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Assessment and methodological support
Scientific Opinion on a quantitative estimation of the public health impact of setting a new target for the reduction of Salmonella in broilers
This assessment relates the percentage of broiler-associated human salmonellosis cases to different Salmonella prevalences in broiler flocks in the European Union. It considers the contribution and relevance of different Salmonella serovars found in broilers to human salmonellosis. The model developed to provide quantitative estimates, which is based on the microbial-subtyping approach, considers 22 Member States, four animal-food sources of Salmonella (broilers, laying hens, pigs and turkeys) and 23 Salmonella serovars. The model (called the ‘Broiler Target Salmonella Attribution Model’ or BT-SAM model) employes data from the EU Baseline Surveys and EU statutory monitoring on Salmonella in animal-food sources, data on incidence of human salmonellosis and food availability data. It is estimated that around 2.4%, 65%, 28% and 4.5% of the human salmonellosis cases are attributable to broilers, laying hens (eggs), pigs and turkeys respectively. Of the broiler-associated human salmonellosis cases, around 42% and 23% are estimated to be due to the serovars Salmonella Enteritidis and Salmonella Infantis respectively, while other serovars individually contributed less than 5%. Different scenarios are presented showing changes in the percentage of broiler-associated human salmonellosis cases under different prevalences of Salmonella in broiler flocks. Compared to 2006, the 2009 Salmonella in broiler flocks prevalence has achieved a reduction of 69% in the number of broiler-associated human salmonellosis cases. When comparing the results of the adjusted prevalences for Salmonella Enteritidis and Salmonella Typhimurium as reported in 2009 with a theoretical combined prevalence of 1% for these two serovars, the difference between the percentages of broiler-associated cases is small. However, when adjusting the combined prevalence of all serovars to 1%, a large reduction in the percentage of broiler-associated cases compared to the one achieved with the two previous serovars only is expected. Uncertainty and data limitations are discussed, including recommendations on how these could be overcome.
© European Food Safety Authority,2011
Following a request from the European Commission, the Scientific Panel on Biological Hazards (BIOHAZ) was asked to deliver a Scientific Opinion on a quantitative estimation of the public health impact of setting a new target for the reduction of Salmonella in broilers. Specifically, EFSA was asked to assess the relative public health impact if a new target for reduction of Salmonella is set in broilers being 1% or less remaining positive for all Salmonella serovars with public health significance, compared to (1) the theoretical prevalence at the end of the transitional period (1% or less flocks remaining positive for Salmonella Enteritidis and/or Salmonella Typhimurium), and (2) the real prevalence in 2009 reported by the Member States (MSs).
For this task, the BIOHAZ Panel was supported by the work of a contractor that developed a source attribution model providing estimates for the quantitative contribution of broilers and other major animal-food sources to the estimated true burden of human salmonellosis in the EU. The model was based on the so-called microbial subtyping approach, which allows for distinguishing between the different serovars. The basic principle is to compare the serovar distributions observed in different animal-food sources with the serovar distribution found in humans. The full Technical Report submitted to EFSA by the contractor provides detailed information on the modeling approach and results.
The model considered the following data: (i) the EU-wide Salmonella Baseline Surveys on broiler flocks, broiler carcasses, turkey flocks and slaughter pigs, (ii) the results from the harmonised EU monitoring in broiler and laying hen flocks in 2009, (iii) the reported cases of human salmonellosis in EU in 2007 to 2009 by MSs as provided by the European Centre for Disease Prevention and Control (ECDC), and (iv) the amount of each food source available for consumption by MS as estimated from EUROSTAT data on production, import and export. The model included data from 22 MSs, four animal-food Salmonella sources (broilers, laying hens, pigs and turkeys) and 23 individual serovars. To take account for differences in underreporting of human salmonellosis cases between MSs, MS-specific underreporting factors were calculated and applied in the model. Some sources of Salmonella (e.g. cattle/beef) were not included in the model due to lack of data. The possible influence of this is discussed.
Initially, a model applying prevalence data from the Baseline Survey conducted in broiler flocks in 2005-2006 was developed. This model is referred to as the ‘Broiler Target Salmonella Attribution Model’ or BT-SAM model throughout the Opinion. In order to answer the Terms of Reference, different scenarios where Salmonella prevalences in broiler flocks were changed were developed and the results compared to the results of the BT-SAM model.
The BIOHAZ Panel concluded that based on the results of the BT-SAM model 2.4% (95% CI: 1.8-3.4) of all human salmonellosis cases (i.e. estimated true number of cases when accounting for underreporting) in the EU were attributed to broilers. Around half of the broiler-associated human salmonellosis cases were caused by serovars other than the currently regulated serovars. Salmonella Enteritidis and Salmonella Infantis constituted 42% and 23% of all broiler-associated cases respectively. Salmonella Hadar, Salmonella Typhimurium, Salmonella Kentucky and Salmonella Virchow constituted individually between 4% and 5% of all broiler-associated cases. Other serovars constituted less than 4% on an individual basis.
For the other Salmonella sources included, the model estimated that around 65% (95% CI: 63-67), 28% (95% CI: 27-30) and 4.5% (95% CI: 4-5) of the estimated number of human salmonellosis cases could be attributed to laying hens (eggs), pigs and turkeys, respectively. The results of the model indicate that the majority of the Salmonella Enteritidis infections are related to the laying hen reservoir (i.e. consumption of eggs), whereas Salmonella Typhimurium infections originate primarily from the pig reservoir.
The Panel concluded that the situation in 2009 already showed a considerable improvement compared to the results of the BT-SAM model with a reduction of 69% in the number of broiler-associated human salmonellosis cases compared to the situation in 2006. The Panel further concluded that: (1) Considering that the prevalence of Salmonella Enteritidis and Salmonella Typhimurium is as reported by the Member States in 2009 (but keeping the prevalence for the other 21 serovars as per the 2005-2006 Baseline Survey in broiler flocks), an estimated reduction in the number of broiler-associated human salmonellosis cases of 26% compared to the situation in 2006 is expected; (2) considering that the current target of the EU control programme of Salmonella in broiler flocks would be met (i.e. the combined prevalence of Salmonella Enteritidis and Salmonella Typhimurium being 1% or less), and keeping the prevalence for the other 21 serovars as per the 2005-2006 Baseline Survey in broiler flocks, an estimated reduction in the number of broiler-associated human salmonellosis cases of 25% compared to the situation in 2006 is expected; (3) Considering that an EU-wide target of maximum of 1% of flocks remaining positive for the all the Salmonella serovars considered in the model would be met, an estimated reduction in the number of broiler-associated human salmonellosis cases of 93% compared to the situation in 2006 is expected.
The Panel emphasised that the EU statutory monitoring in the Member States is likely to have a lower sensitivity in detecting positive flocks than the conducted EU-wide Baseline Surveys. For this reason, the estimated reductions in number of human salmonellosis cases are overestimated at the EU-level. Furthermore, the individual MS contributions to the estimated reductions vary greatly.
The BIOHAZ Panel finally concluded that the main factors contributing to the uncertainty of the model results beyond the statistical uncertainty are the lack of harmonised monitoring of human salmonellosis in the EU as well as the different levels of serovar detail reported in both the human and animal food source data. These uncertainties could not be statistically quantified with the model employed to support this Scientific Opinion.
The Panel makes a series of recommendations related to the establishment of active surveillance of human salmonellosis in all MSs, including efforts to quantify the level of under-ascertainment and underreporting. For both human and animal-food surveillance data it is recommended to make available more comparable Salmonella subtyping data for more accurate future modeling and trend analyses. In addition, it is recommended to repeat the subtyping modelling approach on a regular basis (i.e. every 3 to 5 years) in order to follow the progress of Salmonella control and the trends in the sources of human salmonellosis.
Salmonella, poultry, broiler, source attribution, microbial subtyping, targets