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From NPR News, this is ALL THINGS CONSIDERED. Im Melissa Block.

ROBERT SIEGEL, host:

And Im Robert Siegel.

In Iowa, 20 investigators from the Food and Drug Administration are going through records and chicken houses at two large egg producers. Wright County Eggs and Hillandale Farms are suspected of causing a nationwide outbreak of salmonella poisoning.

Theyve recalled 500 million eggs. No one seems to remember a larger egg recall in U.S. history. In a few minutes, we'll talk about the company linked to both those farms and its record of violations.

First, though, NPR's Dan Charles has the story of how scientists traced the outbreak to central Iowa, and he begins in his own kitchen.

DAN CHARLES: Millions of Americans this week are doing what I am doing right now - opening the refrigerator, pulling out a pack of eggs and looking for the information that'll tell me these are the eggs that are no longer considered safe to eat. Here are some numbers on the side: 225 P 1979. What does that mean?

Dr. PAT CURTIS (Director, National Egg Processing Center, Auburn University): The first numbers are the Julian date. Like for January 1st, it would be 001. Or for December 31st, it would be 365.

CHARLES: Pat Curtis is director of the National Egg Processing Center at Auburn University. She explains thats the date those eggs were packed. And the other number, after the P, is the plant number.

Dr. CURTIS: That's a code that tells you which actual processing plant processed those eggs, and each plant has an individual number.

CHARLES: You can learn which numbers to look for and which eggs to throw out at the website FoodSaftey.gov. But these numbers also help scientists identify the farms that now are under investigation.

The trouble started last May. By August, at least a thousand more people than usual all around the country had gotten very sick with salmonella poisoning, but no one knew why. Hospitals reported these cases to state health authorities. They took a kind of genetic fingerprint of the bacteria and passed it along to the federal Centers for Disease Control and Prevention in Atlanta.

Ian Williams, who's in charge of the outbreak response and prevention branch at the CDC, says in this case, the genetic fingerprint was not very helpful. It was a very common strain of salmonella bacteria, so investigators couldn't tell if all those people really were getting sick from the same thing.

Dr. IAN WILLIAMS (Director, OutbreakNet Team, Centers for Disease Control and Prevention): So one of the approaches we did to try to address this was to try to identify clusters of cases who had eaten at a common restaurant or event in a similar time period. You all went to a prom together, or you went to a church picnic - something like that.

CHARLES: Investigators then searched for the common link: What did all of these people eat? And did people in other clusters eat it too?

Dr. WILLIAMS: And the real breakthrough actually came sort of towards the end of July, with our colleagues in California had actually identified six of these clusters. And they simply asked the simple question of: We know there was salmonella enteritidis; common sources are either chicken or eggs. And they simply asked the question of: I wonder where these six clusters are getting their eggs from.

CHARLES: The egg cartons carried dozens of different brand names, from Albertson to Wholesome Farms. But the numbers on the cartons, the plant numbers, told the real story. Those eggs came from just three different plants operated by two companies in Iowa. And when the CDC checked, people who'd gotten sick in other parts of the country also had eaten eggs from those producers.

FDA investigators now are trying to confirm the presence of salmonella at those operations and others that supplied them with feed. Often, salmonella comes from mice getting into chicken feed and leaving droppings, and that infects the chickens, who pass it on to the eggs.

Each of these enterprises has more than a million chickens. Some critics of the food industry say such mammoth operations are giving us more contaminated eggs.

But Hongwei Xin, director of the Egg Industry Center at Iowa State University, says scientists have not found that to be true.

Dr. HONGWEI XIN (Director, Egg Industry Center, Iowa State University): You know, whether you have a flock of 100 birds or 100,000 birds, if mice get into your system you'll be equally vulnerable.

CHARLES: But if your chicken house is huge and it gets contaminated, you have a really big problem. FDA officials say they cannot remember a bigger recall of eggs.

Dan Charles, NPR News, Washington.

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