One person has died and dozens have been infected with E. coli after eating a McDonald's Royal Cheeseburger in 10 states, the U.S. Centers for Disease Control and Prevention said Tuesday.
An outbreak linked to one of McDonald's most popular menu items has sickened 49 people and hospitalized 10, officials said.
The strain of E. coli O157:H7 that was identified can cause serious illness and was the source of an outbreak in 1993 that killed four children after eating undercooked hamburgers at Jack in the Box restaurants.
Shares in the world's largest fast-food chain fell nearly 6% in after-hours trading. A livestock trader said the outbreak could also weigh on U.S. cattle futures on Wednesday, threatening beef demand.
All those interviewed for the investigation reported eating at McDonald's before becoming sick, and most mentioned the Royal Cheeseburger, according to the CDC.
The specific ingredient linked to the illness has not been identified, but investigators are focusing on chopped fresh onions and beef patties.
Most of those infected are residents of the states of Colorado and Nebraska.
"Initial findings from the investigation indicate that some of the illnesses may be linked to sliced onions used in the Royal Cheeseburger, which come from a single supplier that serves three distribution centers," said McDonald's North America chief supply chain officer Cesar Piña.
McDonald's has already removed sliced onions and beef patties used to make Royal Cheeseburgers from stores in affected states while the investigation continues, the CDC said.