Doctors say the West Nile virus, which has killed 43 people in the United States, may have reached the U.S. West Coast.
Doctors are treating a Los Angeles woman for aseptic meningitis, an irritation of the membrane that covers the brain. Preliminary tests suggest she has the West Nile virus, but tests to confirm its presence will take several days.
More than 800 people across the United States are known to have been infected, but this is the first probable case in the Western United States.
Los Angeles county health official Jonathan Fielding said doctors are unsure how the woman contracted the virus. "She has no travel history, does not remember being bitten by mosquitoes, has no other factor that would lead us to believe a specific source," he said.
The West Nile virus entered the United States three years ago. U.S. health officials say before that, there was record of it in the Western Hemisphere. The virus is found in Africa, West Asia and the Middle East, and is thought to be spread by mosquitoes that are infected by birds. The mosquitoes then transmit the disease to humans.
Health officials say four people who received donated organs from a Georgia car crash victim have also been infected, suggesting that the virus may also be transmitted in other ways.
In some patients, the West Nile virus causes no symptoms. Others experience fever, headaches and muscle pains. In the most serious cases, the virus can be deadly.
Officials at the U.S. Centers for Disease Control and Prevention say it appears that the West Nile virus is now permanently established in the Western Hemisphere. They say U.S. infections are likely to peak in coming weeks, but will drop off as winter approaches and the weather gets colder.