West Nile virus cases jumped nearly 1,000% in Sacramento County last year. Here’s why

West Nile virus cases rose 980% in Sacramento County in a one-year period after record-breaking storms drenched California in 2023, triggering local officials to request residents to practice mosquito mitigation as the insects this year threaten another intense season.

Two people died and 54 people contracted the West Nile Virus last year in Sacramento County, according to the California Department of Public Health. In 2022, the county recorded one death and five cases, the data show.

West Nile virus killed seven people last year across the four-county region of Sacramento, El Dorado, Placer and Yolo — making 2023 the deadliest year for the disease in the past decade, state health data show.

The viral infections recorded last year in Sacramento, Yolo and El Dorado counties were also the highest going back to at least 2013, according to CDPH data. In Yolo County, the virus infected 45 people and killed three last year, up from three cases and zero deaths in 2022, according to CDPH. One person also died last year from West Nile virus in each of Placer and El Dorado counties.

“It was a very, very bad year for West Nile virus last year,” said Gary Goodman, the executive director of the Sacramento-Yolo Mosquito and Vector Control District, during Tuesday’s Sacramento County Board of Supervisors meeting. “We attribute that to the fact that we had so much water and so many new sources that we … had to deal with.”

State water officials recorded one of the deepest snowpacks in California’s history last year after a series of atmospheric rivers buffeted the golden state.

Statewide, 428 people contracted West Nile virus in 2023, nearly double the case total recorded in 2022, according to the district’s data.

But there may have been 12,000 to 30,000 statewide cases last year because the disease is severely under-reported, Goodman said.

Placer County saw six cases in 2023, up from two cases each of 2020, 2021 and 2022, according to the Placer Mosquito and Vector Control District. Placer County reported 35 instances in 2005, the district’s data show.

El Dorado County experienced three viral infections in 2023 and zero in 2022, according to CDPH data.

Goodman said residents may not believe the virus is serious, but it can leave long-lasting, debilitating effects or result in death. Serious symptoms include encephalitis, blindness and paralysis.

CDPH found more than 50% of West Nile virus patients experienced symptoms one year after blood suckers deposited the virus, he said.

The control district sets about 100 mosquito traps daily to catch insects and test them for the endemic disease, Goodman said. It also breeds about 4,000 pounds of mosquitofish, which eat larvae in waters, he said.

But it’s difficult for district officials to peer into residents’ backyard to spot pools of water in which female blood suckers can lay their eggs, Goodman said. Any item — from large pools to small tire swings — holding water for about seven days can attract breeding mosquitoes, he said.

Residents are asked to dump water collected in their backyards frequently.

“That would make a big … difference for their neighbors specifically,” he said.

West Nile virus originates in birds and is transmitted by mosquitoes who suck their blood and release the virus into humans, Goodman said.

He asked residents to report dead birds or other concerns by calling 1-800-429-1022 or on www.fightthebite.net.