West Nile Cases Surge Across California as El Niño Looms

California’s warm, wet summer and fall brought the state’s highest West Nile virus activity in five years. The virus had seriously sickened 528 people and killed 32 by November 6, state health officials said. Los Angeles County accounted for more than half of California’s reported human cases.

Mosquitoes carrying the virus surged after September rain washed larvicide from more than 3,000 miles of underground storm drains in Los Angeles County. At one point, over 70% of mosquitoes caught for testing there carried West Nile; the proportion had fallen to about one-third in recent weeks. Cooler nights may now slow mosquito breeding and viral growth.

Most infected people do not develop symptoms. About one in five may experience fever, headache and body aches, while roughly one in 150 develops serious illness that can affect the brain. There is no vaccine or cure, and treatment is supportive.

Climate patterns may add to the risk. A federal forecast gives a very strong El Niño more than a 90% chance this winter, raising the possibility of heavy Southern California rain. If floodwater remains into spring as temperatures rise, mosquito numbers could increase. Researchers also point to a temperature threshold: West Nile was more likely to appear in mosquitoes when average temperatures exceeded about 73 degrees.

Outbreaks do not rise steadily, however. Bird immunity contributes to a three-to-five-year cycle, and both drought and flooding have coincided with major outbreaks. California recorded 433 human cases in 2023 after Tulare Lake re-formed, and more than 1,600 people contracted the virus during the drought years of 2014 and 2015. Experts say the mix of weather, birds and mosquitoes makes next year difficult to predict.

West Nile Surge Shows How a Warming Climate Raises Health Risks

California’s West Nile surge is a warning about the public-health costs of a warming climate. By November 6, the virus had seriously sickened 528 people and killed 32, the state’s highest activity in five years. Los Angeles County accounted for more than half of reported human cases.

In coastal Los Angeles, summer temperatures already sit near the range where West Nile becomes more likely to circulate in mosquitoes. Research found the virus was less likely below average temperatures of about 70 degrees and more likely above roughly 73. This year’s warmth helped push conditions into that risk zone.

September rain then washed mosquito-killing treatments from thousands of miles of underground storm drains. At one point, more than 70% of mosquitoes sampled in Los Angeles County carried the virus. A strong El Niño is forecast as highly likely this winter, bringing concern that lingering floodwater and warmer spring temperatures could create another surge.

The consequences are not abstract: about one in five infected people may become ill, and roughly one in 150 can develop severe neurological complications. There is no vaccine or cure. While bird immunity can create a three-to-five-year cycle and outbreaks are hard to predict, extreme weather can tilt conditions toward disease. Treating climate-driven health risks as inevitable would ignore the warning this year has delivered.

West Nile’s Cycles and Weather Matter More Than Simple Forecasts

California’s West Nile outbreak was severe, but the virus does not follow a simple, steadily worsening path. The state counted 528 serious illnesses and 32 deaths by November 6, while Los Angeles County accounted for more than half of human cases. Experts say bird immunity helps produce a natural cycle of higher and lower activity every three to five years.

Weather can amplify risk in different ways. Heavy rain in 2023 revived habitat around Tulare Lake and coincided with 433 human cases. Drought also preceded a major outbreak: more than 1,600 people contracted West Nile in 2014 and 2015. The shifting relationships among birds, mosquitoes and weather make precise predictions difficult.

Los Angeles mosquito samples showed how quickly conditions can change. After September rain washed larvicide from storm drains, the share of sampled mosquitoes carrying West Nile topped 70% at one point; it later fell to about one-third. Cooler nights may bring some relief.

A very strong El Niño is considered more than 90% likely this winter and could bring heavy Southern California rain. If water remains into spring as temperatures rise, mosquito numbers may grow. That risk merits attention, but the reported evidence also shows why one weather forecast cannot determine next year’s outcome: outbreaks depend on multiple forces, including whether local bird populations remain susceptible.