Because people don't keep their vegetables around to study.
Scientists say the outbreak of E. coli in Europe that has already killed at least 17 people and sickened over 1,500 may turn out to be the deadliest ever. The bacteria combine a deadly toxin with a special binding agent, or "glue," that sticks to a patient's intestines. While it's likely that the strain was carried by contaminated vegetables in Germany, the exact source is still unknown. Spanish cucumbers were originally suspected but have now been ruled out. Why is it so difficult to pinpoint the source of this outbreak?
Two main reasons: One, this particular strain of E. coli is particularly difficult to study; two, vegetables don't tend to stick around for very long.
There are many strains of Escherichia coli bacteria, most of them harmless when ingested. But some produce a dangerous Shiga toxin that can cause severe abdominal pain, hemorrhagic diarrhea, and even an acute syndrome that can cause kidney failure. The most commonly observed dangerous strain of E. coli is O157. This strain can easily be detected either in a stool sample from a patient or from the contaminated food using a culture medium called sorbitol-MacConkey agar. Unlike other, non-harmful strains of E. coli, O157 ferments the agar, producing observable white spots.
The strain currently infecting people in Europe, however, is a version of O104, meaning that it doesn't produce a reaction in the agar and is much more difficult to observe. Although rare, O104 has been seen before: There was a small outbreak in Helena, Montana, in 1994. But the current outbreak is a much more dangerous variant of the strain, which has been labeled O104:H4 and has been seen only once before in a patient in South Korea in 2005. In addition to its other nasty qualities, the strain also appears to be highly resistant to several classes of antibiotics.
Cattle are the most common carriers of E. coli, and the majority of outbreaks are the result of consumption of improperly cooked meat, contaminated with feces. However, a variety of other foods can carry the bacteria as well. There was an outbreak carried by a particular brand of apple cider in Maryland last year, which sickened about a dozen people, as well as the highly publicized Nestlé cookie dough outbreak of 2009, which sickened more than 60 people throughout the United States. (Both outbreaks were of the more common O157 strain.)
If, as is suspected, the strain making people ill in Europe is being carried by fresh produce, that's bad news for epidemiologists. Unlike meat, beverages, or packaged products, people don't tend to keep leftovers of fresh fruits and vegetables around long after they've had the original meal that made them sick. That means fewer samples for researchers to study in order to determine exactly which product consumers should be avoiding.
This has already had economic consequences -- Spanish farmers have seen their export market drop precipitously, for instance. There are political ramifications, too: Russia has already issued a blanket ban on vegetable imports from the European Union.
If an answer isn't found soon, this food fight might just be getting started.
Thanks to Shannon Manning, professor of microbiology and molecular genetics at Michigan State University.
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