Salmonella is and remains an important issue in the poultry industry. Egg and meat producers work hard to control this bacteria to guarantee these products to be safe food for humans. Intensive prevention programmes prove to be successful but it has a cost.
In addition to the environmental assays prior to transfer of pullets and at the depletion of flocks, monitoring is required at 45 weeks of age and at the completion of molting which are both regarded as periods of stress with the potential for vertical transmission of SE.
The US egg production industry, working collectively with its representative association, the United Egg Producers (UEP) has actively complied with the requirements of the US Food and Drug Administration to successfully control Salmonella Enteritidis (SE) egg-borne infection. In 1979 the index outbreak of SE which occurred in the New England states was documented following a comprehensive epidemiological investigation. This was followed by sporadic outbreaks through 1983 as cases were identified in the Mid-Atlantic states and in the Midwest. Following the emergence of the infection among consumers in the Pacific states and the Southwest in 1992, public health authorities established monitoring programmes contributing to the 1995 FoodNet surveillance system. This initiative organised by the centres for Disease Control and Prevention (CDC) linked the results from individual states and regional laboratories into a comprehensive database.
In 2008 the new Obama Administration instructed the FDA to develop a programme to eliminate SE infection in consumers, although it was acknowledged that the incidence rate of eggborne infection was obviously declining (Table 1). The Centers for Disease Control and Prevention, an agency of the Department of Health and Human Resources estimated that there were as many as 109,000 egg-attributable incident cases of SE in the US in 2007 (Table 2). This projection which has been questioned, incorporated a multiplication factor of 38 which was based on the presumption that only a defined proportion of cases of SE were diagnosed and confirmed by laboratory assay.
Industry response to emergence of SE
Following the emergence of egg-borne SE many of the egg producing states initiated Egg Quality Assurance Programs (EQAPs) which mandated improved biosecurity, monitoring of the SE status of flocks and vaccination. The specific details varied among states and compliance was usually voluntary. The United Egg Producers introduced a "Five-Star Programme" which required compliance with the National Poultry Improvement Programme (NPIP) regulations confirming that chicks were derived from parent flocks and hatcheries free of SE. The programme also required high levels of rodent control and encouraged vaccination. The UEP monitoring programme commenced with an assay of chick-box liners to ensure that day-old pullets were free of vertically-transmitted SE. The environment of rearing houses was tested two weeks before the flock was transferred to egg production units and laying flocks were tested two weeks before depletion. Effectively there was no monitoring of the SE status of flocks from onset of production at 18 weeks through to depletion, frequently in a second cycle at 110 weeks of age. The UEP programme did not incorporate auditing for compliance. In contrast Eggland's Best, a franchisee marketing organisation distributing a range of branded enriched eggs nationally through a network of producers imposed a more rigorous prevention and monitoring programme in 2003. In addition to the environmental assays prior to transfer of pullets and at the depletion of flocks, monitoring was required at 45 weeks of age and at the completion of molting which are both regarded as periods of stress with the potential for vertical transmission of SE. Obligatory vaccination was required with two live mutant Salmonella Typhmurium vaccines at the hatchery or within 48 hours of placement and during the 14 to 21 day age periods. In addition, certain flocks including those placed on litter were required to be protected with an inactivated SE emulsion vaccine. Compliance with obligatory management and biosecurity procedures including rodent control was audited annually on all producers' farms.
The FDA final rule on prevention of SE
In response to Administration directives, the FDA published a Final Rule applicable to all flocks of over 30,000 hens which took effect in June 2010. The Final Rule required placement of chicks in accordance with the NPIP certification programme, suppression
of flies and rodent control involving exclusion and baiting. Flocks are monitored two weeks before transfer to laying houses and at 40 to 45 weeks of age and where applicable, four to six weeks after molting. The FDA Final Rule suggested vaccination but this was not incorporated as a requirement. Previous to the Final Rule, the FDA had required refrigeration of eggs at or below 8C within 36 hours of lay and extending forward to post-pack storage and distribution through to point-ofsale. Effectively with in-line units of 0.8 to 2.5 million hens, daily collection and processing results in packed eggs entering coolers within 24 hours of production. Even in remote small cage free units eggs are collected daily and placed in coolers awaiting transport to packing plants for off-line processing at two to three day intervals. Procedures were mandated in the event of a positive environmental assay. Three successive assays of pools comprising 1,000 eggs must be assayed for the presence for SE at two week intervals. If positive, either the flock must be depleted or eggs should to be diverted to breaking and pasteurisation. If a flock yields a positive environmental result on feacal drag swabs, indicating intestinal colonisation, but with a negative SE in egg-pool assay, product may be distributed subject to monthly re-testing for the presence of SE in egg pools. In effect producers generally elect not to distribute eggs from suspect flocks due to the legal implications and consequences of product liability claims. These arise from trace-back investigations following detection of infection of consumers. Producers are aware of the diagnostic capability of the public health authorities at the state and national levels. Regional laboratories have access to the FoodNet system maintained by CDC and the the database posted by PulseNet (pulse-field electrophoresis assay). Epidemiologists investigating outbreaks also have the option to apply extremely specific molecular assay techniques including gene sequencing to correlate human isolates with a farm or plant suspected to be the source of infection.
During the second half of 2010 and most of 2011 the FDA undertook field investigations to ensure compliance with the Final Rule. All these inspections involved a review of documentation including the mandated Salmonella Prevention Programme specific to the farm, flock records, environmental assays for SE, rodent and fly control data and confirmation of acceptable biosecurity precautions. In addition, the comprehensive inspections on "high risk" complexes and farms also included environmental swabbing of manure in an attempt to detect the presence of SE. The FDA Final Rule was extended to farms holding from 3,000 to 30,000 hens in July 2012. Small-scale producers frequently function as contractors to large integrators. Accordingly they have been provided with guidance and have had sufficient time to prepare farms and procedures to comply with the Final Rule. The prevalence rate on these small flocks has yet to be ascertained.
Cost of compliance with the FDA rule
The categories generating costs to comply with the FDA Rule are shown in Table 3 The typical US in-line complex of 1.25 million hens would generate a total cost approaching €120,000 annually which approximates a unit cost of 0.44 cents per dozen sold.
The assays conducted by the FDA have demonstrated a prevalence rate of 2% to 5% in US flocks investigated. Assuming the higher value, reflecting the preponderance of high-rise units which accumulate manure for up to six months and are more susceptible to rodent infestation, the total cost to the US industry for the FDA Final Rule was estimated to be €24.7 million per annum (Table 4). These cost projections are based on a typical complex of 1.25 million hens. The actual value will vary depending on SE prevalence, value of shell-eggs and products and the capacity of available breaking and pasteurisation.
Vaccination as a component of SE prevention
Eggland’s Best imposed a rigorous prevention and monitoring programme. Most US producers follow a simple but effective program of vaccination. It is accepted that two consecutive doses of a mutant live Salmonella Typhimurium vaccine derived by application of transduction technology, will protect the immature intestinal tract from colonisation, if the environment of the pullet flock is contaminated at the time of placement. Tissue immunity in the intestinal tract can be enhanced by administering a vaccine containing a high concentration of antigen specifically formulated for pullet flocks. Live mutant SE vaccines are not yet available in the US. Administration of at least one dose of an oil-emulsion inactivated SE vaccine is now a routine practice. This is usually injected as a trivalent product incorporating antigen against Newcastle disease and infectious bronchitis. In cases where farms or complexes have a previous history of SE, more intensive vaccination programmes are followed. These might include two sequential oilemulsion vaccines at four-week intervals during rearing. This is costly both from the point of view of handling and administration and also due to the cost of the product (€65 per 1,000 doses). Extralabel administration of live mutant highantigen titer ST vaccine by the spray route is occasionally used in mature flocks especially prior to molt. Concurrent with control and prevention measures for SE at the farm, packing and distribution levels, the UEP and the USDA have implemented an extensive educational and awareness programme to prevent SE. Printed and video material has been directed to consumers and the operators of commercial kitchens and food service establishments. The National Restaurant Association and similar organisations have released manuals describing appropriate procedures to limit cross contamination during food preparation and to ensure that eggs are thoroughly cooked.
The impact of the FDA Final Rule on SE will only become apparent in 2015 when the epidemiologic results from 2012 and 2013 are collated and published. Unfortunately the incidence rate of SE in consumers is increasing although this does not appear to be associated with egg-borne transmission. Regrettably SE is emerging as a major problem in the poultry meat industry and the pathogen has also been detected in fresh produce. The impact of a major egg recall is confirmed by the episode in August 2010 after introduction of the Final Rule. Figure 1 depicts the epidemiologic curve for this outbreak. A large Midwest operation housing in the region of five million hens knowingly distributed contaminated eggs which resulted in a trace-back to the complexes of origin and eventually led to the depopulation and sale of the enterprise. The resulting adverse publicity caused a drop in purchase of generic product with a precipitous drop in price reminiscent of the Edwina Curry misstatement of Salmonella in the 1980s. It is estimated that the US shell-egg production industry collectively lost in the vicinity of €1.1 billion over the two month period following the outbreak. Consumption was restored when assurances were provided that the egg supply was free of this source of contamination and seasonal demand increased. The implementation of a comprehensive SE prevention programme has subjectively eliminated Salmonella Enteritidis infection as a deterrent to consumption of eggs. It is to the credit of the industry that the programme was rapidly introduced and deployed for the benefit of both producers and consumers.
Monitoring flocks for SE
The standard and approved sampling method in the US to detect environmental Salmonella Group D involves drag swab assays of manure in high-rise houses or from manure belts on compact and enriched systems or from litter in cage-free units. The procedures used to collect and transport swabs and their subsequent processing and assay by certified laboratories are clearly defined in the NPIP procedures manual. All samples are subjected to a two-stage enrichment process. Screening is generally accomplished using an immuno-based, solid-state lateral-flow
antigen capture strip. Under practical laboratory conditions the SDIX Rapid Check Select system has a sensitivity of 100%, specificity of 93%, a negative predictive value of 100% and a positive predictive value of 91%. This means that if suitably enriched samples are negative on the SDIX
screening assay the environment of the flock can be regarded as free of Salmonella Group D1. The actual status of the environment of a flock is however subject to inherent sampling error due to variables associated with the collection of swabs. Commercial laboratories in the US offer 48-hour turnaround times with release of results on a dedicated web-database. If the test strip discloses the presence of the target antigen the test is designated as “presumptive positive”. Confirmation of status (positive or negative for Salmonella Group D) follows by applying routine microbiology using the FDA-approved BAM procedure which requires an additional 7 to 10 days. Some laboratories, including the FDA still use time- consuming conventional BAM microbiology which delays release of results, imposes a cost of €65 per sample and may in fact be less sensitive and specific for the purposes of screening than immunobased systems which are offered at €15 per test.