All foods may become contaminated with bacteria or viruses, but eggs pose a particularly high risk for transmitting salmonella bacteria. The nutrient-rich yolk provides a near-perfect environment for bacterial growth. Commercial eggs undergo strictly regulated cleansing and pasteurization procedures, reducing the occurrence of salmonella contamination to near zero, says the American Egg Board. Proper handling eliminates the risk of salmonella infection from eggs.
Egg Contamination
The shell protects the egg from most contaminants, explains the Egg Board. Bacteria and other contaminants may reside on the outside of the shell, but government regulations require all commercial eggs to be washed with detergents and sterilized. Salmonella bacteria, however, are commonly found in the intestinal tract of animals, including chickens. A hen may be infected with salmonella without showing any signs of illness. The bacteria infect the hen's ovaries and enter the egg before the shell is formed, says the Centers for Disease Control and Prevention. A contaminated chicken may lay uninfected eggs and only occasionally lay an egg contaminated with salmonella.
Incidence
Relatively few eggs are contaminated with salmonella. The Egg Board estimates that one in 20,000 eggs is infected. More commonly, eggs may be contaminated on the outside of the shell through improper handling or contact with unclean hands, other foods or surfaces. Eggs with bacteria inside the shell have defenses to prevent the bacteria from reaching the yolk. Salmonella inside the shell usually becomes trapped in layers of the white material. The white is slightly alkaline and binds to nutrients, making them unusable to the bacteria. The dense structure of the white helps prevent the bacteria from migrating to the yolk. The layer of white closest to the yolk consists of ropey strands containing little water for the bacteria. However, as eggs age, the white begins to break down and the membrane separating the yolk from the white thins, allowing the salmonella easier access to the nutrients in the yolk.
Salmonellosis
Eggs contaminated with salmonella are safe to eat if cooked thoroughly and eaten immediately after cooking. If contaminated eggs are eaten without proper cooking, an infection called salmonellosis develops within six to 72 hours after eating the egg, notes the Egg Board. Salmonellosis causes nausea, vomiting, diarrhea, abdominal cramps, chills, fever and headache. The symptoms usually last one to two days. The infection is rarely fatal, although it may cause complications in very young children, elderly people, pregnant women and those with a major illness or weakened immune system.
Prevention
Despite the uncommon incidence of in-shell salmonella, a few precautions can eliminate the risk of infection, suggests the Egg Board. Keep eggs properly refrigerated and handle with clean hands. Cook eggs thoroughly and serve promptly. Throw away any eggs with cracked, broken, leaking or dirty shells.
Treatment
Treat salmonellosis with antidiarrheal medications and antibiotics, advises MayoClinic.com. Antidiarrheals reduce abdominal cramping and discomfort, but they may also extend the period of diarrhea.
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