CDC says slivered onions are likely cause of McDonald's E. coli infections as cases rise to 90

The number of E. coli cases linked to McDonald's Quarter Pounders has risen to 90, the Centers for Disease Control and Prevention said Wednesday, up from 75 reported cases last Friday.

The slivered onions used on the burgers are the "likely source" of the outbreak, the CDC said.

A spokesperson for McDonald's said Tuesday that Quarter Pounders in affected states had been pulled from the restaurants by Oct. 22, when the outbreak was first announced. The CDC said that any further risk to the public is "very low."

On Sunday, Colorado Agriculture Department officials ruled out the beef patties used in the Quarter Pounders as the outbreak’s source after testing found no traces of E. coli.

One person has died in the outbreak, and 27 have been hospitalized so far, the CDC said. At least two people developed a rare kidney illness called hemolytic uremic syndrome, including a 15-year-old girl in Grand Junction, Colorado.

The high school freshman's initial symptoms — fever and stomach pain — began just days after she went to her local McDonald's several times for a Quarter Pounder. She experienced vomiting and diarrhea with blood.

She's since been hospitalized with kidney damage and has had to undergo multiple rounds of dialysis.

The true number of cases linked to the Quarter Pounders is likely much higher than the 90 reported so far. Most people with E. coli infections are never tested for the bacteria and recover on their own.

Thirteen states have reported cases: Colorado, Iowa, Kansas, Michigan, Missouri, Nebraska, New Mexico, Montana, Oregon, Utah, Washington, Wisconsin and Wyoming.

The Food and Drug Administration has also said that the slivered onions, supplied by food producer Taylor Farms Colorado Springs, are the most likely source of contamination.

According to McDonald's, 900 restaurants across 12 states received the onions from Taylor Farms.

The restaurant chain said it stopped sourcing onions from the company indefinitely on Friday. It'll start selling the Quarter Pounder without onions in affected stores this week.

Other restaurant chains in Colorado, including Taco Bell, Burger King, KFC, Pizza Hut and Illegal Pete’s, removed onions from their menus out of an abundance of caution. There are no signs of people getting E. coli after eating at those restaurants.

This story first appeared on NBCNews.com. More from NBC News:

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