It turns out that a Stanislaus County man who became sick in September was stricken by St. Louis encephalitis, a virus that has reappeared in the Northern San Joaquin Valley this year.
The county Health Services Agency made the case public Wednesday but did not identify the individual, who is in his 70s.
Like the West Nile virus, the St. Louis disease is transmitted by mosquitoes. Both of the viruses may cause encephalitis, or inflammation of the brain, in people who are infected.
The Stanislaus County man had symptoms on and off for a month and was not hospitalized. He suffered from fatigue, fever and abdominal pain and sought care from his physician, said Anuj Bhatia, a Health Services Agency spokesman.
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Samples of his blood were tested. Additional tests to confirm the virus were done by the state Department of Public Health and Centers for Disease Control and Prevention.
The county health agency had to wait for the verification tests before reporting the case, Bhatia said. The local man was the second person to test positive for St. Louis encephalitis in California this year.
California had three cases of St. Louis encephalitis illness last year. Mosquitoes have tested positive for the virus in counties in the San Joaquin Valley and other places in Southern California in 2017.
In August, St. Louis encephalitis was detected in Stanislaus County for the first time in decades. Mosquito abatement districts stopped seeing the virus after West Nile took hold in California about 15 years ago.
David Heft, general manager of Turlock Mosquito Abatement District, said a small amount of St. Louis encephalitis activity was detected in Culex mosquitoes near Grayson and Newman, where most of the river flooding occurred last winter.
St. Louis was not detected near the man’s home in the eastern part of Stanislaus County, but the patient recalled getting mosquito bites at other places where he owns property, Heft said.
St. Louis was one of the viruses targeted by mosquito abatement control efforts historically. “Once West Nile showed up, we did not see any St. Louis, and all of a sudden it came back last year,” he said.
Heft said early indications from genetic testing suggest the St. Louis virus might be from South America and a different strain than what was previously found in the San Joaquin Valley.
Red flags were raised in Merced County in August when a chicken near Gustine tested positive for St. Louis encephalitis.
Most people who are bitten by an infected mosquito will not have symptoms. The St. Louis virus may cause a fever, headache, nausea and tiredness in some of those infected, a county press release said.
An infection resulting in encephalitis is more common in older adults and may be life-threatening. There is no treatment other than supportive care.
Mosquitoes are not very active with the colder temperatures this month. In the warmer months of the year, county residents can avoid mosquito bites that can transmit West Nile and the St. Louis virus, by wearing insect repellant and staying indoors in the early morning and evening when the mosquitoes are active.
Ken Carlson: 209-578-2321, @KenCarlson16