• McDonald's shares fell in extended trading after the CDC said an E. coli outbreak was linked to the chain's Quarter Pounder burgers.
• The outbreak has led to 10 hospitalizations and one death, the CDC said.
• The restaurant chain said initial findings from the investigation show some of the illnesses may be linked to onions that are used in the Quarter Pounder.

McDonald's shares dropped in extended trading Tuesday after the Centers for Disease Control and Prevention said an E. coli outbreak linked to McDonald's Quarter Pounder burgers has led to 10 hospitalizations and one death.

The agency said 49 cases have been reported in 10 states between Sept. 27 and Oct. 11, with most of the illnesses in Colorado and Nebraska. "Most" sick people reported eating a McDonald's Quarter Pounder, the CDC added.

One of the patients developed hemolytic uremic syndrome, which is a serious condition that can cause kidney failure. An older adult in Colorado died. 

McDonald's shares dropped about 7% in after-hours trading Tuesday.

In a statement Tuesday, McDonald's said it is taking "swift and decisive action" following the E. Coli outbreak in certain states.

The company said initial findings from the ongoing investigation show that some of the illnesses may be linked to slivered onions — or fresh onions sliced into thin shapes — that are used in the Quarter Pounder and sourced by a single supplier that serves three distribution centers. McDonald's has instructed all local restaurants to remove slivered onions from their supply and has paused the distribution of that ingredient in the affected area.

Quarter Pounder hamburgers will be temporarily unavailable in several Western states, including Colorado, Kansas, Utah and Wyoming, and portions of other states, McDonald's said. It added that it was working with suppliers to replenish ingredients.

The majority of states and menu items are not affected by the outbreak, McDonald's USA President Joe Erlinger said in a video. The company's other beef products, including the cheeseburger, hamburger, Big Mac, McDouble and the double cheeseburger, are not affected, he added. Those sandwiches use a different type of onion product.

"We are working quickly to return our full menu in these states as soon as possible," Erlinger said. "I hope these steps demonstrate McDonald's commitment to food safety."

Quarter Pounder hamburgers are a core menu item for McDonald's, raking in billions of dollars each year. In 2018, McDonald's launched fresh beef for its Quarter Pounders across most of its U.S. stores.

The CDC said the number of people affected by the outbreak is "likely much higher" than what has been reported so far. The agency said that is because many people recover from an E. coli infection without testing for it or receiving medical care. It also typically takes three to four weeks to determine if a sick patient is part of an outbreak, the CDC added. 

E. coli refers to a group of bacteria found in the gut of nearly all people and animals. But some strains of the bacteria can cause mild to severe illness if a person eats contaminated food or drinks polluted water.

Symptoms, including stomach cramps, diarrhea and vomiting, usually start three to four days after swallowing the bacteria, according to the CDC. Most people recover without treatment after five to seven days.

There have been several past reported cases of E. coli at McDonald's restaurants.

In 2022, at least six children developed symptoms consistent with E. coli poisoning after eating McDonald's' Chicken McNuggets Happy Meals in Ashland, Alabama. Four of the six children were admitted to a hospital after experiencing severe adverse effects.