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Quantitative Risk Assessment (QRA) of Salmonella in Sheep Meat Produced in New Zealand

Specific research projects funded by MIRINZ Inc . (Meat NZ & NZ Meat Industry Association) & NZFSA

A quantitative risk assessment model for Salmonella in sheep meat produced in New Zealand (NZFSA, 2000-2003)

The outcomes of the research project “A quantitative risk assessment of Salmonella in sheep meat produced in New Zealand” are dependent on development of a quantitative risk assessment (QRA) model to estimate risks to human health under varying conditions. QRA models provide a description of all pathways for contamination of food from farm-to-plate, and allow risk management strategies to be put in place that are solidly backed by science. NZFSA (previously MAF Food) will develop a QRA model over a period of 3 years that provides the basis for effective risk management of Salmonella in sheep meat according to specific stakeholder goals. The project will:

• Compare the effectiveness of different sanitary measures applied at different steps in the food chain in negating market access / compliance problems and reducing risks to human health (including the value of microbiological monitoring programmes), and select optimal measures on this basis.

• Develop cases for judgement of the equivalence of sanitary measures applied in New Zealand that may be different to those that are applied in importing countries and that are a condition of market access.

• Provide information to assess the relative importance of sheep meat as a vehicle for food-borne salmonellosis compared with other foods.

• Provide a QRA platform for other microbial hazards transmitted by faecal contamination of fresh carcasses e.g. S. Brandenburg in bobbies, E. coli O157:H7 in adult cattle, and Campylobacter.

Retrospective case-control study of factors associated with outbreaks of Salmonella Brandenburg abortion (Massey University EpiCentre, 2000-2001)

There has not been any systematic epidemiological investigation of S. Brandenburg abortions on sheep farms. This project will provide the first information on factors associated with the occurrence of outbreaks on farms. The project will compare, on a flock, farm and regional basis, environmental, management and animal factors in clinically affected (S. Brandenburg outbreak during the 2000 season) and unaffected flocks. The results will be used to identify management interventions to reduce the risk of disease.

Pilot studies of Salmonella Brandenburg prevalence in lambs and ewes (Massey University EpiCentre, 2000-2001)

The project will compare the prevalence of Salmonella in groups of ewes and lambs on farms that experienced clinical outbreaks of S. Brandenburg abortion with that on farms in the same region that had no disease outbreak during the 2000 season. Faecal samples on-farm and caecal samples after slaughter will be collected in order to evaluate the effect of pre-slaughter transportation and lairage on Salmonella carriage rates.

Retrospective case-control study of strip-grazing use in farms associated with outbreaks of Salmonella Brandenburg abortion (Massey University EpiCentre, 2001-2003)

The 2000 studies indicated that the practice of strip-grazing was the most important individual risk factor associated with the risk of disease outbreaks. However, the results also indicated that many flocks using strip grazing remained unaffected through the course of the regional epidemic. The project will compare on a flock, farm and regional basis, environmental, management and animal factors in clinically affected (S. Brandenburg outbreaks during the 1999 and 2000) and unaffected flocks (no outbreaks in 1999 and 2000) that practice strip grazing in order to identify additional factors that may explain the patterns of disease observed.

Association of severity of disease to carcass microbiology (Massey University EpiCentre, 2001-2003)

The 2000 “targeted” studies indicated that the occurrence of disease on farms was a determinant of risk of carcass contamination. Hence, the results of carcass sampling should be a reflection of disease status in the flocks from which the carcasses were derived. As the disease profile of supplying flocks is prone to variation over time, it is useful to retrospectively determine the prevalence and severity of clinical disease on farms supplying carcasses to the “random” carcass surveys in the 2001 and 2002 seasons.

Evaluation of a flock-screening test for confirming S. Brandenburg infection (Massey University EpiCentre, 2002-2003)

A flock-screening test based on bacteriological culture of vaginal swabs from aborted ewes is to be evaluated for confirming S. Brandenburg outbreaks in the wake of an outbreak of abortions. In addition, active surveillance, in collaboration with regional veterinary practices and LabNET, Invermay, is to be carried out to obtain improved data on the temporo-spatial pattern of the 2002 epidemic and to facilitate selection of study farms during 2003.

Enhancement of disease surveillance in collaboration with veterinary practices in the southern region (Massey University EpiCentre, 2002-2003)

Improved disease surveillance was identified as a necessary development in the original Gore meeting on the QRA project to facilitate diagnosis of affected flocks and gain insight into the nature of the epidemic. This project will encourage improvements in surveillance and reporting in the epidemic area through liaison with veterinary practices, laboratories and experts. Practices will be encouraged to maintain logs of clients reporting suspected outbreaks and funds budgeted for tests will be allocated to support laboratory diagnosis of suspected outbreaks.

Salmonella Brandenburg in sheep faeces (ESR Food, 2003-2004)

The goal of this project is to determine the prevalence and levels of Salmonella and E. coli (generic) in faecal pellets from clinically normal ovines from farms with S. Brandenburg abortions immediately prior to or after slaughter.  The objectives of this project are to quantify the exposure of the ovine carcass to S. Brandenburg, and to determine if there is proportionality between Salmonella and E. coli levels in faeces such that the relative faecal contamination of different locations on the carcass, as judged by E. coli levels, can be used to predict the distribution of Salmonella. 

Validation of methods for quantitative detection of Salmonella in Sheep Meat (AgResearch/MAF Food, 2000-2001)

The selection of standardised sample collection and analytical methods for the QRA programme has been rigorously and openly discussed through the TCG. A preliminary study to assess the effect of 72h refrigeration in pre-enrichment broth prior to quantitative analysis and the comparability of cultural and PCR-based methods is required. Similarly, it is necessary to perform an ongoing, although limited, validation study of the procedures for pre-enrichment of faecal/caecal samples.

Survey of premises processing characteristics (AgResearch, 2000-2002)

Industry will be surveyed and a database maintained of process characteristics of different sheep slaughter premises in New Zealand. Industry will be asked to make the information available to the QRA programme on a strictly confidential basis. A postal questionnaire will be utilised to document and categorise the process characteristics should this information not be made available, or if supplementary information is required.

Establishment of an estimate of the prevalence and concentration of Salmonella on freshly pelted carcasses and boned cuts on a national basis (AgResearch, 2000-2003)

A series of targeted and random carcass and cut sampling surveys for S. Brandenburg will be carried out on representative sample groups from (a) farms reporting abortions in the previous season (b) farms not reporting abortions in the previous season and (c) farms never reporting abortions. Samples will be collected at the start of the processing season (closest to the period of greatest number of abortions) and later in the processing season. Samples will be collected from animals from farms within the epidemic area (generally Southland) and outside the epidemic area (generally the North Island). A component of this study will be the determination of Salmonella contamination rates on associated pelts. These studies will be integrated with the on-farm, and other, Salmonella research projects and the National Microbiological Database.

Microbiological mapping of the surface of lamb carcasses (AgResearch, 2000-2001)

In order to model the numbers of Salmonella actually consumed on individual cuts of lamb, this project will map the distribution of E. coli (location and relative numbers) on lamb carcasses after processing using inverted dressing. Assuming that the sources of Salmonella and E. coli contamination are the same (i.e. indirect faecal via carcass/fleece contact) then the distribution of the two organisms has a high probability of similarity and can therefore be inferred.

3-D Interactive display of microbial contamination (AgResearch, 2003-2004)

An epidemic of Salmonella Brandenburg abortions in sheep in the South Island and consequential increase in human cases and detections on sheep meat resulted in a "Quantitative risk assessment of Salmonella in sheep meat produced in New Zealand (Sal-QRA)". The Sal-QRA programme has identified the prevalence and levels on carcass meat of Salmonella after slaughter, and demonstrated a significant decrease during processing. While preliminary interpretation of the Sal-QRA model suggests that a foodborne route of infection is not contributing significantly to the burden of S. Brandenburg cases, more accurate modelling of carcass contamination is required to provide a definitive answer. The goal of this project is to overlay accurate anatomically referenced descriptions and data from previous carcass microbiological surveys onto a generic 3-D lamb carcass model using a texture mapping technique. The 3-D lamb carcass model will take the form of a stand-alone executable that will allow the user to display and hide various elements of the model, to move the model around in 3-D space, and to show data and experimental designs in contextual drop-down boxes. The objective of this project is to graphically display microbial contamination of ovine carcasses in a 3-D interactive graphical model.

Pelting aerosols as a cause of carcass and cross contamination (AgResearch, 2002-2003)

This project evaluates the contribution of aerosols generated during depelting to cross-contamination of carcasses with S. Brandenburg. Evidence to hand points to the fleece as being a major immediate source of carcass Salmonella contamination. Direct fleece-carcass contact certainly occurs in the vicinity of opening cuts, and indirect contamination occurs through fleece-hand-carcass contact or carcass-hand-carcass redistribution. However, it does not explain contamination of carcass sites where such direct or indirect (contact does not occur. Low levels of contamination of the carcass being “pelted” or neighbouring carcasses on the chain could result from aerosol formation and fallout. A knowledge gap presently surrounds the question of in-plant aerosols that may impact on both the Risk Assessment Model and the requirement, or otherwise, for antimicrobial interventions.

Effect of chilling on Salmonella viability and recoverability (AgResearch, 2001-2003)

This project examines the reduction of Salmonella numbers over the chilling process by experimental means, in particular, relative effect of the sampling methodology itself, temperature, drying and induction of the viable but non-culturable (VNC) state. Should the results point to a VNC concern, then that concern must be further evaluated and the relative risk to human health of infection by normal and VNC organisms investigated. Meat surface pH is but one of the variables of interest with respect to both pathogen survival and induction of the VNC state.

Water activity on the surface of lamb carcasses during chilling (AgResearch, 2003-2004)

The Sal-QRA programme has identified the prevalence and levels on carcass meat of Salmonella after slaughter, and demonstrated a significant decrease during processing, in particular during chilling. While, the reasons for this decrease are not fully understood, it is probable that drying on the carcass surface would not only prevent microbial growth but also cause microbial death. However, until recently, measurement of the water activity (Aw) on carcass surfaces has not been possible. The goal of this project is to use the method of Lovatt and Hill (1998) to measure the Aw of lamb carcasses during chilling. The apparatus designed for beef carcasses will be assembled, programmed for lamb chilling, calibrated using representative gels, and used to determine the Aw at sites of greatest microbial contamination on the carcass. The objective of this project is to measure the Aw of the surface of lamb carcasses during chilling to enable more accurate modelling of the growth and death of S. Brandenburg on the surface of lamb carcasses during chilling.

Consumer/health component of a quantitative risk assessment of Salmonella spp. in sheep meat produced in New Zealand (ESR Epidemiology, 2002-2003)

This project includes a detailed epidemiological description, using combined laboratory and notification data, of human salmonellosis in New Zealand for the period 1995-2002. Data on isolates referred to the enteric reference laboratory from animal health laboratories will also be analysed. A literature review will be carried out to estimate the degree of under-ascertainment of the Salmonella surveillance system. Finally a national case-control study will be carried out to estimate the contribution of food, water, environmental, animal and human exposures tot he incidence of S. Brandenburg.

PFGE analyses of Salmonella Brandenburg isolates from non-epidemic regions (ESR Communicable Diseases, 2003-2004)

While the epidemic of S. Brandenburg has been restricted to the South Island, human cases of S. Brandenburg have occurred in the North Island. The pulsed field gel electrophoresis (PFGE) DNA profile of the South Island strain is distinctive, and it is not known if the North Island human cases are this PFGE sub-type or the more ubiquitous sub-types. This project will PFGE type a selection of isolates of S. Brandenburg from human cases (1996-2003) from geographic locations of New Zealand outside the epidemic area, PFGE type all isolates of S. Brandenburg from the ESR case/control study, and use the National Typing Database to identify clonal lineages of S. Brandenburg. The objective is to provide an authoritative overview of the population structure of the “South Island” clonal lineage S. Brandenburg in NZ, especially outside the epidemic region, and to identify possible epidemiological relationships and potential transmission routes given that sheep meat from the epidemic area is transported to, sold, and consumed in the North Island.