Three people in Fresno County learned they were infected with West Nile virus after they donated blood and it underwent routine testing. The Fresno County Department of Health reported Wednesday that all three cases were asymptomatic, meaning they showed no symptoms of being ill.

The virus is also showing up in mosquito samples. Mosquito abatement districts in Fresno County reported that 56 samples have tested positive for West Nile virus this year.

The disease is carried by birds and transmitted when mosquitos bite infected birds and then bite humans.

Watch: Mosquito Education and Prevention

According to the California Department of Public Health, in 2023 there were 433 symptomatic human cases of West Nile reported, including 19 deaths, and 40 asymptomatic infections in blood donors. Since the virus first arrived in California in 2003, there have been more than 8,000 human cases and more than 390 deaths related to the virus statewide.

“We need to stay vigilant as mosquitoes may continue to cause severe illnesses, including WNV infection, which can require hospitalization and intensive care,” Dr. Rais Vohra, Fresno County interim health officer, said in a news release.

Take Precautions

To reduce the risk of being bitten and contracting West Nile virus, residents are urged to take precautions that include:

• Using an EPA-registered insect repellant that contains DEET, picaridin, oil of lemon eucalyptus, or IR 3535, especially at dawn and dusk when mosquitos are most active.
• Make sure doors and windows have tight-fitting screens with no holes.
• Eliminate standing water and drain vessels that can hold water and that are near your home.
• Contact your mosquito abatement district to report swimming pools or water features that contain stagnant or green water.

To report dead birds and for information on West Nile virus activity in California, go to the California Department of Public Health website at https://westnile.ca.gov or call 1-877-WNV-BIRD.