A severe E. coli outbreak tied to McDonald’s Corp.’s Quarter Pounders sickened dozens of people in the U.S., mainly in Colorado and Nebraska, and killed one, the U.S. Centers for Disease Control and Prevention said. 

Ten people have been hospitalized, including a child with complications from hemolytic uremic syndrome that damages small blood vessels and can lead to deadly clots, the agency said. All reported eating at McDonald’s before they fell ill, and most specifically mentioned having a Quarter Pounder, the agency said.

The restaurant chain’s shares dropped more than 10% in postmarket trading before paring losses.

McDonald’s said it was taking “swift and decisive action” to control the outbreak. It removed the Quarter Pounder from restaurants in the affected areas and instructed all local restaurants to remove the slivered onions that it identified as the likely culprit from their supply chains.

“We are working in close partnership with our suppliers to replenish supply for the Quarter Pounder in the coming weeks,” the company said in a statement. All of its other menu items remain available, it said.

E. coli is a bacteria that’s normally found in the intestines of people or animals, and is often harmless. The latest outbreak was caused by a virulent form, and all the infections were tied to the same strain: E. coli O157:H7. Symptoms typically start three to four days after swallowing the bacteria and can include severe stomach cramps, diarrhea and vomiting.

Major outbreaks can haunt restaurant chains for years. In 2015, an E. coli and norovirus outbreak across Chipotle Mexican Grill locations in multiple states led to a drastic drop in sales, as well as fines.

Contaminated food

It can be spread through contact with contaminated foods, often raw or undercooked meat. It can also be transmitted via fruits and vegetables, which may be contaminated via tainted irrigation water. Cooking typically kills the bacteria.

While the infections occurred in people from more than 10 states, 27 were from Colorado and nine were from Nebraska.

The CDC said the specific ingredient causing the outbreak hadn’t been identified and investigators were focused on two ingredients: fresh, slivered onions and fresh beef patties. McDonald’s told the CDC it removed both those ingredients for Quarter Pounders from stores in the affected states while the investigation continues.

A preliminary analysis by the U.S. Food and Drug Administration suggested the slivered onions were a likely source of the contamination, the agency said. It’s working to confirm the findings and determine if the onions were served or sold at other businesses.

—Bloomberg News