Yolo County residents warned after St. Louis encephalitis virus is found in mosquitoes

A mosquito sample collected in Yolo County has tested positive for St. Louis encephalitis virus, which has the same symptoms as West Nile virus and is transmitted to humans by the same type of bug.

Laboratory staff detected the St. Louis encephalitis virus while performing routine testing of mosquito samples collected from an area north of Woodland, the Sacramento-Yolo Mosquito and Vector Control District announced Friday in a news release.

Mosquitoes infected with the St. Louis encephalitis virus are not commonly found in Northern California, but district officials said this virus has been spreading. So far this year, the virus has been detected in San Joaquin and Stanislaus counties.

Mosquito and Vector Control District manager Gary Goodman said this is another reminder of how important it is to wear an effective insect repellent while outdoors, especially at dawn or dusk.

“The finding of St. Louis encephalitis along with a record amount of West Nile virus activity this season highlights the importance of mosquito control programs to protect residents from these and other mosquito transmitted diseases that pose a threat,” Goodman said in the news release.

District officials said the initial symptoms of St. Louis encephalitis include headache, fever, nausea, vomiting and fatigue from five to 15 days after being infected.

Some people may develop severe neuroinvasive disease, such as encephalitis (an inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord), according to the U.S. Centers for Disease Control and Prevention.

District officials said severe neuroinvasive disease occurs most commonly in older adults, those with a previous health condition or a weakened immune system.

CDC officials said long-term disability or death can occur in rare cases, but most people infected with the virus do not have symptoms. There were 32 human cases of St. Louis encephalitis virus in the United States last year; 17 in 2021 and 16 in 2020, according to the CDC. Prior to 2022, the total hadn’t been above 20 human cases since 2015, when 23 were recorded.

CDC data shows only one human case of St. Louis encephalitis virus reported so far this year in the U.S. prior to the one in Yolo County; it was in California. Kern County public health officials announced on July 7 the county’s first reported human case of St. Louis encephalitis virus.

The mosquitoes that carry St. Louis encephalitis virus in Northern California are the same mosquitoes that are able to transmit West Nile virus.

Health officials on Thursday announced the first West Nile virus death this year in Yolo County. It’s the county’s first reported human fatality from the mosquito-borne disease in five years. The first West Nile virus death this year in Sacramento County was reported on Wednesday.

Yolo County Public Health Officer Dr. Aimee Sisson said on Thursday that 2023 has been “a bad mosquito and West Nile virus year.” Sisson said one in four mosquito samples in the county is testing positive for West Nile virus.

Sacramento-Yolo Mosquito and Vector Control District offered these tips to reducing the threat of mosquitoes:

▪ Eliminate stagnant water that may produce mosquitoes, such as water in plant saucers, buckets, wheel barrows, children’s toys, tires and other containers.

▪ When outdoors, apply an insect repellent that contains DEET, Picaridin, IR3535 or oil of lemon eucalyptus.

▪ Ensure swimming pools, spas, fountains and ponds are properly maintained.

▪ Change water in pet dishes, bird baths and other small containers every 2 to 3 days.

▪ Request mosquitofish for ornamental ponds and fountains.

▪ Report neglected swimming pools and other mosquito breeding sources.

To report a mosquito breeding source or for more information, call the Sacramento-Yolo Mosquito and Vector Control District at 1-800-429-1022 or visit FIGHTtheBITE.net.