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WASHINGTON (CNN) -- Investigators combed through farm fields in Monterey County, California, on Tuesday, searching for the source of an outbreak of illness linked to the consumption of raw spinach.
A federal health official said Tuesday that the disease has affected 131 people in 21 states.
That's up from 114 cases in 21 states on Monday, Dr. David Acheson, chief medical officer for the Food and Drug Administration's Center for Food Safety and Applied Nutrition, told reporters in a conference call.
A 22nd state, Colorado, also reported a confirmed case, which the FDA had not yet added to its list. (Watch why the scare is widening -- 2:09)
"There are no confirmed positives, but it's too early to have those yet," Acheson said.
Typically, about 25 percent to 30 percent of cases of E. coli infection require hospitalization. In the current outbreak, 66 of the 131 people infected by the 0157:H7 strain -- more than half -- have been hospitalized, and 20 of them have hemolytic uremic syndrome, a form of kidney failure, he said.
That could indicate the strain involved is particularly virulent, or it could mean simply that people with milder forms of the disease did not require medical attention and were not included in the case count, he said.
About three quarters of the cases in the outbreak are female, and two-thirds of the cases are among people ages 20-64.
Acheson said three to four more days would elapse before the field tests would be able to confirm any positives on the nine farms initially tested, but he acknowledged that authorities may never find the source. (Watch as the CDC works to pinpoint the E. coli source -- 2:02)
The farms all had contracted with Natural Selection Foods LLC, of San Juan Bautista, California, which last week issued a recall of its products containing spinach in brands packed with "Best if Used by Dates" of August 17, 2006 through October 1, 2006.
A second recall, also announced last week, covered products containing spinach in brands sold by River Ranch, of Salinas, California. That company obtained its spinach from Natural Selection.
The FDA has not changed its recommendation that consumers steer clear of any raw spinach until they have been able to pinpoint the source.
The ban has hit hard in Monterey County, where nearly 125,000 tons of spinach were harvested from 10,451 acres of farmland in 2005.
Acheson said he hoped, within 48 hours, to limit the search to a specific region in California "to see if we can get spinach not implicated back on the market in a safe way."
"It's critical that we get our arms around this as fast as we can, and we're really trying to do that," he said.
The states involved in the outbreak and the number of cases are California (1), Connecticut (2), Idaho (5), Illinois (1), Indiana (8), Kentucky (6), Maine (2), Michigan (4), Minnesota (2), Nebraska (6), Nevada (5), New Mexico (2), New York (9), Ohio (15), Oregon (5), Pennsylvania (6), Utah (16), Virginia (1), Washington State (2), Wisconsin (32) and Wyoming (1).
The first case occurred Aug. 2; onset of the most recent case occurred Sept. 9, Acheson said.
Symptoms of E. coli infection include diarrhea and vomiting, and the bacteria can lead to more severe complications, including anemia and kidney failure, particularly in the very young or elderly. Given that the maximum incubation period from time of exposure until symptoms is about a week, "you can figure that, by the end of this week, anybody who is going to get sick will have got sick," he said. (Watch why some fear the food supply is vulnerable -- 2:34)
Federal health official have advised the public not to eat fresh spinach. No cases have been linked to frozen or canned spinach.
Possible sources of contamination include water, agricultural practices, improper refrigeration, inadequate washing or any combination of those, he said.
FACT BOXNatural Selection Foods has recalled all packages of its fresh spinach and any salad with spinach in a blend because they are possibly contaminated with E. coli. The affected packages have "Best if Used by Dates" of August 17 through October 1. Consumers with questions can contact the company at 800-690-3200.
The recalled brands are:
Fresh and Easy
Mills Family Farm
Natural Selection Foods
Pride of San Juan
Tanimura & Antle
The Farmer's Market
Source: Natural Selection Foods LLC