One person died and 10 were hospitalized with E. coli infections tied to McDonald's Quarter Pounder hamburgers, the Centers for Disease Control and Prevention said Tuesday.
The outbreak began Sept. 27 and has sickened a total of 49 people in 10 states in the Midwest and Western U.S through Oct. 11, according to the CDC.
But more recent illnesses may not have been reported and the true number is likely much higher, the CDC said.
"This is a fast-moving outbreak investigation," the CDC said. "McDonald's has pulled ingredients for these burgers, and they won't be available for sale in some states."
Infections have been reported in Colorado, Iowa, Kansas, Missouri, Montana, Nebraska, Oregon, Utah, Wisconsin and Wyoming.
Most of the cases are in Colorado, which has 27, followed by Nebraska, with nine.
McDonald's stock plunged nearly 10% in after-hours trading, to a low of $284.48 a share, following the CDC's announcement.
In a statement, the company said that initial findings suggested some cases were tied to onions from a single supplier and that it had had stopped distributing the slivered onions, according to the Associated Press.
It's also stopped selling Quarter Pounders in the affected states and parts of Idaho, Nevada, New Mexico and Oklahoma.
"We take food safety extremely seriously and it's the right thing to do," the statement said.
Those infected range in age from from 23 to 88 and the person who died was an older adult in Colorado, the CDC said.
E. coli bacteria are found in the intestines of people and animals, and can be spread through contaminated food and water, according to the CDC.
Some strains can cause diarrhea, urinary tract infections, pneumonia, sepsis and other illnesses including hemolytic uremic syndrome, or HUS, a serious condition that can lead to kidney failure, permanent health problems or death.
One of the people who was hospitalized developed HUS but isn't the person who died, the CDC said.
Eighteen people were interviewed by investigators and all said they had eaten at a McDonald's restaurant, with 16 saying they ate a hamburger and 12 saying it was a Quarter Pounder, the CDC said.
Symptoms of E. coli infection usually take three to four days to develop and include severe stomach cramps, diarrhea and vomiting.
The CDC advised anyone who's eaten a Quarter Pounder to call their healthcare provider if they suffer diarrhea and a fever higher than 102° F, vomiting that prevents them from keeping liquids down or exhibit signs of dehydration such as a dry mouth and throat, lack of urination or dizziness when standing.
