How has Salmonella prevalence changed in poultry since the implementation of Hazard Analysis and Critical Control Point regulations by the Food Safety and Inspection Service in 1998? Nobody can say with assurance.

Comparing test results over time demands continuity in testing methods. Unfortunately, the lack of continuity in regulatory sampling of raw meat and poultry makes it impossible to say with any confidence exactly how Salmonella prevalence has changed in the last 15 years.

“First, it should be possible to compare the results of the new baseline to the old baseline to determine if the situation is improving, worsening, or staying the same,” according to “Scientific Criteria to Ensure Safe Food,” an expert report produced by the Institute of Medicine and National Research Council in 2003. The Food Safety and Inspection Service conducted baseline studies in the 1990s that were used to set Salmonella performance standards for raw meat and chicken. More recent baseline studies have been compared to the previous results even though the methodology changed.

Sampling methods changed  

The accompanying table shows the sampling methods used by Food Safety and Inspection Service to test chicken carcasses for Salmonella. In the broiler chicken baseline sampling in 1994 and 1995, for example, chilled carcasses were removed from processing plants and were shipped overnight to Food Safety and Inspection Service labs where the carcasses were rinsed in Butterfield’s phosphate diluent on the following day, followed by incubation of a portion of the recovered rinse. In the broiler chicken baseline studies conducted in 1999-2000 and 2007-08, however, Food Safety and Inspection Service rinsed chilled chicken carcasses in buffered peptone water in processing plants and a portion of the recovered rinse liquid was then shipped overnight to labs where the samples were incubated on the following day. The differences between the methods were length of time after chilling before the rinse was done (0 versus ~24 hours), diluent used (BPD versus BPW), and length of time between rinsing and incubation (0 versus ~24 hours).

USDA’s National Advisory Committee on Microbiological Criteria for Foods has stated that Food Safety and Inspection Service “conducts extensive validations prior to implementing new methods," but nothing has been published to justify comparisons of results obtained with such different methods. Salmonella cells that were shipped to the lab on chicken carcasses might have been better protected from pH stress due to the additional buffering capacity of the skin and carcass. There should have been more nutrients available as well, with a possible effect on cell survival, as compared to cells removed from carcasses and shipped in buffered peptone. In addition, some more recently approved antibacterial chemicals do not rely on pH effects alone and may not be adequately neutralized by buffered peptone water, so some cell injury and death could occur during overnight shipment rather than in the processing plant where effects are assumed to occur.

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Screening method added  

Besides the change in the rinse liquid and time and place of sampling, a screening procedure was added in the 2007-08 chicken baseline study. Samples were not subjected to cultural testing for Salmonella unless a PCR-based assay indicated first that a sample was Salmonella positive. Testing positive by two methods rather than one is usually a more demanding requirement for a sample to be considered positive.

Salmonella sampling under chicken HACCP is usually compared to the 20 percent prevalence rate found in the 1994-95 baseline, so changes in HACCP methodology compared to that baseline are important. Chicken HACCP testing has always been based on chilled carcasses rinsed in buffered peptone water in the processing plant, with recovered rinse liquid shipped overnight to Food Safety and Inspection Service labs. PCR-based screening of HACCP samples was started in 2003, with an immunological screening method used for some time before 2003. As with the case of comparison between different baseline studies, comparison of HACCP results with the original 20 percent baseline prevalence, or comparison with other HACCP years with different methods, has uncertain validity. Despite occasional disclaimers, FSIS makes those comparisons routinely.

Office of Risk Assessment and Cost-Benefit Analysis  

At about the same time that the Pathogen Reduction: HACCP Final Rule was issued in 1996, USDA created the Office of Risk Assessment and Cost-Benefit Analysis to analyze the impact and cost of major new regulations. Few people seem to know that the first regulation analyzed by the office was the new PR:HACCP rule, but the review is a fascinating document that contains a lot of interesting commentary on the Food Safety and Inspection Service version of HACCP for raw meat and poultry.

The first issue listed by the review panel was the assumption that baseline levels of E. coli and Salmonella could be used to set detection standards that would be used for samples using a different methodology. The review panel wrote, “There are major differences in the carcass sampling method used in the national baseline survey and the method carried out at the plant [referring to HACCP samples].” The Office of Risk Assessment and Cost-Benefit Analysis was also concerned about differences in measurement errors between the two methods and the implication for control charts.

Previous articles in this series have discussed the seasonality of Salmonella in poultry and the interaction of seasonality with the irregular sampling schedules used by Food Safety and Inspection Service. Changes in sampling and lab methodology cause even more uncertainty in trying to compare Salmonella prevalence in chilled chicken carcasses over the 15 years that Food Safety and Inspection Service HACCP has been in operation.