THURSDAY, Aug. 26 (HealthDay News) -- U.S. health investigators say they've found positive samples of salmonella bacteria in feed given to chickens at the two farm enterprises implicated in the ongoing egg recall.

The finding, at Iowa-based Wright County Egg and Hillandale Farms, suggests that feed or feed ingredients might be the source of the salmonella outbreak, government experts said at a press conference Thursday.

"These are the first set of positive samples we believe are significant," said Sherri McGarry, emergency coordinator for the U.S. Food and Drug Administration's Center for Food Safety and Applied Nutrition at a Thursday teleconference. "The DNA fingerprint [of the samples] matches the outbreak fingerprint."

"This finding of matching the DNA footprint in feed indicates that Wright County Farms and Hillandale Farms are likely sources of the contaminated eggs and that feed and feed ingredients perhaps were the sources but maybe not the only sources," McGarry added.

Thus far, problems have been confined to these two enterprises, officials said.

"We do not know at this point how, when or where the feed may have been contaminated. It's part of our ongoing investigation but the finding essentially raises a lot of additional questions to answer at this point," added Dr. Jeff Farrar, associate commissioner for food protection at the FDA's Office of Foods, U.S. Food and Drug Administration. "On poultry operations they do get at least some ingredients from outside the farm. That would be normal procedure, so we are going to follow that trail back where it leads."

Dr. Christopher R. Braden is acting director of the Division Foodborne, Waterborne and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, part of the U.S. Centers for Disease Control and Prevention. He told reporters that between May 1 and August 25, there have been a total of 2,403 salmonella cases reported, 1,470 of which may be related to the current outbreak.

He said he expects additional reports of illnesses and new "sub-recalls" in coming weeks and days.

Wright County Egg and Hillandale Farms issued the egg recall earlier this month after receiving reports that salmonella had sickened nearly 2,000 people.

Meanwhile, two new brands of eggs were added Thursday to the recall tied to the two Iowa farms.

Wright County Egg said it has found salmonella in the Cardenas Market brand and is beginning a voluntary recall. Affected cartons have the plant number 1026 on the side and Julian (packaged) dates between 136 and 228, CBS News reported Thursday.

And Trafficanda Egg Ranch reported salmonella in some of its eggs from Wright County. Affected plant numbers are 1026, 1413, 1720, 1942 and 1946, with Julian dates between 136 and 229, the news network said.

At least 550 million eggs have been recalled so far, according to federal officials. Experts stress that any shell eggs that have been recalled from store shelves are being destroyed.

To find out if any eggs in your fridge might be affected, check the carton for the "Sell By" date and the two numbers below it, federal health officials say, to see if your eggs are involved in the recall. One number is the plant number, and the other is the packaged date, or Julian date, showing what day of the year the eggs were packaged. For example, Jan. 1 is 001 and Dec. 31 is 365. The U.S. Food and Drug Administration has a list of what numbered designations are included in the recall.

In healthy people, salmonella can cause fever, abdominal cramps and diarrhea and usually lasts four to seven days. However, contamination can cause serious and sometimes fatal infections in young children, frail or elderly people, and others with weakened immune systems.

Harmful bacteria such as salmonella are the most common cause of foodborne illnesses, according to federal health officials.

More information

Learn more about salmonella at the Centers for Disease Control and Prevention.