A mosquito sample in Stanislaus County tested positive for the St. Louis encephalitis virus, which is capable of causing a potentially deadly illness.
The mosquitoes infected with St. Louis encephalitis were southwest of Modesto in the Grayson area.
The St. Louis variety is similar to the West Nile virus and is carried by the same kind of mosquitoes. Like a West Nile infection, a person bitten by an infected mosquito may come down with encephalitis, or inflammation of the brain, with headache, dizziness, confusion, seizures or paralysis.
Most people infected won’t feel sick but some will have mild flu-like symptoms, health officials said. Older people and those with weakened immune systems are more susceptible to getting the serious illness. No county residents are known to have come down with the St. Louis illness thus far.
“A small number get the infection of the brain,” said Dr. Julie Vaishampayan, public health officer for Stanislaus County. There is no treatment for the disease, so it is “important that people take precautions to protect themselves and their families from mosquito bites,” she said.
Officials were unable to find any record of St. Louis encephalitis in Stanislaus County in the past 40 years, although it was present earlier.
The virus has shown up in mosquitoes this year in Fresno, Kings and Kern counties in the San Joaquin Valley and Riverside County in Southern California.
Vaishampayan said there were three cases of St. Louis encephalitis illness in California last year, the first in almost 20 years.
Stanislaus County’s Health Services Agency will send an advisory asking that health care providers test both for St. Louis encephalitis and West Nile if patients have the symptoms.
Both viruses are carried by the same Culex mosquitoes and are controlled by the same surveillance and abatement methods, said David Heft, general manager of Turlock Mosquito Abatement District.
St. Louis encephalitis was a virus historically controlled by mosquito abatement districts in California before West Nile virus came on the scene about 15 years ago. St. Louis seemed to disappear after West Nile became the dominant virus, but it has reappeared in some counties in the last two or three years, Heft said.
Heft said the flooding earlier this year resulted in large numbers of Culex mosquitoes, which may carry St. Louis and West Nile, and “that is helping to drive (the St. Louis) virus.” Mosquitoes spread the two viruses by biting an infected bird and then biting a person.
Residents can guard against mosquito bites by using insect repellent, draining standing water outside their homes and patching holes in window screens. The bugs are more active in early morning and evening, so avoid going outdoors at those times.
Mosquito problems, including neglected swimming pools, that occur north of the Tuolumne River should be reported to Eastside Mosquito Abatement District at (209) 522-4098. Problems south of the Tuolumne River are the responsibility of Turlock Mosquito Abatement District, (209) 634-1234.
Ken Carlson: 209-578-2321. @KenCarlson16