California vector control districts are on the lookout for the St. Louis Encephalitis Virus after it appeared in Placer, Yuba and Stanislaus counties last week.
Kern, Kings, Fresno and Riverside counties have all seen positive mosquito samples this year.
Culex mosquitos can pass the virus from bird to human. No one has gotten sick this year, but symptoms can include fever, headache and dizziness.
There were three human cases in California last year. Before that, the state hadn't seen a case since 1997.
Joel Buettner is tracking the disease at the Placer Mosquito and Vector Control District. He says they’ve never seen St. Louis Encephalitis there before. He doesn't know why it's appearing now.
Climate change and people's travel habits could be a factor.
“Diseases that we thought went away decades ago are now coming back, and one of the questions I have is, 'Is it the same one? Is it going to present new or different public health threats? And what do we have to do to help prevent those?'”