Officials in Riverside are trapping mosquitoes along the Santa Ana River to determine how many are carrying encephalitis. State health officials notified the Northwest Mosquito Abatement District that the St. Louis encephalitis virus was found in blood samples taken from a chicken kept by the district. Additional blood samples will be drawn to determine if any other chickens are carrying the mosquito-spread virus, district manager L. Lino Luna said. “None of the mosquitoes have tested positive for the disease yet,” Luna said. A mild bout of encephalitis can cause flu-like symptoms, such as headaches, fever and stiffness. However, in later stages, the illness can lead to paralysis and coma. No cases of encephalitis have been reported among humans in California this year, health officials said.