A farmworker carries heads of romaine lettuce. (Gregory Bull/Associated Press)

The largest outbreak of E. coli in more than a decade is over, federal authorities said Thursday, after five people died and more than 200 others were sickened in three dozen states.

Although investigators determined that the E. coli came from contaminated romaine lettuce grown in Arizona's Yuma region near the border with Southern California, the Food and Drug Administration has not been able to link the outbreak to one farm, processor or distributor. New evidence showed bacteria taken from several canal water samples in the Yuma growing region to be a genetic match to the strain of bacteria that caused the outbreak, according to a statement from FDA Commissioner Scott Gottlieb on Thursday.

“More work needs to be done to determine just how and why this strain of E. coli 0157:H7 could have gotten into this body of water and how that led to contamination of romaine lettuce from multiple farms,” he said. FDA officials, who have been investigating the source of the outbreak since mid-March, are trying to determine whether canal water was used to irrigate the lettuce fields. The Yuma region — which includes farms across the Colorado River in southeastern California — grows the overwhelming majority of the lettuce and other leafy greens consumed in the United States in the winter months. The harvest season there has ended, and contaminated lettuce that made people sick in this outbreak should no longer be available, authorities said.

Bill Marler, a prominent food-safety lawyer who represents 105 patients sickened by the lettuce, said he was not surprised that the probable link was broad environmental contamination, such as by water, given the number of people sickened and the number of unnamed farms and fields implicated.

The illnesses began in mid-March. The latest tally from the Centers for Disease Control and Prevention says 96 people were hospitalized, including 27 who developed a type of severe kidney failure that can be life-threatening to people with weak immune systems, such as young children and the elderly. Single deaths were reported from Arkansas, California  and New York, and two people died in Minnesota. The latest reported illness started  June 6. Some people who became sick did not report eating romaine lettuce but had had close contact with someone who fell ill from eating it.

Authorities initially said that only bagged and pre-chopped romaine lettuce that had been distributed to retailers across the country was contaminated with E. coli, but a number of inmates at a prison in Alaska also became ill after eating whole-head lettuce.

The outbreak is the worst E. coli outbreak since 2006, when 205 people became ill and five died after contracting E. coli from baby spinach.

E. coli is a type of bacteria found in undercooked beef, raw milk, soft cheeses made from raw milk, raw fruits and vegetables, and contaminated water. The infection can also spread from person to person through germs on hands. Most E. coli bacteria is not harmful, but one type, known as E. coli O157: H7, produces a toxin called Shiga, which destroys red blood cells  and causes kidney failure and bloody diarrhea. Other kinds of E. coli cause urinary tract infections, respiratory illness and pneumonia.

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