(USA TODAY) -- The Centers for Disease Control and Prevention has added 40 illnesses to the estimated 1,300 that have occurred already in the salmonella enteritidis outbreak that forced the recall of half a billion eggs.

Meanwhile, the more than 20 Food and Drug Administration staffers on site at the two implicated farms -- Wright County Egg and Hillandale Farms, both in Iowa -- have completed their initial investigations, says Jeff Farrar, the agency's associate commissioner for food protection.

"We're evaluating potential causes for contaminations on the farms associated with this outbreak," he says.

There is currently "no evidence that other farms are involved in this outbreak," but it's a continuing investigation and the FDA will follow all sources where they lead, says Sherri McGarry, the FDA's coordinator for food-borne outbreaks.

The agency hopes to have information available about what has been found within the next week, Farrar says. For now, FDA Commissioner Margaret Hamburg is telling consumers to avoid undercooked eggs -- no runny yolks.

The two farms shared a supplier of chicks and feed, Quality Egg.

The FDA is now investigating the pullet operations of Wright County Egg. Pullets are young chickens being raised until they're large enough to begin laying. The FDA says the supplier that provided them with newly hatched chicks had been certified as free of salmonella enteritidis for more than a decade. This might indicate that the infection that developed later among the pullets was the source of the infection in the laying hens, which then went on to infect the eggs.

Fallout from the outbreak and recall is renewing calls for Congress to pass food-safety legislation.

Rep. Rosa DeLauro, D-Conn., has sent a letter to the U.S. Department of Agriculture and the FDA asking what they knew about Wright County Egg and Quality Egg and when they knew it. Rep. Henry Waxman, D-Calif., a longtime advocate for food safety, has sent a letter to the companies seeking more information.

The DeCoster family, which owns Wright County Egg and Quality Egg, has a history of trouble with environmental, safety and employment issues.

Wright County Egg recalled 380 million eggs Aug. 13 after it was linked to cases of salmonella poisoning. A week later, Hillandale Farms recalled 170 million eggs. Hamburg said Monday that no eggs would be shipped from the farms until the FDA is satisfied that they are operating safely, The Des Moines Register reported.

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