Updated: 4:02 PM

An outbreak of E. coli, believed to be linked to McDonald’s Quarter Pounders, has been felt particularly hard in Western Colorado, where more than a dozen residents have fallen ill and one person has died.

In a news release Wednesday and a subsequent interview, Mesa County Public Health noted that at least 10 of the state’s 26 confirmed cases of E. coli were counted in Mesa County, including the nation’s only fatality. The province is still waiting on laboratory genome results from several other sick people to confirm whether they also have E. coli in their systems.

The state health department suspects that all Colorado McDonald’s locations statewide received ingredients from the same suppliers, and therefore believes all of the chain’s locations in the state may have been affected.

The confirmed Colorado cases to date come from nine counties in the state: Arapahoe, Chaffee, El Paso, Gunnison, Larimer, Mesa, Routt, Teller and Weld. The infected people may not have eaten at a McDonald’s location in the county where they live, according to the agency, which said it expects the numbers to change as more cases are investigated.

Federal health data shows that the first case linked to the outbreak was recorded on September 27, and no new cases have been confirmed since October 11.

“The person who died was an older adult,” said Dr. Rachel Herlihy, state epidemiologist. “We know that older adults and young children are most susceptible to serious illness outcomes when it comes to these types of E. coli infections, and are more likely to develop complications from the infection.”

She said the eight hospitalizations occurred across a wide range of age groups, “but we have been fortunate in that we have not had any cases among young children in the state.”

She said the youngest cases were in their teenage years: “But we do know that the youngest children will be most susceptible to serious consequences. So it’s certainly a good thing that we didn’t see that.”

She said the health department began investigating because it saw cases of E. coli rising above the usual baseline level and began investigating, including asking questions of people reporting similar symptoms.

“We are conducting interviews with cases to identify any foods they may have eaten leading up to the period when they became ill,” she said. Researchers will ask questions about what those foods were, when they ate them and where they ate them “to get a sense of whether there are any possible links between these cases.”

The agency then matches that data with laboratory data, whole-genome sequencing or genetic fingerprinting data, giving it even more information and telling researchers whether “these bacteria are genetically related in a way that helps us further build a picture of a possible outbreak,” she said.

The outbreak was first reported Tuesday and is believed to be linked to onions served at the Quarter Pounder, Mesa County officials said in the release, citing the Food and Drug Administration. However, researchers are still investigating the possibility that beef patties were contaminated.

“Although the specific source of contamination remains under investigation, early information from the Food and Drug Administration indicates that the onions on McDonald’s Quarter Pounders may be a source of this outbreak. Fresh-cut onions are primarily used on Quarter Pounder burgers and not on other menus.” items,” Erin Minnerath, deputy director at Mesa County Public Health, said in the release. “USDA is also investigating the beef patties on the Quarter Pounder burgers.”

Mesa County Public Health is conducting interviews with residents to further identify the source of infection.

“We have local experts on hand with an efficient system to deal with these situations. This contributes to the speed of investigations to prevent even more people from becoming ill,” said Minnerath.

E. coli is a bacterium that occurs in many places in nature, especially in the intestinal bacteria of animals and humans. There are many different types, many of which are not harmful, and some are even theorized to be potentially beneficial as normal intestinal flora.

“However, there are certain strains of e.coli that are particularly pathogenic or likely to cause disease. And that’s what we’re seeing here with this outbreak at McDonald’s,” said Dr. Daniel Pastula, professor of neurology, infectious diseases and epidemiology at the University of Colorado School of Medicine and Colorado School of Public Health. “This has proven to be pathogenic or cause disease in the past.”

He said infections could prove to be serious in some cases, especially for children under five, people over 65 and people with weakened immune systems. It can cause gastroenteritis, nausea, vomiting and diarrhea.

“This is not the type of E. coli you want to introduce into your body,” he said.

The bacteria make toxins that can settle in your intestinal tract. That can cause some bleeding and bloody stools in severe cases, he said. Additionally, due to fluid loss through diarrhea and vomiting, it can cause severe dehydration. Sometimes a person is hospitalized with severe symptoms, and although rare, these types of infections can cause kidney failure.

“We think it’s caused by those toxins that cause a big inflammatory cascade in the smallest blood vessels and cause small blood clots. So in some cases this can be quite serious,” he said.

The species is often transmitted by food. Therefore, according to Pastula, it is crucial to cook meat at a high enough temperature, avoid cross-contamination of food during food preparation, avoid unpasteurized dairy products, wash hands and clean surfaces.

All researchers interviewed reported eating at McDonald’s before their illness started, and specifically said they had eaten a Quarter Pounder.

McDonald’s told the CDC that it was proactively removing the sliced ​​onions and beef patties used for the Quarter Pounder burgers from stores in the states reporting cases as the investigation continued. Quarter Pounder burgers may be temporarily unavailable in some states, the agency said.

According to the Cleveland Clinic, most strains of E.coli spread through fecal-oral transmission.

“This happens when bacteria from poop, which is too small to see, enters your mouth and digestive tract,” the website says. Some forms can also be transmitted through undercooked meat and unpasteurized drinks.

It lists the possible ways you can get E. coli from contaminated food, drinking unpasteurized beverages, drinking contaminated water or getting it in your mouth, touching poop or contaminated surfaces, and not wiping properly after you went to the toilet.

“What you see with contaminated ground beef or cross-contamination with vegetables or whatever could be little bits of fecal material at the microscopic level” getting into the food, Pastula said.

He said pinpointing the exact source of the contamination often requires a complex investigation with local and state public health agencies working with the CDC.

“It just shows how important it is to have a robust public health response system because these types of outbreaks are never going to go away completely,” Pastula said. “We must be able to respond quickly and prevent future cases as much as possible.”

This is a developing story and may be updated.