The German E. coli outbreak is demonstrating the difficulty of identifying a “smoking gun” in outbreaks of foodborne illness.

Some are asking, “Where is the evidence?” By “evidence,” I believe they mean identifying the outbreak strain of bacteria in a food source that is genetically identical to clinical isolates by Pulse-Field Gel Electrophoresis (PFGE).

There is another kind of evidence that is just as important … it’s the evidence provided by shoe-leather epidemiology. Shoe-leather epidemiologists (the name is used to describe applied epidemiology performed mostly in the field, outside of an office, or “wearing a hole in one’s shoe soles”) work in local and state health departments and at the Centers for Disease Control and Prevention (CDC) and conduct outbreak investigations to try to find that smoking gun.

They design case-control studies, develop questionnaires, interview case patients and controls, and analyze the data to determine statistically significant associations with food or other exposures. These identified food items are then traced back to restaurants, food distributors, and manufacturers where food and environmental sampling can be done.

Samples of the food from the same batch eaten are rarely still available by the time the connection to the outbreak is made. You can’t sample an entire farm or factory.  Epidemiology may be the only evidence you have.

Some are criticizing the German health authorities for blaming cucumbers from Spain too soon and for not focusing on sprouts earlier. Sprouts are served in many sandwiches and dishes at restaurants and many people don’t even recall eating them.

In 2009, a multi-state outbreak of Salmonella Saintpaul infection was tied to alfalfa sprouts from a local grower in Omaha, Nebraska by shoe leather epidemiology (Outbreak of Salmonella Serotype Saintpaul Infections Associated with Eating Alfalfa Sprouts — United States, 2009).  The outbreak strain was never grown from sprouts or seeds.  Epidemiology was the sole evidence that prompted a recall of sprouts.      

We interviewed 32 case patients as part of the case control study conducted by health departments in Nebraska and Iowa.  Only 8 answered “yes” to the question, “Did you eat any sprouts?” But upon further inquiry, 27 had actually eaten them. Most of these sprouts were included in sandwiches or salads eaten at restaurants.

Also, according to the Control of Communicable Diseases Manual, the incubation period for Salmonella is from 6 to 72 hours.  In 13 case patients where the date of exposure to sprouts could be determined, 7 had eaten them more than 72 hours prior to becoming ill, up to 8 days.  It would have been very easy to miss the sprout association.

In another national outbreak of Salmonella Saintpaul infection in 2008 (Outbreak of Salmonella Serotype Saintpaul Infections Associated with Multiple Raw Produce Items — United States, 2008), CDC initially identified fresh tomatoes as the probable source of food contamination.

But as the outbreak continued on, more evidence surfaced that pointed to jalapeño peppers, which are commonly eaten with tomatoes in items like fresh salsa.  I can imagine many case patients who were interviewed answering “no” to “Did you eat jalapeño peppers?” even if they had eaten salsa at a restaurant.

Identifying the culprit in a foodborne outbreak is not an easy task.  Much focus has been placed on finding a genetically matched outbreak-causing organism in a food source, or “the smoking gun,” but not on the epidemiology that points us to that source. This has prompted arguments for increased spending on food safety agencies to increase regulation and monitoring of food production.  While that is an important area, we should not forget about the shoe-leather epidemiologists in our public health agencies who produce the evidence that points us to those food sources in the first place.


Dr. Anne O’Keefe is the Senior Epidemiologist at the Douglas County Health Department in Omaha, Nebraska.

  • Doc Mudd

    My fundamental (and constructive) criticisms of German authorities are that they seem to have been entirely too lax in preventive measures, too slow to accept the seriousness of the outbreak and too hesitant to begin investigating in earnest. They seemed relatively unconcerned and inactive during the first two weeks of May (when the mainstream media hadn’t yet picked up the story) and responded with rushed, dishevelled finger pointing when finally approached by the press.
    Rather like a drowsy nightwatchmen awakened by the screeching tires of burglars making off with the loot.
    I suspect they were lulled by dreamy (but incorrect) assumptions of the innate safety of organic & local produce, and perhaps by smug over-estimation of their own capabilities — simply too full of themselves; an image of Prince Charles naturally comes mind when contemplating the recent European E. coli outbreak.

  • ICBM

    Ah… and highly “suspect” and very smug anti local and organic criticisms at that — an image of Muddiness as Usual (not Prince Charles, actually) comes to mind when contemplating it…..
    Of course, with this contamination event still going on unresolved, any One-Eyed Man is King. There’s even a bioterrorism theory that says because this novel virulent strain is resistant to 8 different antibiotics it must have been genetically engineered — theories abound!
    But what it all comes down to is innocent people are sick, dying and dead and the answers are still hard to come by…

  • Doc Mudd

    Heh, heh. More obtuse finger pointing plus a little name-calling and a conspiracy theory thrown in at no extra charge! From Prince Charles’ own organic sycophant, no less. I am flattered.
    Dung worshiping organic fanatics have some ‘splainin to do. In the meantime, keep pitching the manure to the veggies, old boy.

  • ICBM

    Funny thing, the name-caller ( a real Prince) doesn’t like being called the names he calls others. Hmmmm…… SOMEBODY has got some ‘splaining to do….
    And although it might upset some pet Doc prejudices — a lot of organic vegetable crop growers don’t use any manure-based compost at all — not that it’s not effective or unsafe — but due to lack of access to any quality local product. Organic doesn’t allow what comes out of most industrialized dairies that have gone to liquid systems, for example, storing it in holding ponds and spraying it on fields after tillage — or on snow in the spring when they’ve got to make room for more (and guess where That goes with the melting snow…?)
    But then we wouldn’t want the facts to get in the way of a good ole’prejudice now would we?

  • Doc Mudd

    So there’s a fair criticism of European organic growers and purveyors who smeared fecal E. coli on the food – they were reckless with their excrement handling etiquette.
    Oh, we have nothing to fear from any fecal faux-pas here in the US, according to self-proclaimed organic manure geniuses like ‘ICBM’. Good thing we have strong investigative teams like Minnesota Department of Health if prevention is in the hands of tens of thousands of Prince Charles’ loopy disciples.

  • Michael Bulger

    Mudd seems to be forgetting that manure fertilizers are not confined to organic production. In fact, raw manure is more strictly regulated here in the US when used in certified organic production, as compared to non-organic.

  • Doc Mudd

    The battle-scarred practitioner Mudd has forgotten more about medical microbiology, production agriculture and feeding the world than the perky, wet-behind-the-ears Bulger will cherrypick, spin and fling against the wall during the entire two short years of his NYU masters program in fanatic industry bashing under Marion Nestle’s biased tutelage.
    Ahh, impressionable youngsters today – so many are misguided Keen “amateurs” and much, much too proud of it.
    First, learn to know your “manure”, Bulger.

  • Michael Bulger

    Oh “Mudd”, our anonymous internet amateur, 2012 is a leap year. So, actually, my year will be long.

  • ICBM

    Not surprising that the Muddite would quote his altar-egotist, Anthony Trewavas — “one of the United Kingdom’s strongest supporters of genetically modified food crops and a vocal critic of organic food.” Such expertise at not seeing the forest for the trees….