A deadly E. coli outbreak linked to McDonaldThe Quarter Pounders have led to 75 cases in 13 states, the Centers for Disease Control and Prevention said Friday as it investigated the source of the spread.

The outbreak has resulted in 22 hospitalizations and one previously reported death of an older adult in Colorado.

Of the 61 patients for whom information is available, 22 were hospitalized and two people developed a serious condition that can lead to kidney failure, called hemolytic uremic syndrome. All 42 people surveyed by the CDC said they had eaten at McDonald's, while 39 people said they had eaten a beef hamburger, the agency said.

According to the CDC, the people with infections ranged in age from 13 to 88. The agency reiterated that the number of cases in the outbreak is likely much higher than previously reported. The CDC added that the outbreak may not be limited to the states with corresponding cases. That's because many patients don't get tested for E. coli and recover from an infection without receiving medical care, the CDC said. Additionally, it typically takes three to four weeks to determine whether a sick person is part of an outbreak.

Shares of the restaurant chain fell 2% on the update. The stock has fallen 6% since the CDC announced the outbreak on Tuesday, initially citing 49 cases and one death in 10 states.

McDonald's declined to comment on the update, citing the company's statement when it first announced the outbreak.

Quarter Pounder hamburgers are a key menu item for McDonald's, bringing in billions of dollars annually.

Health officials are closely examining the onion slices used in Quarter Pounder as a likely contaminant. McDonald's has ordered restaurants in the affected region to remove sliced ​​onions from their offerings and has stopped selling the ingredient in the region.

McDonald's locations in Colorado, Kansas, Utah, Wyoming and parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma have temporarily stopped using Quarter Pounder slivered onion and beef patties, according to the CDC.

McDonald's identified California-based food giant Taylor Farms as a supplier of the sliced ​​onions that the company had removed from its supply chain. Taylor Farms has issued a recall for four raw onion products due to possible E. coli contamination. Burger King, Pizza Hut, KFC and Taco Bell have removed onions from select restaurants in response to the outbreak.

But federal authorities are also investigating the Quarter Pounders beef patty as a potential culprit.

As the CDC and other federal agencies track cases and work to contain the outbreak, McDonald's has pulled Quarter Pounders from restaurants in affected areas. About a fifth of McDonald's restaurants in the U.S. do not sell Quarter Pounder burgers.

McDonald's spokespeople said Wednesday that it was too early to say whether the outbreak would have any impact on traffic to its restaurants.

The company is expected to report its third-quarter results on Tuesday and could provide investors with more details about the situation on the conference call.

The outbreak comes after several quarters of sluggish U.S. sales at McDonald's. Price-sensitive consumers are no longer going to restaurants as often, leading McDonald's and other fast-food chains to turn to inexpensive meals to boost sales. According to StreetAccount estimates, Wall Street analysts expect the company to report third-quarter comparable sales growth of 0.5% in the United States.

For now, McDonald's is trying to reassure customers that its menu items are safe to eat and drink and that it is taking the outbreak seriously. Experts told CNBC that barring a more serious crisis, damage to the brand could be minimal, such as a related E. coli outbreak Wendy's two years ago.