US health authorities said Friday that the number of E. coli cases potentially linked to McDonald's (MCD) Quarter Pounder hamburgers increased to 75 across 13 US states as of Thursday, adding that the actual number of sick people could be "much higher."

Late Tuesday, the Centers for Disease Control and Prevention said the outbreak had resulted in 49 cases across 10 states, one death and 10 hospitalizations. E. coli is a group of bacteria commonly found in places like intestines of people and animals, and can make people sick with diarrhea and urinary tract infections, among other illnesses, according to information posted on the the agency's website.

As of Thursday, "of 61 people with information available, 22 have been hospitalized, and 2 people developed hemolytic uremic syndrome, a serious condition that can cause kidney failure," the CDC and the US Food and Drug Administration said in separate Friday statements.

All 42 people interviewed by public health officials said they ate at McDonald's in the week before they got sick, with 39 eating a beef hamburger, according to the CDC. Of 36 people who remember the specific beef hamburger they ate at the fast-food giant, 31 named Quarter Pounder.

"The true number of sick people in this outbreak is likely much higher than the number reported, and the outbreak may not be limited to the states with known illnesses," the CDC said.

Separately, the FDA said 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 onions and beef patties. The FDA said it's currently not aware of any cases or affected product outside of the US.

Taylor Farms, which supplied slivered onions for the impacted McDonald's locations, has recalled yellow onions sold to additional food service customers, the FDA said.

The Quarter Pounder burger will be temporarily unavailable for sale in some states as the investigation continues, according to the CDC. "Due to the product actions taken by both companies, CDC believes the risk to the public is very low," the agency said.

McDonald's shares were down 2.3% in Friday afternoon trade. The company declined to comment on the latest development, while Taylor Farms didn't respond to a request for comment.

"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 centers," McDonald's Chief Supply Chain Officer Cesar Pina said in a statement published on the company's website earlier in the week.

"As a result, and in line with our safety protocols, all local restaurants have been instructed to remove this product from their supply and we have paused the distribution of all slivered onions in the impacted area," Pina said at the time.

McDonald's is scheduled to report its third-quarter financial results Tuesday.