Is nothing safe anymore, now that the U.S. government is warning of the danger of leafy greens?

The alert not to eat romaine lettuce came just in time to foil plans some Americans might have made for a light, healthy item to add to their traditionally heavy Thanksgiving dinners.

While people should take seriously the recent outbreak of E. coli food poisoning that triggered the alert, the fact that it was flagged early shows that the agency once known as the CDC deserves its upgraded name as the Centers for Disease Control and Prevention.

This is the third romaine-related scare in two years. Is there something wrong with the way we’re growing lettuce?

I checked in last week with Cornell University professor Martin Weidman, a food safety expert, to find out why we’re seeing so many outbreaks. He attributes the increase in outbreaks to better technology — now scientists can quickly and easily identify the specific strains of bacteria infecting people, thus tying scattered cases to a single source.

At the time of the Nov. 20 warning, the outbreak had led to just 32 reported cases of illness, spread over 11 states, which had begun Oct. 6. As recently as 20 years ago, it would have been impossible for the CDC to identify an outbreak at all, let alone trace its source, from such a small scattering of people reporting gastrointestinal problems.

DNA testing can also detect unusually dangerous strains. While many forms of E. coli are harmless, this latest outbreak is caused by a strain called O157:H7, which produces a toxin that in previous cases has led to kidney failure.

So even though only a small fraction of U.S. lettuce is likely to be contaminated, CDC leaders made the judgment call to warn people to throw away any romaine they happen to have bought, and suggested restaurants stop serving it.

The romaine is still just a strong suspect in this case, Weidman said. Once the government scientists found a common DNA signature among the infections of those seeking medical help, they asked people to recall what they had eaten over the last week. More than half had eaten romaine lettuce, which is far from proof of its guilt, given that surveys done a decade ago showed that about half of Americans eat romaine in a given week.

But romaine is a likely suspect.

Some foods are known to be more likely to harbor dangerous bacteria. Thorough cooking kills E. coli, so foods eaten raw are a more likely source of infection, and for reasons scientists are still trying to puzzle out, romaine seems to be unusually prone to picking up E. coli from contaminated soil, water or fertilizer.

The next step is to find the 0157:H7 on romaine, which means sorting through a lot of uncontaminated lettuce in pursuit of some contaminated lettuce. That’s reassuring for those of us who ate salads in the days before the warning.

Earlier this year, CDC tied the previous outbreak to farms in Yuma, Arizona, where lettuce had been contaminated by irrigation water. In the latest news on the current outbreak, experts are zeroing in on a source in California.

It’s a good thing that more Americans want to eat leafy green vegetables. It may be impossible to have perfect safety in a system that has to mass produce food in ever-increasing quantities.

The fact that food safety experts at the Centers for Disease Control and Prevention can flag an outbreak early, before anyone has died, is something to be thankful for.

Bloomberg Opinion columnist Faye Flam has written for the Economist, the New York Times, the Washington Post, Psychology Today, Science and other publications.

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