Rapid Screening System Contributes to Reduction of U.S. Egg-borne SE

Egg-borne Salmonella Enteritidis* (SE) infection has been drastically reduced in the U.S. commercial egg industry. Following cases in the EU in the early 1970s, the index outbreak occurred in 1979 in New England. This was followed by extension through the Mid-Atlantic States in the early 1980s and ultimately emergence in the Southwest and Pacific regions in the 1990s.

Total SE cases in the U.S. peaked at 10,200 in 1995 but had declined to 5,000 by 2005. Incidence rates in the Mid-Atlantic States, clearly the worst affected region, attained 9.1 per 100,000 in 1994. By 2007, the level was lower at 2.5 per 100,000 population.

The US Centers for Disease Control and Prevention projected that the 5,333 confirmed cases of SE actually represented 170,000 cases nationwide, of which 108,000 were attributed to consumption of infected eggs. Irrespective of the validity of these projections, based on a series of contested assumptions, it was evident that a problem existed, stimulating Federal intervention.

Egg quality assurance programs

During the late 1990s, individual states introduced the Egg Quality Assurance Programs requiring producers to conform to minimal standards of biosecurity, post-pack refrigeration, vaccination, assay of flocks for environmental contamination and diversion of eggs from infected flocks to breaking and pasteurization. The elimination of vertically transmitted infection through successive generations, extending from pure lines to parent stock, was critical to the attainment of a meaningful reduction in  the prevalence of SE in flocks.

The National Poultry Improvement Plan (NPIP) established in the early 1930s is responsible for the coordination of Federal, state and industry activities to eradicate vertically transmitted infections. The Plan mandates operating procedures, approves diagnostic tests and certifies hatcheries supplying day-old chicks and poults.

Originally focused on the eradication of Salmonella pullorum infection, the NPIP is now concerned with SE in laying strains, in addition to other pathogens in a wide range of avian species. Adherence to the requirements of the National Poultry Improvement Plan ensures a supply of commercial replacement pullet chicks from hatcheries certified free of SE. This is an essential foundation of industry efforts to prevent SE in commercial flocks.

In 2003, a nationally distributed brand of eggs introduced a rigorous SE prevention program involving vaccination and enhanced biosecurity required of all franchised egg producers. The use of lateral-flow immunoassay test kits was central to the successful implementation of the program. Since flocks were screened five times from the day of placement to the end of the second cycle of production, a rapid test with appropriate parameters was required in place of the conventional Food and Drug Administration (FDA) Bacteriological Analytical Manual (BAM) microbiological procedure.

RapidChek^® SELECT™ - a highly sensitive test

The Romer Labs RapidChek^® SELECT™ lateral flow immunoassay has become the standard screening procedure to determine the presence of a Group D1 Salmonella in manure drag swabs or in egg pools. The procedure is regarded by the FDA as equivalent to the more laborious and expensive BAM methodology. Following the initial evaluation of the RapidChek^® method, it was determined that the test demonstrated a sensitivity of 100 percent, critical for a screening test which should not provide false negatives.

Concurrently in the approval process assaying 141 Group D1 Salmonella isolates and 210 non-Group D1 Salmonella serotypes, the specificity of the test was determined to be 93 percent. This is an important parameter since a false positive assay from a 45-week old flock of 125,000 hens will incur substantial costs and disrupt operations.

Applying the BAM method a commercial laboratory examined 2,412 drag swab samples in 2010, yielding 1,477 “suspicious” colonies requiring further examination. From this complement only two SE positive samples were confirmed. The combined results from two mid-West state laboratories applying the RapidChek^® assay in 2010 and 2011 yielded 106 “presumptive” positives from approximately 14,000 drag swabs examined.

Confirmatory bacteriologic assays identified 16 SE isolates from the routine samples submitted by producers. Data from a Western state laboratory on 1,162 drag swabs compared the ability of the RapidChek^® assay to detect Group D1 Salmonella against the conventional BAM microbiological assay. Eleven “presumptive” positives were identified using lateral-flow immunoassay compared to three SE positives applying the BAM procedure.

Practical considerations

The specificity of a screening assay is important from the perspective of potential financial loss. It is calculated that if a false positive environmental assay was obtained from a flock at the mandatory 40 to 45 weeks age period, withholding eggs from the market or diverting to breaking and pasteurization over a four-week period during which confirmatory test are performed could cost as much as $75,000 for a flock of 125,000 hens, depending on the prevailing market price of shell eggs and breaking stock.

In 2010, shortly after the introduction of the FDA Final Rule on Prevention of Salmonella, a widespread outbreak associated with one complex in Iowa resulted in an extensive recall ultimately numbering 500 million eggs. This case was an aberration and was due to gross mismanagement and did not reflect current practices in the industry.

In this 2010 outbreak, the initial round of assays derived from the audits conducted by the FDA applying the FDA BAM methodology showed an infection rate of 2.5 percent of flocks examined. This has since declined to negligible levels. There have not been any reported cases of SE attributed to eggs from 2011 onwards. This is due to diligent compliance with the principles of the FDA Final Rule and is based in large measure on the frequency and intensity of environmental monitoring. This would not have been possible without the application of the RapidChek^® SELECT™ SE test, which is used by commercial and state laboratories to conduct screening and compliance assays for the industry.

Epidemiologic evidence demonstrates that SE infection in the poultry industry is now associated with broiler meat and that SE isolates comprise an increasingly greater percentage of Salmonella serotypes recovered from carcass washes. Accordingly, programs of suppression and elimination are being applied, with the SE kit as the preferred screening technique for parent and growing flocks in the U.S. broiler industry.