EU Scientific Risk Assessment of Salmonella in Slaughter and Breeder Pigs

June 2, 2010 — 

Following a request from the European Commission, the Panel on Biological Hazards was asked to deliver a scientific opinion on a Quantitative Microbiological Risk Assessment (QMRA) of Salmonella in slaughter and breeder pigs. The assessment would provide the input for a future cost/benefit analysis of setting a target for reduction in slaughter pigs at EU level. EFSA commissioned a QMRA modelling the pig meat food chain from farm to fork. The QMRA model was based on input data from the baseline studies of Salmonella in breeder and slaughter pigs, and other relevant data. The QMRA represents a major step forward in terms of modelling Salmonella in pigs from farm to consumption as it takes into account the variability between and within EU Member States (MSs). Transmission of Salmonella was analysed using the individual pig as the unit of interest. There are data gaps and critical assumptions in the model, and these should be carefully considered when interpreting the results of the model.

The fraction of human salmonellosis cases attributable to Salmonella in pigs and pig meat will vary considerably between MSs and will mainly depend on i) the Salmonella occurrence (prevalence and numbers) in pigs and pig meat, ii) consumption patterns and preferences and iii) the relative importance of other Salmonella sources. From the descriptive and comparable analysis of the serovar distribution in animal sources and humans, a cautious assessment would be that around 10-20% of human Salmonella infections in EU may be attributable to the pig reservoir as a whole.

However, the use of this estimate necessitates caution due to the lack of MS-specific data on the distribution of serovars in humans. From the QMRA analysis it appears that an 80% or 90% reduction of lymph node prevalence should result in a comparable reduction in the number of human cases attributable to pig meat products.

Breeder pig herd prevalence is a major determinant of slaughter pig lymph node prevalence at EU level. The importance appears to be more obvious in high prevalence countries as a 90% reduction of the breeder pig herd prevalence could theoretically result in a reduction in an order of magnitude of two thirds of slaughter pig lymph node Salmonella prevalence. The major sources of infection for breeder pigs are the same as for slaughter pigs; infected incoming pigs and Salmonella contaminated feed, plus other external and internal sources. Salmonella control in breeder pig farms need to focus on the following key control measures (1) control of Salmonella in nucleus and multiplier herds; (2) control of Salmonella in incoming pigs (knowledge of Salmonella status); (3) control of Salmonella in feed; and (4) biosecurity programs should include the control of Salmonella.

To achieve control of Salmonella in slaughter pigs the two major sources should be controlled: Salmonella-infected breeder pig herds, and Salmonella-contaminated feed.

Theoretically, according to the QMRA following scenarios appear possible (a) by ensuring that breeder pigs are Salmonella-free a reduction of 70-80% in high prevalence MSs and 10-20% in low prevalence MSs can be foreseen; (b) by feeding only Salmonella-free feedstuffs, a reduction of 10- 20% in high prevalence MSs and 60-70% in low prevalence MSs can be foreseen; and (c) by preventing infection from external sources of Salmonella (i.e. rodents and birds) a reduction of 10- 20% in slaughter pig lymph node prevalence can be foreseen in both high and low prevalence MSs. A hierarchy of control measures is suggested - a high prevalence in breeder pigs needs to be addressed first, followed by control of feed and then control of environmental contamination. Also according to the QMRA, for each MS, a reduction of two logs (99%) of Salmonella numbers on contaminated carcasses would result in a 60-80% reduction of the number of human salmonellosis cases attributable to pig meat consumption. A reduction of one log would result in a 0-40% reduction of human cases. This could be achieved through measures preventing direct and/or indirect faecal contamination during transport, lairage and, particularly, slaughter and dressing processes; and/or by effective carcass decontamination.

Control of Salmonella in pig meat as a public health problem should be based on the individual MSs situations and include combinations of following interventions: Salmonella-free (low risk) breeder pigs, Salmonella-free feed, cleaning-disinfection between batches both on-farm and during lairage, avoidance of faecal contamination during slaughter and decontamination of the carcasses. Efficient vaccination will also be useful to control Salmonella on farm, but might interfere with the interpretation of serological test results in monitoring/surveillance programmes. The QMRA results could give some guidance on appropriate combinations. From the current evidence, it would appear that specific slaughterhouse interventions are, at present, more likely to produce greater and more reliable reductions in human illness, at least in a shorter timeframe, than can be achieved at the farm in high prevalence MSs. However, the hypothetical reductions and multiple interventions investigated with the current risk assessment model suggest that MSs can achieve more effective reductions in human cases by targeting both farm and slaughterhouse. MSs should have the possibility to assess their national pig meat food chains using this QMRA model.

The slaughterhouse remains a critical step of the pig meat chain in respect to pig and carcass contamination and numerous aspects (e.g. airborne transmission of Salmonella in the abattoir) still remain unknown. Therefore studies need to be performed to properly assess the ways carcasses become contaminated.

The control of Salmonella in pig reservoir in the EU is a reasonable objective. The EU Salmonella control strategy in pigs should be continuously evaluated to identify possible improvements. EFSA Panel on Biological Hazards; Scientific Opinion on a Quantitative Microbiological Risk Assessment of Salmonella in slaughter and breeder pigs. EFSA Journal 2010; 8(4):1547. [80 pp.]. doi:10.2903/j.efsa.2010.1547. Available online: http://www.efsa.europa.eu European Food Safety Authority (EFSA), Parma, Italy