After dozens of E. coli infections were linked to McDonald’s Quarter Pounders, the company temporarily pulled them off the menu in several states. But McDonald’s said on Sunday that it will be bringing the burgers back in the coming days since preliminary data from the Colorado Department of Agriculture implicates slivered onions used on Quarter Pounders — not the beef patties themselves — as a possible source.
Whether it’s E. coli from onions at McDonald’s or Listeria from Boar’s Head deli meats, foodborne illness is a common problem worldwide. In the United States alone, the CDC estimates there are over 47 million cases and over 127,000 hospitalizations per year. Here’s how public health authorities use basic epidemiology to pinpoint the source of a foodborne outbreak to a specific food or vendor.
Any outbreak investigation starts with a notification that people are ill to health authorities, such as the local health department, Centers for Disease Control and Prevention, the U.S. Department of Agriculture or the Food and Drug Administration. That can come through a phone call, email, text or media report. The source of notification is frequently an astute clinician who recognizes an unusual pattern of illness in his or her patients and then sounds the alarm. Other potential sources could include the patients themselves, laboratories noting unusual numbers of positive lab samples or pharmacies noting large numbers of prescription or over-the-counter medications being purchased.
A notification triggers public health authorities to seek more information to determine whether, indeed, an outbreak is happening. They then follow a series of steps and use epidemiological methods to hunt down the cause. They will first establish a case definition to define the signs and symptoms of the illness and use that definition to go out and find additional cases. For instance, the case definition in the McDonald’s outbreak might include someone with a fever, diarrhea and a positive stool test for E. coli. As more information is collected, a timeline of dates of illness might be included as well as possible exposure sources.
It is important then to query the cases about possible exposures. This is usually done with a battery of questions, such as where people ate during a specific time frame, what they ate, who else ate with them and whether anyone else they know is ill. This information could give an early indication of a potential cause. When McDonald’s was identified as a common place patients ate prior to illness, for instance, that could be the trigger to ask more specific questions related to McDonald’s.
In order to conduct what is known as a case control study, the first thing would be to obtain a menu of all possible food options at McDonald’s and then to ask the case patients what they ate as well as what they didn’t eat to help rule in or rule out the different foods. It is also useful to query other individuals as controls who are not ill but also ate at McDonald’s during the same time period as the cases. The goal is to identify a common source of infection.
Investigators then make mathematical calculations to compare the odds of infection from eating different foods. This helps to pinpoint not only that the culprit was Quarter Pounders, but to delineate more specifically whether the meat in the Quarter Pounders or something else on the Quarter Pounders, such as onions, is the likely cause.
At this point, epidemiologists will double check whether their suspected source of the epidemic stands up to scrutiny. This includes a reassessment to ensure that their data is not missing anything important and that the implicated foods make biological sense. In a situation with E. coli, the organism is found in the guts of animals and can contaminate meat or soil, so having either hamburgers or onions implicated is biologically plausible. Genetic testing of the E. coli can also be used to further compare a possible source found in implicated foods and in patient samples.
Researchers linked the McDonald’s outbreak to E. coli O157:H7, a known pathogen that produces a toxin that can cause fever, abdominal cramps, severe and even bloody diarrhea and kidney failure.
Public health authorities want to be reasonably certain they are correct about the outbreak cause before putting out any press releases. There are a number of reasons for this: this type of investigation is potentially politically charged and it has huge implications for the businesses that might be affected. As was seen, the stock price of McDonald’s fell off a cliff on the date of the outbreak announcement. Affected businesses can lose money or go bankrupt, because frightened customers choose to dine elsewhere.
Other fast-food chains, including Taco Bell, Pizza Hut and KFC also pulled onions from their menus and the onion supplier, Taylor Farms, issued a voluntary recall.
The goal is to identify the source rapidly and to mitigate the problem by notifying the public to stop eating the implicated food. Ultimately, the true test of whether they’ve hit the mark is that illnesses decline. And if that occurs, then, bon appétit.
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