One person has died and dozens were sickened from E. coli infections linked to McDonald’s Quarter Pounder hamburgers in 10 states, the U.S. Centers for Disease Control said on Tuesday.

Shares of the company were down about six per cent in extended trading after the CDC said 49 people were sickened by the E. coli O157:H7 strain, which can cause serious illness and is perhaps most often associated with a 1993 outbreak that killed four children who ate undercooked hamburgers at Jack in the Box restaurants.

Everyone interviewed has reported eating at McDonald’s before their illness started, and most mentioned eating a Quarter Pounder hamburger, according to the CDC.

The specific ingredient linked to the illness has not yet been identified but investigators are focused on fresh, slivered onions and fresh beef patties, the CDC said.

Most of the illnesses were reported in Colorado and Nebraska.

“The initial findings from the investigation indicate that a subset of illnesses may be linked to slivered onions used in the Quarter Pounder and sourced by a single supplier that serves three distribution centres,” Cesar Piña, McDonald’s North America chief supply chain officer, said in a statement.

McDonald’s removes slivered onions, beef patties

McDonald’s is temporarily removing the Quarter Pounder from restaurants in the impacted area, including Colorado, Kansas, Utah and Wyoming, it said in a statement, adding it was working with suppliers to replenish supply in the coming week.

Top U.S. food safety attorney Bill Marler, who represented a victim in the Jack in the Box outbreak, said onions are the most likely source of contamination and have been linked to prior E. coli O157:H7 outbreaks.

Beef seems unlikely, because those outbreaks have become uncommon due to food safety measures, said Marler, a founder of Marler Clark in Seattle.

“You’d have to have multiple restaurants undercooking the meat,” said Marler, who expects to see more cases reported in the outbreak.

A spokesperson for McDonald’s Canada said the E.coli concern does not extend to the restaurant’s Canadian locations.

“We want to reassure our Canadian guests that the situation affecting some McDonald’s restaurants in certain U.S. states does not impact our Canadian restaurants, or menu items,” the spokesperson said in an email to CBC News. 

“Across the McDonald’s system, serving customers safely in every single restaurant, each and every day, is our top priority and something we’ll never compromise on.

Symptoms for E. coli include severe stomach cramps, diarrhea and vomiting.

In 2015, burrito chain Chipotle saw its sales battered and reputation hit due to E.coli outbreaks in several states. That outbreak was linked to the E. coli O26 strain that usually causes less severe illness than E. coli O157:H7.