At least 49 people sickened after E. coli outbreak linked to McDonald’s quarter pounders, health officials say.

An E. coli outbreak linked to McDonald’s quarter pounder hamburgers has sickened dozens of people in the United States, including one fatally, health officials have said.

At least 49 people have fallen ill in 10 states, with most cases concentrated in Colorado and Nebraska, the US Centers for Disease Control and Prevention said on Tuesday.

Most of the people who fell ill reported eating the quarter pounder hamburger at McDonald’s before getting sick, the CDC said.

Health authorities have yet to determine the specific ingredient that may have caused the outbreak.

“McDonald’s is collaborating with investigation partners to determine what food ingredient in Quarter Pounders is making people sick,” the CDC said in a statement.

“McDonald’s stopped using fresh slivered onions and quarter pound beef patties in several states while the investigation is ongoing to identify the ingredient causing illness.”

McDonald’s shares plunged about 9 percent in after-hours trading.

McDonald’s USA President Joe Erlinger said that most states and most menu items were not affected.

“Food safety is so important to me and everyone at McDonald’s,” Erlinger said in a video statement.

“We’ve taken steps to proactively remove slivered onions, which are used in Quarter Pounders, in select states. We’ve also made the decision to temporarily remove the Quarter Pounder from restaurants in select states.”

“We will continue to update you on the actions we are taking,” Erlinger added.

“At McDonald’s you can count on us to do the right thing.”

The news follows a tough year for the fast-food giant.

In July, the Chicago-based chain reported its first drop in quarterly same-store sales in about four years as customers stayed away for reasons ranging from rising inflation to the brand’s perceived support for Israel amid its war in Gaza.

Chief executive officer Chris Kempczinski earlier this month warned that 2025 looked set to be “another challenging year” for the company as lower-income customers were expected to see their wallets stretched into next year.