Mosquitoes collected in Anaheim and Westminster have tested positive for Saint Louis encephalitis -- the first occurrence of SLE-positive mosquitoes in those cities in three decades, county officials announced Tuesday.

The mosquitoes were collected along Old Bosa Chica Road in Westminster and near Dale and Orange avenues in Anaheim on Thursday, according to Heather Hyland of the Orange County Mosquito and Vector Control District.

The last time any SLE-positive mosquitoes were found in those areas was 1987, according to the district. Mosquitoes testing positive for SLE were found in 2017 near the OCMVCD offices in Garden Grove, according to the agency.

Most people bitten by an SLE-infected mosquito do not become ill, but those who do will experience symptoms such as fever, headache, nausea, vomiting and fatigue. Older adults can experience inflammation of the brain, and in rare cases a person can sustain a long-term disability or die.

The last time anyone in Orange County was afflicted with SLE was in the fall of 1984. There were six positive samples of SLE in mosquitoes from 1984 through 1987 in Orange County.

Vector Control officials encouraged residents to focus on dumping any standing water around their homes to stymie breeding grounds for the insects.