Time to Dump the Myth of 1 in 20,000 Eggs Infected with SE

Jan 31, 2014

That 1 in 20,000 eggs in the U.S. is “infected” with Salmonella Enteritidis (SE) is an oft-repeated preamble in many technical articles and a centerpiece in web postings opposing intensive production. The 1 in 20,000 eggs figure is nonsense.  If a flock were not infected, the proportion of eggs potentially carrying SE would be zero. Period.

Environmental monitoring in accordance with the FDA Egg Safety Rule has revealed a prevalence rate of approximately 2% contamination with SE among the flocks surveyed in 2012. This would theoretically represent approximately 4.4 million potentially infected eggs produced each day, given a U.S. population of approximately 280 million hens.

This is not a reflection of the true situation.  The 1 in 20,000 figure was based on trials conducted with high levels of infection, possibly in the region of 107 to 108 cfu (colony forming units).  Under practical conditions hens might be exposed to less than102 cfu.  The trials on which the obsolete assumption is based were conducted using non-vaccinated and therefore susceptible hens devoid of antibody protection.  Even prior to the introduction of the FDA Egg Safety Rule in 2011, farmers were vaccinating their pullets with at least two doses of Salmonella Typhimurium live attenuated vaccine to avert intestinal colonization.  Subsequent administration of inactivated Salmonella Enteritidis polyvalent-strain inactivated oil emulsion vaccine stimulates humoral (circulating antibody) immunity.  This would markedly reduce the probability of systemic infection and therefore colonization of the ovary and oviduct necessary for vertical transmission of SE to consumers through the egg.

Effectively the vertical transmission rate, which in any event is low, might be reduced to 1 in 30,000 eggs. This means that the residual environmentally infected 2% of U.S. flocks might collectively produce 133 eggs per day or 24,000 dozen eggs per year.  In point of fact farmers with known environmentally positive flocks are required to submit successive 1,000-egg samples for assay to determine SE status with respect to vertical transmission. Infected flocks are either depleted or their eggs are diverted to pasteurization depending on age and prevailing production costs and revenue from breakers. Under practical circumstances eggs from infected flocks should not enter the market. Even if a few eggs from an infected but non-diagnosed flock were to be packed, refrigeration would inhibit multiplication of SE in the egg. The possible 101 to 102 cfu level of contamination would persist under refrigeration until purchase. Cooking according to FDA recommendations would destroy this low level of remaining pathogens.   

The misquoted “1 in 20,000 eggs” figure wrongly implicating all eggs produced in the U.S. is obviously outdated and does not reflect the reality of current operation.  This is borne out by the absence of recent documented outbreaks of SE attributed to eggs.  The prevalence rate of SE in the U.S. population has declined since the single noteworthy outbreak associated with DeCoster flocks in Iowa in 2012. The vehicles of infection with SE are now other than table eggs produced in accordance with the FDA Salmonella Prevention Rule which extends to all flocks over 3,000 hens.