How investigators quickly traced a deadly E. coli outbreak to McDonald’s Quarter Pounders

On Monday, October 7, Silas Mayes pulled into his local McDonald's drive-thru during lunch and ordered his regular: a Quarter Pounder, fries, and a Sprite.

By Thursday morning, he was wracked with such severe stomach cramps that he could barely stand to reach the bathroom.

"It was agonizing. And every time I went to the bathroom, there was blood," said Silas, 17, from Grand Junction, Colorado. "It was terrifying."

His mother, Lera Davidson, rushed him to St. Mary’s Regional Hospital after the bleeding continued. Doctors took a stool sample for testing, gave him IV fluids, and administered pain relief. He was kept in the ER most of the day before being sent home.

He couldn’t keep anything down. Even a small sip of water or a single cracker bite would send him running for the bathroom, with more blood pouring out.

His stool sample tested positive for a dangerous strain of E. coli that produces Shiga toxin, which damages cells and causes tissue destruction. A serious complication of such infections is hemolytic uremic syndrome, a condition that can lead to kidney failure and even death, particularly in young children and the elderly.

All 50 states mandate that labs report any cases of Shiga toxin-producing E. coli (STEC) to state health departments when detected.

When a foodborne illness is identified, a series of routine actions by local and state health officials follows. These steps usually happen quietly, behind the scenes. Without them, outbreaks may go unnoticed, and more people could fall ill or even die.

"We treat every case as if it could be part of an outbreak. We investigate each one with the goal of stopping disease spread and identifying the source," said Julie Hartshorn, a disease surveillance specialist at Mesa County Public Health in Grand Junction.

Scientists from the US Centers for Disease Control and Prevention, in collaboration with the Colorado Department of Public Health and Environment, helped investigate the recent E. coli outbreak and pinpoint its source. They emphasize that the swift identification and resolution of the cases is a significant achievement.

On October 22, the CDC and the US Food and Drug Administration announced that 90 cases of STEC infection across 13 states have been traced to McDonald's Quarter Pounders. More than 20 people have been hospitalized, and one person has died.

Dr. Heather Carleton-Romer, chief of the CDC's Enteric Diseases Laboratory Branch in Atlanta, notes that only about 10% of the 6,000 E. coli infections genetically sequenced in the US each year are tied to outbreaks.

"I believe I’m in one of the rare fields where I can go home and say my work saves lives, because it truly does," she said.

The race to uncover the source of the outbreak intensifies

By the time the report on Silas’ case reached them, Hartshorn and two other staff members at Mesa County Public Health had been working non-stop, trying to figure out why so many people in Grand Junction were contracting STEC infections.

"Normally, we might see one or two cases of STEC a month, but this time we had an unusually high number. That immediately raised concerns," said Hartshorn.

Mesa County, one of the outbreak’s initial hotspots, has reported 11 cases linked to the outbreak. However, Hartshorn mentioned that more people are being investigated. Some of these cases won't be officially confirmed until their stool samples undergo genetic sequencing at the state lab, which can take some time.

Even in the best of circumstances, it’s challenging for people to remember all the details of what they’ve eaten. That’s why health departments move quickly to interview people while their memories are still fresh.

After receiving a report of a foodborne illness, a specialist—usually someone trained in conducting detailed interviews—reaches out to the sick individual. They ask about everything the person ate over the past week, including the ingredients. They also inquire about recent travel and any contact with animals, even asking about the types of treats fed to pets.

The Mesa County team began reaching out to individuals with STEC infections during the week of October 7, roughly three weeks before the CDC officially launched the investigation. Within two days, it became clear that most of those they contacted had eaten at McDonald's. The health department sent its environmental health team to inspect the local restaurant, but they didn’t find any obvious issues.

"What we found was that the issue wasn’t with how the food was prepared at the restaurant," Hartshorn explained. Employees were properly washing their hands, food was cooked to the correct temperatures, and all surfaces were cleaned and disinfected as required.

Still, people were falling ill after eating there, which led them to suspect that the contamination might have occurred before the food reached the restaurant, Hartshorn said. "So, we thought this might be a larger issue."

The investigation expands

And they were right.

At the state level, the Colorado Department of Public Health and Environment was also overwhelmed with a surge of STEC reports from across the state.

In some cases, local health departments, like Mesa County, conduct their own foodborne illness interviews, while in other areas—both urban and rural—the state health department steps in to provide assistance.

Alayna Younger, an epidemiologist who leads the state agency's four-member Enteric Disease Interview Team, is in charge of assigning those interviews. She was the first to raise the alarm about the unusually high number of STEC cases, according to her supervisor, Rachel Jervis, who jokingly refers to herself as a 'scientist ... of diarrhea' on LinkedIn.

"I said, 'Can you compare this to past years? I need to see the numbers,'" said Jervis, who oversees the Foodborne, Enteric, Waterborne, and Wastewater Diseases Program at the Colorado health department.

By early October, the agency had already recorded as many STEC cases as it would typically expect for the entire month, Jervis reported.

On October 10, the same day Silas became ill, the Colorado health agency alerted the CDC, suspecting a STEC outbreak. They noted that many of the individuals being interviewed had mentioned eating fast food and ground beef.

Jervis explained that they also contacted neighboring states to see if they were experiencing similar spikes in cases. "Some reported similar trends, others did not," she said.

On October 11, the CDC received the first genetic sequences from Colorado via its PulseNet system. The DNA profiles from the cases were strikingly similar, differing by only one gene, signaling a clear link between the cases, according to Jervis.

PulseNet is a system that compiles and analyzes over 60,000 genomes from individuals who suffer from foodborne illnesses each year. Each genome holds the complete genetic blueprint for the bacteria or virus responsible for the infection, and together, they form a vast reservoir of information. Every month, PulseNet processes enough data to fill the Library of Congress.

"Imagine assembling a 5,000-piece puzzle, and then comparing pieces from different cases to spot even the smallest difference, because that’s what helps us solve the puzzle," Carleton-Romer explained.

Traditional methods crack the case

Typically, the scientists overseeing PulseNet are the first to identify when cases in the system align to form an outbreak investigation.

However, in the case of the STEC outbreak linked to McDonald's Quarter Pounders, it was Younger and her team of epidemiologists in Colorado, working tirelessly on the phones, who first made the connection between the cases—a testament to the effectiveness of traditional epidemiological methods.

This style of case investigation dates back to the mid-1800s, when Dr. John Snow, often regarded as the father of modern epidemiology, halted a cholera outbreak in London by tracing it to a contaminated water pump that was responsible for the illness.

As more E. coli genetic sequences were added to PulseNet, the CDC team noticed matching DNA fingerprints appearing in other states, suggesting that a widely distributed food source was causing the outbreak.

The CDC team collaborated with the Colorado health department to create a second round of questions, focusing specifically on fast-food restaurants, menu items, and the consumption of beef or onions. The follow-up questionnaire was 13 pages long.

"The best approach for these supplementary outbreak interviews is to limit the number of interviewers, as it helps to identify patterns more clearly," Jervis explained. "So, these extra interviews are centralized at the state health department."

Davidson tried to assist the Mesa County health workers in answering questions about Silas, as his condition had worsened, with bloody diarrhea occurring every minute or so. He was too ill to help much. However, during her initial interview, she wasn’t aware that her son had eaten at McDonald's.

"You’re trying to remember everything you’ve eaten over the past week," Davidson said. "I could recall the meals I had prepared, but figuring out where Silas had been with his girlfriend, eating out and such, was the real challenge."

When state investigators followed up with a supplemental interview on October 12, they asked Davidson to search for receipts for any ground beef purchases and to review bank statements for any clues that could help fill in the blanks.

It wasn’t until news reports revealed the connection to McDonald's that Silas remembered he had eaten lunch there.

Eventually, they found a transaction on the bank statement from October 7 that showed a purchase at McDonald's.

"Your memory is triggered when you’re asked for something specific," Jervis said. "We definitely had cases where people called us back and said, ‘You know what, after our conversation, I checked my credit card statement, and I went to McDonald’s on this date.’"

Even so, the official count is likely an underestimate. Many individuals who fell ill may not have been sick enough to visit a doctor, so they never got tested. Others might have been treated without undergoing a stool test.

McDonald’s reported that during any two-week period in the area where cases were identified, they sell over 1 million Quarter Pounders.

Because the majority of sick individuals had eaten Quarter Pounders, investigators were able to inquire about what made this specific menu item stand out from the rest. It turns out that Quarter Pounders are made with fresh beef that’s cooked to order, along with freshly sliced onions. The only other item on the menu that used slivered onions was a breakfast item, McDonald's said.

McDonald’s temporarily halted the sale of Quarter Pounders in about one-fifth of its locations until tests confirmed that the beef patties were not the source of the E. coli contamination.

Taylor Farms, the supplier of the fresh onions, recalled batches sent to McDonald’s and other restaurants and institutions. Ongoing testing of the onions led the CDC and FDA to announce on Thursday that they were the likely source of the E. coli outbreak that caused the illnesses.

Thanks to the swift narrowing of the investigation, companies were able to swiftly pull the suspected ingredients from their restaurants. Carleton-Romer stated that they do not expect any new cases, although the investigation may expand as lab tests link historical cases to the outbreak strain.

Lasting consequences

McDonald’s issued a statement saying, "The health and safety of our employees and customers is our top priority."

"Although McDonald’s removed all slivered onions from this facility as of October 22, 2024, in response to widespread concern and our strong commitment to food safety, we have decided to halt sourcing onions from Taylor Farms’ Colorado Springs facility indefinitely," the statement read.

"At McDonald’s, we will never compromise on food safety. Customers can trust McDonald’s to take the right actions, and public health authorities can count on our continued close collaboration. We appreciate all the efforts of health authorities," the statement concluded.

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Silas’ family has enlisted attorney Ron Simon to represent them in a lawsuit against the fast-food chain.

Although they are certain that Silas ate at McDonald’s and was diagnosed with E. coli, they are unsure whether his case is officially tied to the outbreak. They have requested his lab test results from the state health department but have yet to receive them, according to Simon.

Silas is still recovering from the infection, having lost 10 pounds during his illness, which was significant for his already-thin frame. He continues to battle severe fatigue, requiring frequent breaks when exercising or working at his job installing tile. He describes it as a struggle to get through daily tasks.

Silas is no longer on IV fluids and has returned to work with his father. However, he says he won't be eating McDonald’s or any other fast food for a long time.

'I don’t trust fast food anymore after how badly it made me sick,' he said.

Jervis, from the Colorado health department, said solving the case was professionally rewarding.

'It’s always this feeling of, 'Wow, this is fascinating. It’s so exciting. I really want to crack this,’' she said.

'But at the same time, these are real people who are becoming seriously ill,' she added.

In the coming weeks, she anticipates that more cases will be linked to the outbreak strain as additional test results come in.

'It’s difficult to look at the numbers and realize that behind each one is a person who has suffered, and their families have been affected,' she said.