• A deadly E. coli outbreak linked to McDonald's Quarter Pounders has led to 75 cases in 13 states, the Centers for Disease Control and Prevention said Friday.
• It is the latest tally of people affected since the agency announced the outbreak on Tuesday.
• Health officials are closely examining the slivered onions used in the Quarter Pounders as a likely contaminant.

A deadly E. coli outbreak linked to McDonald's Quarter Pounders has led to 75 cases in 13 states, the Centers for Disease Control and Prevention said Friday, as it investigates the source of the spread. 

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

Out of 61 patients with information available, 22 have been hospitalized and two people have developed a serious condition that can cause kidney failure, called hemolytic uremic syndrome. All of the 42 people who were interviewed by the CDC reported eating at McDonald's, while 39 people reported eating a beef hamburger, the agency said.

Those with infections ranged between ages 13 and 88, according to the CDC. The agency reiterated that the number of cases in the outbreak is likely much higher than what has been reported so far. The CDC added that the outbreak may not be limited to the states with related cases. That is because many patients do not test for E. coli and recover from an infection without receiving medical care, the CDC said. It also usually takes three to four weeks to determine if a sick person is part of an outbreak.

Shares of the restaurant chain closed down 3% on Friday. The stock has fallen 7% since the CDC announced the outbreak on Tuesday, initially citing 49 cases and one death across 10 states.

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

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

Health officials are closely examining the slivered onions used in the Quarter Pounder as a likely contaminant. McDonald's has instructed restaurants in the affected area to remove slivered onions from their supply, and has paused the distribution of that ingredient in the region.

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

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

But federal agencies are also investigating the Quarter Pounder's beef patty as a potential culprit.

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

McDonald's spokespeople said Wednesday that it is too early to tell if the outbreak is having any effect on traffic to its restaurants.

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

The outbreak comes after several quarters of sluggish U.S. sales for McDonald's. Price-sensitive consumers have not been visiting restaurants as much, leading McDonald's and other fast-food chains to turn to value meals to boost sales. Wall Street analysts are expecting the company to report U.S. same-store sales growth of 0.5% for the third quarter, according to StreetAccount estimates.

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, the damage to its brand may be minimal, as with an E. coli outbreak linked to Wendy's two years ago.