It was in the 1980s that scientists and public health officials first recognized that a type of Salmonella bacterium, Salmonella enteritidis, could be lurking on the inside of perfectly good-looking whole shell eggs. Outbreaks identified at that time were primarily limited to the Northeast, but over the years, the occurrence seems to have spread far and wide. In fact, the 550 million eggs recently recalled due to a surge in Salmonella-related illnesses were from two farms in Iowa.
In the past, officials estimated that one in 20,000 eggs was infected with Salmonella enteritidis, but that may not be the case today -- the actual number could be much higher.
Not everyone who encounters an egg with Salmonella will become sick. But those who do may experience abdominal cramps, diarrhea, nausea, vomiting, chills, fever and/or headache anywhere from six to 72 hours after being exposed. Children, the elderly or people with a chronic illness are most at risk and could experience serious illness.
Eggs can become contaminated in a number of ways, says Ahmed Yousef, food microbiologist with the Ohio Agricultural Research and Development Center in the Department of Food Science and Technology. If the chicken is infected, the bacterium could enter the egg as it is being formed. But even an infected hen doesn't always lay eggs that are contaminated -- most authorities estimate that the rate is normally under 5%.
Eggs could also become contaminated through the shell. Eggshells are actually porous -- that could allow microscopic bacteria on the exterior to work their way inside.
Most of the time, Salmonella is found in the egg white, but when it does find its way to the yolk, it can multiply more quickly -- the yolk appears to be just as nutritious for bacteria as it is for humans.
Unfortunately, contaminated eggs look just like eggs that aren't contaminated. To reduce your risk -- especially if you're in a high-risk group -- follow these guidelines:
Keep eggs refrigerated. Salmonella multiplies rapidly at room temperature, but remains pretty stable when chilled. If an egg is contaminated, you want to keep that bacterium from multiplying -- more cells of Salmonella are riskier than less.
Cook eggs and foods containing eggs thoroughly. This means hard-boiled or over-hard for fried eggs -- not even scrambled guarantees all of the egg will be completely cooked through. Egg mixtures, such as custard, should be cooked to 160 degrees F.
Don't eat foods containing raw eggs, such as cookie dough, meringue or Hollandaise sauce.
Wash hands and surfaces thoroughly after handling eggs.