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Risk Informed Management of Salmonella in Deep Tissue Lymph Nodes

20 April 2013

The removal of deep tissue lymph nodes at the processing plant is time-consuming and may not significantly reduce Salmonella contamination in ground pork, reports Scott Hurd, Associate Professor at Iowa State University.

The objectives of this project were to build and parameterise a quantitative risk model of the Salmonella prevalence along the pork production chain, and therefore to apply the model in evaluating the relative impact of potential Salmonella source, especially lymph nodes, to human food-borne risk.

Attention has turned to the potential negative role of deep tissue lymph nodes in Salmonella contamination in pork products, such as ground pork. Previous research has shown a low likelihood of recovery from non-visceral lymph nodes (deep tissue). The occasional recovery of Salmonella from these tissues does not necessarily represent a significant human illness risk. However, needed is a method that could assist decision makers in quantitatively evaluating the relative impact deep tissue lymph nodes on Salmonella contamination in pork products, thereafter the impact on public health. Without a firm understanding on the impact, regulators may enforce further control measures to remove deep tissue lymph nodes during swine carcass harvest, which might actually a waste of time and money.

Quantitative risk assessment is an approach that is able to quantify and compare the impact of potential risk factors on outcomes of interest. The author and his team developed a risk assessment model quantitatively describing the Salmonella distribution and dissemination from chilled swine carcasses to fresh ground pork serving. Deep tissue lymph nodes were simulated in the model as a potential source of Salmonella contamination in ground pork. The other targeted source is contaminated carcass surface. A scenario analysis was used to estimate the reduction in Salmonella contamination of ground pork if deep tissue lymph nodes were removed and if carcass surface was decontaminated. In the scenario analysis, the amount of deep tissue lymph nodes and the proportion of contaminated carcass surface included in ground pork were tentatively modified. The difference in risk of Salmonella contamination in ground pork were then estimated between the baseline and modified levels.

When the amount of deep tissue lymph nodes were changed from baseline value to zero, the probability of Salmonella contaminated ground pork servings only reduced from 8.3 per cent to 7.9 per cent. This reflects a situation if the deep tissue lymph nodes would be completely removed in processing plant.

If the intervention of deep tissue lymph nodes is the one to consider for the future to provide a high level of ground pork safety, the questions are how likely is the intervention to occur, and at what cost?

In practice, the complete removal of deep tissue lymph nodes could be very time- and labour-consuming. However, when the probability of Salmonella contamination on carcass surface at processing plant reduced from baseline value to its half, the probability of Salmonella contaminated ground pork servings would reduce from 8.3 per cent to 6.6 per cent and even below 2.8 per cent if carcass surfaces were completed decontaminated.

The findings reveal that deep tissue lymph nodes have non-significant impact, and Salmonella contamination from carcass surface has a more important influence on Salmonella contamination in ground pork, concludes Dr Hurd. Therefore, compared to intervention strategies, such as mitigation of Salmonella on carcass surface, the intervention of deep tissue lymph nodes at processing plants might not effectively reduce the Salmonella contamination in ground pork.

March 2013

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