How eggs become contaminated

Unlike eggborne salmonellosis of past decades, the current epidemic is due to intact and disinfected grade A eggs. Salmonella enteritidis silently infects the ovaries of healthy appearing hens and contaminates the eggs before the shells are formed.

Most types of Salmonella live in the intestinal tracts of animals and birds and are transmitted to humans by contaminated foods of animal origin. Stringent procedures for cleaning and inspecting eggs were implemented in the 1970s and have made salmonellosis caused by external fecal contamination of egg shells extremely rare. However, unlike eggborne salmonellosis of past decades, the current epidemic is due to intact and disinfected grade A eggs. The reason for this is that Salmonella enteritidis silently infects the ovaries of healthy appearing hens and contaminates the eggs before the shells are formed.

Although most infected hens have been found in the northeastern United States, the infection also occurs in hens in other areas of the country. In the Northeast, approximately one in 10,000 eggs may be internally contaminated. In other parts of the United States, contaminated eggs appear less common. Only a small number of hens seem to be infected at any given time, and an infected hen can lay many normal eggs while only occasionally laying an egg contaminated with the Salmonella bacterium.