McDonald's shares fall after CDC says E. coli outbreak linked to Quarter Pounders

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 from Sept. 27 to 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. 

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's 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. 

Investigators are working to determine which ingredient may be contaminated, and McDonald's has withdrawn ingredients for the burgers, the CDC said. The agency said early information from the Food and Drug Administration shows that onions may be a source of the outbreak.

Quarter Pounder hamburgers will be temporarily unavailable in some states, but the CDC did not clarify which ones. 

McDonald's shares dropped about 9% in after-hours trading Tuesday. The company did not immediately respond to a request for comment on Tuesday.

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.

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.

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