GARDEN GROVE, Calif. (KABC) --A virus known as St. Louis encephalitis has been detected in Orange County mosquitoes for the first time in 30 years, officials said.
The virus was found in adult mosquito samples collected in Garden Grove in the back garden of the Orange County Mosquito and Vector Control District.
It was detected as part of a program that normally looks to monitor for mosquitoes carrying the more-common West Nile virus.
The symptoms of St. Louis encephalitis are similar to those of West Nile and both are transmitted the same way - through mosquitoes - though West Nile is considered deadlier as well as more common.
Most people infected with St. Louis encephalitis show no symptoms, but others could feel like they have the flu - experiencing fever, headaches, dizziness and nausea. Older adults tend to experience more severe symptoms.
Most people recover but in some cases the virus can be fatal, according to the U.S. Centers for Disease Control and Prevention. There is no known vaccine or specific medicinal treatment.
The last time a mosquito tested positive for St. Louis encephalitis in Orange County was 1986 and the last human case was 1984. Why it has returned isn't entirely clear.
Mosquitos can contract St. Louis encephalitis as well as West Nile after feeding on an infected bird.
Officials say when West Nile appeared in California in 2003, the presence of St.Louis encephalitis radically dropped. Birds appeared to develop an immunity after exposure to West Nile. So to have mosquitoes testing positive for both viruses in the same time period is unusual, according to vector control officials.
Vector control officials advise people to take precautions against mosquitoes by draining any standing water on their property, wearing insect repellent when outdoors and making sure windows and door screens are in good condition.