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Mosquitoes found in Anaheim and Westminster have tested positive for a rare virus that can cause brain inflammation, officials announced Tuesday.

It’s the first time mosquito samples from the area have been confirmed to carry the St. Louis encephalitis virus  since 1987, according to the Orange County Mosquito and Vector Control District. The agency said the infected insects were collected from a residential area in Anaheim on Dale and Orange avenues and Old Bolsa Chica Road in Westminster.

Most people bitten by an infected mosquito show no signs of illness, but those who do get sick suffer from fever, headache, nausea, vomiting and fatigue about five to 15 days after the bite. Severe cases, which often involve inflammation of the brain and can lead to long-term disability or death, develop more commonly in older adults, officials said.

Authorities confirmed nine California cases of neuroinvasive disease caused by the SLE virus between 2009 and 2018, according to the Centers for Disease Control and Prevention. So far, O.C. officials have not reported any current cases of human infection in the county. Those who may be experiencing symptoms should consult their health care provider.

The O.C. Mosquito and Vector Control District planned to search for any additional breeding sources of mosquitoes in the areas. Meanwhile, authorities urged locals to take the following precautions:

  • Get rid of mosquito breeding sites by draining containers filled with water and cleaning bird baths and pet water bowls at least once weekly.
  • Dump water from plant saucers.
  • Before going outdoors, apply and reapply mosquito repellent containing DEET, Picaridin, IR3535 or oil of lemon eucalyptus.
  • Wear light-colored, long-sleeved shirts and and long pants.
  • Stop mosquitoes from entering indoor spaces by closing doors and windows that have no screens.
  • Repair damaged screens.

The discovery of SLE-positive mosquitoes came less than two weeks after Los Angeles County authorities reported the first deadly case of West Nile virus this year. Orange and L.A. counties had warned of finding mosquito samples infected with the virus, the top cause of mosquito-borne disease in the continental U.S.

The insects can transmit SLE and West Nile viruses to humans after feeding on infected birds.