West Nile virus, a disease that is typically transmitted to people by mosquitos that have fed on blood from infected birds, has so far killed four California residents in 2025, including three in the San Joaquin Valley.

In the Valley, West Nile deaths have been confirmed in Tulare, Fresno and Kings counties. The death of another person was attributed to West Nile in Butte County, in northern California.

The most recent of the fatalities was confirmed Oct. 2 in Kings County.

The San Joaquin Valley has become something of a hot spot for West Nile virus infections in California in 2025, according to the California Department of Public Health. Of at least 56 reported human infections confirmed statewide through late September, 44 of them – almost 80% – have occurred among Valley residents.

Of California’s 58 counties, 44 have experienced no confirmed human West Nile infections so far this year.

The U.S. Centers for Disease Control reports that so far in 2025, 1,324 human cases of West Nile have been confirmed in 42 states. California accounts for at least 56 of those human cases, according to the California Department of Public Health.

Among Valley counties, data from the CDC, the California Department of Public Health, or county health departments show:

  • Fresno County: At least eight human cases, including one death.
  • Kern County: At least 13 human cases, no deaths.
  • Kings County: Five human cases, including one death.
  • Madera County: At least one human case, no deaths.
  • Merced County: At least two human cases, no deaths.
  • San Joaquin County: No human cases.
  • Stanislaus County: At least two human cases, no deaths.
  • Tulare County: At least 13 human cases, including one death.

Most people who become infected by West Nile experience no symptoms at all. About 20% have mild symptoms that can include fever, headache, body aches, nausea and vomiting.

But for some people, including the elderly or those with compromised immune systems, the virus can cause severe neurological illness and death.

West Nile is the leading cause of mosquito-borne illness in the U.S. The CDC estimates that about 2,000 people are diagnosed each year, but most cases are believed to go unreported because the victims either experience no symptoms or think their symptoms are related to some other cause.

More than 1,300 people each year develop a serious case of West Nile affecting the central nervous system, with about 130 deaths per year. There are no licensed vaccines or medicines to prevent or treat West Nile disease in humans, the CDC reports.

Health officials warn that to protect against West Nile or other illnesses carried by mosquitos, residents can:

  • Use an insect repellent containing DEET, picaridin, oil of lemon, eucalyptus or Insect Repellant 3535 to keep mosquitoes from biting.
  • Wear proper clothing including long sleeves and long pants, as well as using mosquito repellent, during the early morning and evening hours when mosquitoes are most active.
  • Make sure that screen doors and window screens are tight-fitting to keep mosquitoes outside, and to repair damaged or torn screens.
  • Eliminate or drain sources of standing water where mosquitoes can lay their eggs. This includes flower pots, buckets, old car tires and other containers. Swimming pools that are not properly maintained and have green or stagnant water can also be a breeding ground for mosquitoes.

Tim Sheehan is the Health Reporting Fellow and a senior reporter with the nonprofit Central Valley Journalism Collaborative. The fellowship is supported by a grant from the Fresno State Institute for Media and Public Trust. Contact Sheehan at tim@cvlocaljournalism.org.