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

Background

An epidemic of Salmonella Brandenburg abortions in sheep in the South Island, peaking in 2000, 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. Similarly, the failure to identify any cases-control association with consumption of sheep meat suggests that a foodborne route of infection is not contributing significantly to the burden of S. Brandenburg cases.

What is Salmonella Brandenburg?

Salmonella Brandenburg is a member of the Salmonella family of bacterial pathogens. Like most Salmonella, it lives in the intestines of animals and people and is generally spread by contact with animals, contaminated environment, faecal contamination in food and water, or from person to person.

While Salmonella is a leading cause of food-borne illness in New Zealand, S. Brandenburg has, until recently, only infrequently caused human illness in New Zealand. Similarly, it was rarely isolated from farm animals despite being relatively common overseas, especially in pigs.

What’s new about this bug?

In 1996, a subtype of S. Brandenburg, now known as the "South Island strain", was identified as the cause of abortions in sheep in mid-Canterbury. Stock movements through drought in that year introduced the organism to South Otago, and it rapidly spread to epidemic proportions in South Otago and Southland. Since then, the South Island strain of S. Brandenburg has also been associated with diarrhoea in cattle and, more importantly, in calves.

For reasons as yet unknown, the South Island strain has not been associated with sheep infections north of mid-Canterbury and in the North Island, although has been isolated in the North Island from a single pig and occasionally in poultry feed.

What about animals?

Like most Salmonella, S. Brandenburg lives in the intestines of animals, especially pigs, although prior to 1996 had rarely been isolated from New Zealand farm animals.

The South Island strain of S. Brandenburg has most frequently been detected in adult sheep where it causes abortion. There is one report of stillborn pups in a dog.

Most recently, the South Island strain of S. Brandenburg has been associated with diarrhoea in cattle and bobby calves, and domestic animals such as dogs and horses are susceptible to S. Brandenburg. Of note is that lambs are apparently not susceptible to S. Brandenburg infection, and do not exhibit symptoms of animal salmonellosis.

Contact with farm animals and the environment is, therefore, a major source of risk on the farm. This is especially so for children who by nature come into close contact with bobby calves.

Can people get Salmonella Brandenburg?

Like all Salmonella, S. Brandenburg causes gastrointestinal illness in humans. Prior to 1996, cases of human S. Brandenburg gastoenteritis were reported infrequently and sporadically. However, a commensurate increase in human cases was associated with the southern sheep epidemic. Indeed, the peak of human cases occurred at exactly the same time (2000) as the peak in the number of farms in Southland confirmed as having sheep abortions due to S. Brandenburg.

The majority of the human cases have been attributed to contact with infected farm animals, including veterinarians, veterinary workers, sheep and cattle farmers, farm workers, and slink skin handlers. Not one case was reportedly food-borne, and indeed an NZFSA commissioned case-control study recently completed by ESR could not ascribe a single case to the consumption of food.

All cases were associated with farms and/or animal contact.

The greatest risk is for children exposed to calf-rearing and lambing operations, the former being of greatest risk as scouring in bobby calves is a common occurrence.