The department said it received confirmation of the positive tests from UC Davis, which studied mosquito samples collected on July 16.
The mosquitos were found near the Nuevo Fire Station, the San Jacinto River, the Ramona Expressway and the Eastern Municipal Water District wetlands and ponds in San Jacinto, according to the department. Other mosquitos collected last month from the Moreno Valley and Nuevo tested positive for West Nile as well.
In addition, a dead crow from Banning and a chicken from the San Jacinto Wildlife Area both tested positive for West Nile virus antibodies, the department said. Certain species of birds, such as crows, are susceptible to the virus.
The department said its staff has intensified mosquito surveillance and efforts to control its populations to interrupt the disease's transmission. It has also asked residents to take an active role in protecting themselves from mosquitos.
The virus is spread through bites from infected mosquitos. Most people who are infected show no symptoms, though some can develop fever, headaches and body aches. Young children, the elderly and people with lowered immune systems can have more extreme reactions.
Mosquitos carrying the disease are common in parts of Riverside County during the summer, according to the department. Last year, 133 mosquito samples and 63 chickens tested positive for West Nile in Riverside County.
In California this year, the virus has been found in 30 counties, including one human case. Anyone showing symptoms is asked to contact their health care provider.