What to know about the deadly E. coli outbreak at McDonald’s: Are the onions or the beef burgers to blame?

Dozens of people in the U.S. have contracted E. coli traced to ingredients in McDonald’s Quarter Pounders, the Centers for Disease Control and Prevention (CDC) said Tuesday.

As a result, McDonald’s announced that it has proactively removed two ingredients from stores in two affected regions, but the company’s leadership team has assured that most other menu items are not affected, according to the CDC investigation.

What to know about the E. coli outbreak at McDonald’s

McDonald’s says fresh sliced ​​onions or beef patties used in the Quarter Pounder may be behind the outbreak.

If onions are found to be the source of the E. coli outbreak, it would be the first time that onions have been found to carry this specific strain of the bacteria, company spokespeople said Wednesday.

Spokespeople also reiterated that the onions used in Quarter Pounders, as identified by CDC tracking data, came from suppliers that also test for E. coli.

McDonald’s uses several suppliers of sliced ​​onions in the area where the outbreak occurred, but all product has been removed from stock and will no longer supply onions until the investigation is complete.

States affected by McDonald’s Quarter Pounder E. coli outbreak

So far, one person has died and a total of 49 people from 10 states have fallen ill with a strain known as E. coli O157:H7, which causes a serious intestinal infection in humans.

The person who died was a resident of Mesa County, Colorado, in the western part of the state, according to the county health department.

The majority of cases have occurred in Colorado, which has 27 reported cases, and Nebraska, which has nine reported cases, according to the CDC.

The other states are Iowa, Kansas, Missouri, Montana, Oregon, Utah, Wisconsin and Wyoming.

Ten people were hospitalized, including a child with complications from hemolytic uremic syndrome (HUS) – a disease that affects the kidneys.

Number of sick people in outbreak expected to grow

The number of confirmed cases linked to the E. coli outbreak at McDonald’s could increase.

The CDC says the investigation is “moving rapidly,” with new cases being reported on an ongoing basis as the investigation links the people who were declared ill and the specific strain of E. coli in this outbreak.

McDonald’s stopped selling Quarter Pounders in select states recently, so there may be more cases developing. The typical onset of symptoms is 3 to 4 days, and recently sick people may not be part of the current report as it typically takes 3 to 4 weeks to determine if a sick person is part of an outbreak.

The true number of people sick in this outbreak is likely much higher than the number reported because many people recover without medical care and are not tested for E. coli.

Investigation into McDonald’s Ingredients Potentially Responsible for Disease

All of the patients interviewed by the CDC said they ate at McDonald’s before getting sick, and most said they ate Quarter Pounder burgers, specifically.

It’s unclear which ingredient is responsible for the illnesses, but CDC investigators are focusing on two ingredients: fresh sliced ​​onions and fresh beef patties.

“Heat destroys things like bacteria and viruses, but in certain cases, especially when producing large quantities of food – and if you have products that are not going to be subjected to heat, for example vegetables – this leads to an opportunity for contamination,” explained ABC News medical correspondent Dr. Darien Sutton.

McDonald’s told the CDC it has removed sliced ​​onions and patties used in Quarter Pounder burgers from stores in states where cases have been reported, the federal health agency said. As a result, the CDC said Quarter Pounders will not be available for sale in some states.

In a statement Tuesday night, McDonald’s North America supply chain director Cesar Piña said the fast food chain believes the outbreak is linked to chipped onions “used in the Quarter Pounder and supplied by a single supplier that serves three distribution centers”.

“As a result, and in accordance with our safety protocols, all local restaurants have been instructed to remove this product from their supply and we have suspended distribution of all sliced ​​onions in the impacted area,” the statement read, in part. “We will continue to work with the Centers for Disease Control and Prevention and are committed to providing timely updates as we restore our full menu.”

“The decision to do this is not something we took lightly and was made in close consultation with the CDC,” Joe Erlinger, president of McDonald’s USA, said in a statement. “It is important to note that most states and most menu items are not affected.”

Sliced ​​onions may be linked to E. coli strain for the 1st time

McDonald’s said it is actively working with the CDC, the U.S. Food and Drug Administration (FDA) and other health authorities as they investigate.

During the two-week period in which the cases were reported, from Sept. 27 to Oct. 11, the fast food chain sold about 1 million Quarter Pounders, complicating tracking the outbreak, according to company spokespeople.

Because the contamination was not linked to a single restaurant, this likely means the outbreak was not caused by food preparation and handling and likely occurred higher up the chain, the spokespeople said.

If beef burgers are behind the contamination, it would mean that the restaurants did not cook the food properly to the 175F standard. However, the chipped onion topping, which is not cooked, is processed in a facility and then sent to McDonald’s.

Company spokespeople said that if chipped onions are identified as the culprit, it would be the first time that onions have been found to carry the O157:H7 strain of E. coli.

McDonald’s said it believes only Quarter Pounders were affected and that the number of cases would be much higher if other products were linked to the outbreak.

Currently, a fifth of McDonald’s stores do not sell Quarter Pounders. Customers at local restaurants know if their location doesn’t serve Quarter Pounders, and if not, the product is available, spokespeople said.

What to Know About E. Coli Symptoms, Recovery

While most E. coli bacteria are harmless and part of a healthy intestinal tract, some strains of the bacteria can make people sick, according to the CDC.

Symptoms of E. coli usually begin three to four days after ingesting the bacteria and include severe stomach cramps, diarrhea that may be bloody, and vomiting.

Most people recover on their own within five to seven days, but some people may develop HUS and require hospitalization.

The CDC urges those who have severe E. coli symptoms and recently ate a McDonald’s Quarter Pounder to contact their doctor immediately.