolve with rest and hydration, while severe cases may lead to long-term complications or death. Health departments automatically report diagnosed cases to monitor the virus's spread.