E. coli outbreak: A preliminary investigation into the E. coli outbreak in McDonald's Quarter Pounders that killed at least one person and sickened 49 others in the US points to the slivered onions served on the hamburgers, according to the Food and Drug Administration. However, neither the company nor public health officials have publicly informed where the onions were grown or whether they were sent to other restaurants, as well.

According to McDonalds, the onions were brought from a single source - then sliced and packaged as raw vegetables in individual bags and sent to restaurants. "It's a raw onion process at a facility and then sent to McDonald's," the spokesperson told CBS News.

The restaurant also said it has been searching for a new regional supplier of fresh onions. McDonalds has also removed sliced onions and quarter-pound beef patties - both used for the Quarter Pounder burgers, from its menu in affected areas.

McDonald's said it had worked closely with federal food safety regulators since late last week when it was alerted to the potential outbreak. The company said the scope of the problem and the popularity of its products have complicated efforts to identify the contamination source.

There are over 14,0000 McDonald's stores across the country which serve 1 million Quarter Pounders every two weeks in the affected area.

According to the officials, the strain of the E. coli outbreak – known as O157:H7, produces a powerful toxin that damages the lining of the small intestine, If onions get confirmed as the source of infection - it would be the first time that strain has been implicated in an outbreak involving raw onions, the Centres for Disease Control and Prevention said.

At least 49 people have been sickened with the deadly infections linked to the outbreak, which causes extreme nausea and vomiting, diarrhoea, and fevers. One person - an older adult, has died and ten others have been hospitalized, including a child who developed a kidney disease called hemolytic uremic syndrome.

According to news reports, the majority of those who have fallen ill - 26, are in Colorado – mostly Mesa County. Along with that, Nebraska reported nine cases. Other patients have been reported in Iowa, Kansas, Missouri, Montana, Oregon, Utah, Wisconsin, and Wyoming.

The first case linked to a Quarter Pounder occurred in Colorado late last month. State health officials alerted the CDC about an unusual uptick in E. coli cases on October 10.

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