The investigation into the E. coli outbreak in McDonald's Quarter Pounders that has killed at least one person and sickened nearly 50 others increasingly points to the slivered onions served on the hamburgers.
But neither the company nor public health officials have said publicly where the onions were grown or whether they were sent to other restaurants, as well.
The onions in question, according to a McDonald’s spokesperson, come from a single source and are then sliced and packaged as raw vegetables in individual bags and sent to restaurants.
"It's a raw onion process at a facility and then sent to McDonald's," the spokesperson said.
McDonald's has pulled the sliced onions and quarter-pound beef patties, both used for the Quarter Pounder burgers, from its menu in affected areas.
The strain of E. coli in the outbreak, called O157:H7, produces a powerful toxin that can damage the lining of the small intestine, according to the Mayo Clinic.
If onions are confirmed as the source, it would be the first time that strain has been implicated in an outbreak involving raw onions, the Centers for Disease Control and Prevention said.
As of Wednesday, 49 people have been sickened with E. coli infections linked to the outbreak. One person, an older adult, has died. Ten other people have been hospitalized, including a child who developed a kidney disease called hemolytic uremic syndrome.
One person, a Greeley, Colorado man, has sued McDonald's, claiming he got sick and had to be hospitalized.
Matt Wise, chief of the CDC's Outbreak Response and Prevention Branch, told NBC News that he expects the number of illnesses to rise in the coming days as reports of cases come in.
"We think that’s going to be driven by people who ate Quarter Pounders at McDonald’s" before it pulled them from the menu.
"For someone walking into a McDonald’s today," he said, any risk is greatly reduced.
The majority of people who have fallen ill, 26, are in Colorado, including 18 in Mesa County. The older adult who died lived in Mesa County.
Six of the Colorado patients are teenagers, said Dr. Rachel Herlihy, the state epidemiologist.
Nebraska has reported nine cases. Other patients have been reported in Iowa, Kansas, Missouri, Montana, Oregon, Utah, Wisconsin and Wyoming.
The first case linked to a Quarter Pounder occurred in Colorado on Sept. 27. State health officials alerted the CDC about an unusual uptick in E. coli cases on Oct. 10.
“We were seeing a greater-than-expected number of cases,” Herlihy said. “A number of them had eaten at McDonald’s. That’s what really started to put the story together for us.”
The CDC ramped up its investigation on Oct. 15.
Lowell Schiller, a former official of the Food and Drug Administration, said it's not the first time the health agencies have faced an outbreak at a major restaurant chain, citing the E. coli outbreak at Chipotle in 2015.
An outstanding question, Schiller said, is whether the onions were supplied solely to McDonald's or go to other major restaurant chains — which, he said, the FDA is also likely to be actively thinking about.
What's more, if the contamination came from a farm, which Schiller said is most likely, other produce could have been affected.
"If it was going to other supermarkets or restaurants, there could be more future recalls that come out of this," he said.
Neither state nor federal public health officials named a specific farm or supplier of the sliced onions Wednesday, though McDonald's said they are zeroing on a single supplier that serves three distribution centers. McDonald's declined to name the supplier.
There is no indication that diced onions on other McDonald’s menu items are part of the outbreak.
The Food Safety and Inspection Service, an agency of the Agriculture Department, which oversees meat, poultry and egg products, is also working with the FDA and the CDC on the investigation.
An FSIS spokesperson said contamination from the quarter-pound beef patties is “very unlikely,” citing a traceback effort by the FDA, which “strongly points” back to the onions.
"While the slivered onions are the likely source, FSIS continues to verify the safety of the ground beef used," the spokesperson said. "A state partner has collected samples from the ground beef patties for testing."
A spokesperson for the FDA declined to comment Wednesday, saying the outbreak is “still a rapidly evolving situation.”
The FDA said in a statement Tuesday that symptoms can include severe stomach cramps, diarrhea, fever, nausea and/or vomiting, which can begin anywhere from a few days after contaminated food is consumed or up to nine days later. Some infections can cause severe bloody diarrhea and lead to life-threatening conditions, such as hemolytic uremic syndrome, a type of kidney failure.
