A California woman has tested positive for the West Nile virus in what could be the first human case contracted in the western United States, health officials said Friday in announcing preliminary test results.

Conclusive results won't be known for another week. However, Los Angeles County health officials were calling it a "probable case of locally acquired West Nile virus infection." The case suggests that the virus may have completed its journey across the continent.

Nationwide this year, there have been 954 confirmed human cases of the virus, including 43 deaths, according to the Centers for Disease Control and Prevention.

The California woman, who was not identified, became sick in August from meningitis, which is associated with the virus. She has recovered.

She had not traveled outside the region, meaning the possible infection occurred locally, said Maria Iacobo, a spokeswoman for the county Department of Health Services.

The diagnosis surprised health officials because ongoing monitoring of 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 mosquito-borne virus appeared in the United States in 1999 in New York. So far though, no local cases have been confirmed west of the Rocky Mountains. Four confirmed cases in Oregon, Washington, Montana and Idaho contracted the virus in states where the virus was already present.

Human cases have been confirmed this year in 28 states and the District of Columbia.

West Nile is spread by infected mosquitos, which pass the virus to birds, horses or humans. Most people develop a mild flu-like illness, though the elderly and people with weak immune systems can get a potentially fatal brain inflammation.

The CDC confirmed this week that four people became sick from organ transplants. Health officials are investigating whether the organ donor and another woman contracted West Nile virus through blood donations.