Yolo County detects first human case of West Nile virus. How many are in California?

A Woodland resident tested positive for the West Nile virus, the first human case of the mosquito-borne disease reported in Yolo County this year.

Yolo officials announced the discovery Friday of a person who fell ill during the first week of August. No other information about the person, including their condition, was disclosed.

“Most people who become infected do not get sick, and the risk of serious illness to most individuals is low,” said Yolo County’s Deputy Public Health Officer Dr. Mary Ann Limbos. “However, about one percent of individuals can develop a serious neurologic illness.”

There have been 14 other human cases of the virus already detected in California in 2023, an increase of four cases compared to the total in 2022. As of Aug. 11, cases have been reported in Butte, San Joaquin, Merced, Madera, Tulare, Kern, San Bernardino and Riverside counties. Another 22 counties have reported non-human virus activity this year.

No deaths have been reported this year, according to the California Department of Public Health, which collects statewide data. Since West Nile was first detected in California, more than 7,500 people have been found with the disease, of which more than 300 deaths have been reported, CDPH said.

West Nile virus symptoms develop between two days to two weeks after a person has been exposed to an infected mosquito, according to Yolo County officials.

However, eight out of ten people infected with the virus may not experience any symptoms, according to the U.S. Centers for Diseases Control and Prevention. About one in five infected persons may develop a fever along with other symptoms including headache, body aches, joint pains, vomiting, diarrhea and other discomforts.

Yolo County noted that, while most cases clear up within a few days to a few weeks, “serious neurologic effects can occur, making it imperative to remain vigilant.”

“Most people who become infected do not get sick, and the risk of serious illness to most individuals is low. However, about one percent of individuals can develop a serious neurologic illness,” said Dr. Mary Ann Limbos, Yolo County Deputy Public Health Officer, in a news release. “We commend the dedicated efforts of the Sacramento-Yolo Mosquito & Vector Control District and their ongoing work in reducing mosquito populations to safeguard our community’s well-being.”

That effort has included an increase in aerial spraying, specifically around Davis and Woodland. Neighboring vector control districts have employed similar efforts to combat infection, including in Roseville.

Mosquito control officials have reported “record” levels of the virus across the capital region since early July, as hot weather in Northern California continues to hatch large populations spawned due to heavy winter rainfall.

CDPH as of Friday reported nearly 2,000 cases of West Nile virus in mosquito samples and 236 in dead birds so far in 2023 – a 300% increase in samples from mosquitoes in about a month and about double the number of dead birds as compared to the five-year average (122.8).

What are West Nile virus symptoms in humans?

West Nile virus in humans often results in mild or no symptoms, but in severe cases the disease can be deadly. Many people can recover from West Nile virus at home, but about 20% of those bitten develop symptoms of West Nile fever.

Roughly one out of every 150 people bitten by an infected mosquito gets the most severe form of the disease, West Nile encephalitis.

Symptoms of severe West Nile virus include sudden onset of high fever, severe headache, nausea, vomiting, sore throat, backache, joint pain, aches and weakness in prominent muscles, prolonged fatigue and swollen lymph nodes. Those with West Nile fever may also get a rash.

“Residents can play a crucial role in preventing West Nile virus by regularly inspecting their yards and eliminating standing water, a breeding ground for mosquitoes,” Limbos said. “Employing precautions such as wearing protective clothing and using insect repellents containing DEET significantly reduces the risk of mosquito bites.”

In addition to West Nile virus, various species of mosquitoes in the capital region can transmit other illnesses such as Zika virus, dengue, malaria, St. Louis encephalitis virus and yellow fever.

Mosquitoes typically are more active in the early morning and evening, officials said, so it’s especially helpful to wear protective clothing and repellent if you’re outside during these times. If you encounter large numbers of mosquitoes or day-biting mosquitoes, contact the vector control district for help with tracking the source and abating the threat.

CDPH advised using an insect repellent containing DEET, picaridin, oil of lemon eucalyptus, or IR3535. DEET can be used safely on infants and children 2 months of age and older, according to public health officials.

You can fill out a service request at fightthebite.net or call 800-429-1022 to get help. To report a dead bird, fill out an online form at westnile.ca.gov or call 877-968-2473.

More information on how to prevent the spread of West Nile virus and detect possible symptoms is available at westnile.ca.gov.

The Bee’s Cathie Anderson contributed to this story.