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

Salmonella under the Microscope

(Article in Food New Zealand, Oct/Nov 2002)

A major three-year research study examining the scope and spread of a strain of Salmonella in sheepmeat is underway and, it is hoped, will give a model for dealing with other food-borne pathogens in the future.

Expected to be completed later this year, the $1.9 million Salmonella quantitative risk assessment (QRA) has been looking particularly at the Brandenburg strain of Salmonella which has been causing abortions in sheep in the lower South Island. S. Brandenburg can be transmitted to humans. Farmers and veterinarians who are in close contact with affected animals are particularly at risk and the study also aims to assess food-borne routes for human infection.

Coordinated by the New Zealand Food Safety Authority (NZFSA) with majority sponsorship from Meat New Zealand, researchers from the NZFSA, Massey University, AgResearch and the Institute of Environmental Science & Research Ltd (ESR) are working on the project which covers four points in the food chain: on-farm, processing, retail and consumer.

NZFSA project coordinator Dr Roger Cook explained that the project was kicked off in May 2000 with a meeting of all the stakeholders in Gore and research started in November 2000: “Results modelled in the QRA will establish the food-borne risk from the pathogen and also identify strategies for dealing with it,” he said.

Early results show that environmental management factors on-farm may play an important role. In particular, strip-grazing with back-fencing is showing a possible contributory factor and – the most common infection method for sheep - inhalation of Salmonella bacterium, may lead to minimum yarding times being proposed. Improvements are already being seen in on-farm infection rates, possibly as a result of improved management information being fed back to farmers.

The good news for the meat industry is that initial results have shown a substantial decline in positives during processing, from slaughter through cooling to boning. This might be due to a redistribution effect during boning or the drying effect of the chillers, which either affects the viability of the pathogen or increases the adhesion of it, making it harder to remove for sampling. Further research is being done this year to clarify reasons for the observed decrease and to establish ways to reduce the prevalence and level of Salmonella even further.

“While the very low prevalence of positives found at retail (2.8%) in New Zealand suggests that the risk to the New Zealand consumer, and indeed those in our export markets, is low, especially since the sheepmeat will usually be cooked prior to consumption,” Roger Cook said "confirmation of such awaits the results of analyses of product movement from the affected regions to the rest of New Zealand and ongoing human case control studies that identify, or not, any contribution from sheepmeat.