CDC told McDonald's about potential E. coli outbreak late last week

The Centers for Disease Control and Prevention told McDonald's late last week about a potential link to an E. coli outbreak, company spokespeople said Wednesday.

At that time, the number of connected cases was smaller than it is now, though the company did not say how many cases there were then. As of Tuesday, the CDC has attributed 49 cases and one fatality across 10 states to the outbreak, which has been linked to McDonald's Quarter Pounder burgers.

Once notified about the link, McDonald's started working with the CDC, the U.S. Department of Agriculture and the U.S. Food and Drug Administration. By the time the CDC had issued its advisory notice Tuesday afternoon, McDonald's had already decided to pull the Quarter Pounder burgers from restaurants in the affected areas, the spokespeople said.

Roughly a fifth of McDonald's U.S. restaurants are not selling Quarter Pounder burgers at this time.

The CDC has interviewed 18 people with confirmed cases, as of Tuesday. Of those patients, 12 recalled eating a Quarter Pounder burger before falling ill.

The outbreak comes as McDonald's tries to win back diners who balked at years of price increases. It adds to the risks facing the company at a time when it hopes a $5 value meal deal will drive consumers back to its restaurants.

The fast-food chain issued a statement on the outbreak Tuesday evening, shortly after the CDC issued its advisory. Cesar Pina, the company's North American chief supply chain officer, said in the statement that the company is removing the Quarter Pounder from restaurants in the affected area, which includes Colorado, Kansas, Utah, Wyoming and parts of eight other states.

The CDC is investigating both the Quarter Pounder's uncooked onions and its beef patty as the potential culprit for the outbreak. However, McDonald's uses multiple beef suppliers in the region, and its burgers are supposed to be cooked to an internal temperature that would kill the bacteria.

That would leave the onions as the more likely contaminant. In that geography, McDonald's uses a single onion supplier, which washes and slices the vegetable. The company has paused its distribution of the ingredient and asked local restaurants to remove their onion supply.

Based on reported cases so far, the outbreak took place between Sept. 27 and Oct. 11. Over a two-week period, McDonald's typically sells roughly one million Quarter Pounders in the affected region, the company spokespeople said.

Shares of McDonald's are trading down 5% in afternoon trading, as investors worry that the outbreak could lead to a sales slump for the fast-food giant.