California Reports First West Nile Virus Activity of 2026: Here’s What to Know

West Nile virus (WNV) has been detected in California.

The discovery came after three birds were found dead in San Diego, Alameda, and Santa Clara counties and were subsequently tested for West Nile virus. All birds tested positive.

These were the first cases of WNV in California this year.

As of May 6, no other cases of WNV have been reported in the state.

Background Information

WNV is a mosquito-borne virus. Around 2,000 to 3,000 cases are reported in humans in the United States each year.

WNV mostly spreads among birds, but it has also been detected in squirrels, sentinel chickens, and horses. In rare cases, cats and dogs also contract the disease.

People and the aforementioned animals become infected with the disease through bites from infected mosquitoes.

Mosquitoes commonly get WNV from dead birds. Due to this, tracking these deceased animals provides insight into areas to watch out for the virus. It is usually most prevalent in magpies, ravens, sparrows, finches, hawks, jays, and crows.

From 2003 to 2025, in California, more than 8000 people were affected by WNV. Over 400 cases resulted in death.

“West Nile virus is the most common and serious vector-borne disease in California,” a statement on the California mosquito-borne virus, surveillance, and response program’s website warned.

People normally catch the illness during early fall and summer, as this is when infected mosquitoes are most active. Around 80% of those infected with WNV show no symptoms and do not know they have it.

In contrast, about 20% of humans with the virus display symptoms such as nausea, vomiting, fever, body aches, and, in some cases, skin rashes. Patients typically experience symptoms from WNV three to 14 days after being bitten by a mosquito.

In extremely rare cases, around less than one percent, people with WNV pass away or become seriously ill.

According to the CDC, some infected patients have undergone paralysis. Others have experienced encephalitis, which is inflammation of one’s brain tissue, or meningitis, inflammation of one’s membranes that protect the spinal cord and brain.

If California residents find a dead bird in the area, they are encouraged to contact the state’s surveillance and response program on their website.

“Humans are really getting impacted more on an accidental basis,” Shannon Bennett, a virologist at the California Academy of Sciences, told SFGATE. “We’re dead ends for the virus.”