WASHINGTON (AP) — More than a half-billion eggs have been recalled in the nationwide investigation of a salmonella outbreak that Friday expanded to include a second Iowa farm. The outbreak has already sickened more than 1,000 people and the toll of illnesses is expected to increase.

Iowa’s Hillandale Farms said Friday it was recalling more than 170 million eggs after laboratory tests confirmed salmonella. The company did not say if its action was connected to the recall by Wright County Egg, another Iowa farm that recalled 380 million eggs earlier this week. The latest recall puts the total number of potentially tainted eggs at about 550 million.

FDA spokeswoman Pat El-Hinnawy said the two recalls are related. The strain of salmonella bacteria causing the poisoning is the same in both cases, salmonella enteritidis.

Federal officials say it’s one of the largest egg recalls in recent history. Americans consume about 220 million eggs a day, based on industry estimates. Iowa is the leading egg producing state.

The eggs recalled Friday were distributed under the brand names Hillandale Farms, Sunny Farms, Sunny Meadow, Wholesome Farms and West Creek. The new recall applies to eggs sold between April and August.

Hillandale said the eggs were distributed to grocery distribution centers, retail groceries and food service companies which service or are located in fourteen states, including Arkansas, California, Iowa, Illinois, Indiana, Kansas, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, Texas, and Wisconsin.

Thoroughly cooking eggs can kill the bacteria. But health officials are recommending people throw away or return the recalled eggs.

A food safety expert at Cornell University in Ithaca, N.Y., said the source of the outbreak could be rodents, shipments of contaminated hens, or tainted feed. Microbiology professor Patrick McDonough said he was not surprised to hear about two recalls involving different egg companies, because in other outbreaks there have also been multiple sources.

Both plants could have a rodent problem, or both plants could have gotten hens that were already infected, or feed that was contaminated.

“You need biosecurity of the hen house, you want a rodent control program and you want to have hens put into that environment that are salmonella free,” McDonough said.

The salmonella bacteria is not passed from hen to hen, but usually from rodent droppings to chickens, he added. This strain of bacteria is found inside a chicken’s ovaries, and gets inside an egg.

CDC officials said Thursday that the number of illnesses related to the outbreak is expected to grow. That’s because illnesses occurring after mid-July may not be reported yet, said Dr. Christopher Braden, an epidemiologist with the federal Centers for Disease Control.

Almost 2,000 illnesses from the strain of salmonella linked to both recalls were reported between May and July, almost 1,300 more than usual, Braden said. No deaths have been reported. The CDC is continuing to receive information from state health departments as people report their illnesses.

The most common symptoms of salmonella are diarrhea, abdominal cramps and fever within eight hours to 72 hours of eating a contaminated product. It can be life-threatening, especially to those with weakened immune systems.

The form of salmonella tied to the outbreak can be passed from chickens that appear healthy. And it grows inside eggs, not just on the shell, Braden noted.