LOS ANGELES — A Los Angeles County woman has tested positive for West Nile virus in what is believed to be the first case of a person contracting the illness in the western United States, health officials said Friday.
The results of further tests won’t be known for another week, but based on preliminary tests county health officials called it a probable case of locally acquired West Nile virus infection. The victims of four other confirmed cases in Oregon, Washington, Montana and Idaho contracted the virus in states east of the Rocky Mountains.
The diagnosis in Los Angeles surprised health officials because ongoing monitoring of chicken flocks, dead wild birds and mosquitoes had shown no trace of the virus in California.
“The virus’ arrival in California is anticipated, but unexpected at this time since it is not present in any contiguous states,” said Dr. Thomas Garthwaite, director and chief medical officer of the Los Angeles County Department of Health Services.
The unidentified woman had a mild case of meningitis, which is associated with the virus, in early August and later recovered.
She had not traveled outside the region for several months, county Department of Health Services spokeswoman Maria Iacobo said. Officials provided no further information about the woman, including her age or where in the county she lives.
Since West Nile was first detected in New York in 1999, the virus has been found in humans in 27 other states and the District of Columbia. While its push westward had been expected, until Friday there had been no confirmed cases of humans contracting the disease west of the Rocky Mountains.
Nationwide this year, there have been 854 confirmed human cases of the virus, including 43 deaths, according to the Centers for Disease Control and Prevention.
In the four other western cases of confirmed West Nile virus, the victims are thought to have been infected in states where the mosquito-borne virus is already known to be present.
In Salem, Ore., a Grand Rapids, Mich., woman also tested positive for the virus Friday, although state officials said she didn’t contract the disease in Oregon.
Heather Aldridge, 29, was in critical condition undergoing treatment for brain inflammation.
“She did not acquire the infection here in Oregon. She came from a place where we know that the West Nile virus is circulating quite a bit,” state epidemiologist Mel Kohn said.
Earlier, a Washington man was diagnosed with the virus, which health officials said he contracted in Louisiana.
In Idaho, officials said a 47-year-old man had been diagnosed with the virus but said they believe he contracted the disease during a recent trip to the East Coast. The man did not need to be hospitalized and is recovering well, health officials said.
Montana state epidemiologist Todd Damrow said a 23-year-old Sweet Grass County women began showing symptoms of West Nile after returning to Montana from Ohio. He said the woman is doing fine and was not admitted to a hospital.
West Nile virus is closely related to St. Louis encephalitis, already present in California and other western states. It typically causes flulike illness or no symptoms at all in humans. In rare cases, it can lead to deadly inflammation of the brain. The young and the elderly are especially at risk.
Officials estimate there could be 110,000 to 150,000 people who have been infected in the United States, most of whom will never suffer its effects or know they have the virus.
Infected mosquitoes carry the West Nile virus in their saliva and pass it on when they bite. Several mosquito species act as vectors for the virus, and more than 110 North American bird species can serve as hosts, according to the CDC.