Salmonella control starts at slaughter

A major international meeting on food safety has delivered a take-home message that sets out new directions for the veterinary authorities of the world who want to reduce the extent of Salmonella contamination in pork. Rather than continue to focus intensively on infections of the bacteria at pig units, according to keynote presentations at this meeting, the more successful of the national plans to control Salmonella from a food safety aspect would be most likely to achieve an additional reduction by directing their attention to the point of slaughter.

This is not to say that pre-harvest intervention on the farm has outlived its usefulness. Especially those countries with relatively new control programmes and a high level of prevalence for the bacteria in their herds were urged to keep their farm-level actions. A comprehensive study in Canada has demonstrated that intervening on farms to decrease the number of serologically positive pigs can still be extremely valuable in obtaining fewer contaminated carcases. Compared with those from non-infected herds, carcases from units with over 20% of pigs positive for Salmonella were 5 times more likely to be contaminated.

These signals were highlights of the latest SafePork symposium, sixth in a series started 10 years ago and designed to showcase new knowledge about current and emerging food-borne pathogens in pigmeat worldwide. Previous editions in the series had looked mainly at control in the pre-harvest period, meaning the part of the food chain from feedmill to farm. But the one held in California, USA, towards the end of last year widened the reach to post-harvest policies in the time from the animal's arrival at the abattoir or processing plant.

Most especially, it highlighted North American and European evidence that the most cost-effective way of beating Salmonella is to practise slaughterhouse controls possibly even without accompanying on-farm work. This approach contrasts most obviously with the situation that has applied in the European Union. There, legislators have rushed to emphasise farm-level control, having been motivated mainly by the success of the poultry sector in reducing a cascade of infection from breeding stock at the grandparent and parent stages.

The published proceedings of the *SafePork 2005 symposium have now been issued and provide several examples of the pig sector's shift in focus to slaughterhouses. One is in a paper by governmental agri-food safety specialists from Alberta in Canada, who describe their monitoring of Salmonella contamination at 5 points in a Canadian pork plant. They had determined that Salmonella bacteria could be isolated from all 31 of the pig units they sampled and this was later confirmed when relatively high levels of infection were found in the caecal contents of over half of the pigs entering the factory. Following animals from those units through the plant, however, it emerged that the length of time they were held in the lairage before slaughter seemed more important than the Salmonella status of the pig on arrival as a risk factor for contamination of carcases.

Heavily contaminated lairage pens have been blamed previously for adding to the rate of infection in pre-slaughter pigs. Various counter-measures are sometimes proposed. At the symposium, Iowa scientists from the USA discussed cutting out exposure inside the lairage by keeping the pigs in their transporter at the abattoir until they could be slaughtered. In the Canadian case, though, batches were not mixed when they arrived and the holding pens themselves had been washed and disinfected. Bacterial transfer in this instance may therefore have been due to a failure to dry the pens completely before re-stocking them, faults in the surface of the concrete floor, or the use of common races (chutes) to take the pigs to the stunning area.

The other assessment points for the Alberta team involved swabbing carcases before evisceration and in the chilling cooler. Results from the cooler were the most significant of any found by the study, because they represented a sharp contrast to the earlier findings. Where the signs before had amounted to a classic picture of infected pigs leading almost invariably to contaminated carcases, now the cooler gave an opposite impression. Only 2 out of 429 carcases tested positive, a rate of just 0.5%.

It is an extremely low prevalence, the paper in the symposium proceedings notes, particularly when compared with the high rate found in incoming pigs. The most likely conclusion seems to be that processing interventions at the plant were successful at minimising the level of contamination present on carcases.

By extrapolation, this would be where to start a Salmonella reduction process with the greatest chance of early success. It is a conclusion that coincides with remarks to the meeting by Danish meat scientists. Denmark introduced a programme of surveillance and control on pig finishing units in 1995, they pointed out. This initial concentration on pre-harvest measures has been successful in that the number of human salmonellosis cases attributable to pork has declined. The prevalence rate of Salmonella in pork has also dropped markedly from about 7% in 1993. Since 2001, however, it has stayed constant at around 1.4%.

The Danes contend that this situation would hardly change even if the most infected units were somehow excluded from production. In fact, they believe that no single measure aimed at primary production has the capacity to reduce prevalence to the maximum of 1.2% that has been set nationally as a goal for 2006. Further pre-harvest initiatives would not be cost-effective compared with measures targeted at slaughterhouse events after harvesting the pigs.

A simulation model of the pork chain from pig unit to slaughterhouse has been constructed to probe the effect of different intervention measures. This model shows that a typical Salmonella prevalence within the range of 5-10% in slaughter-weight pigs at the time of loading on the farm can rise nearer to 15% when the same animals are unloaded at the abattoir, because of exposure during transport. Then it reaches a maximum of nearly 20% after their stay in the lairage awaiting slaughter. A rapid reduction to only 1-2% may occur at singeing of the carcase. Although subsequent events such as evisceration tend to produce small increases afterwards, the simulation still proposes a rate of under 5% at the stage of chilling.

Extra slaughterhouse hygiene and the use of decontamination procedures have been identified in Denmark as offering the most promise of moving the national prevalence rate in pork towards the 1.2% goal. Carcases could be decontaminated using hot water, but that would be costly in water usage. While the use of steam on critical parts of the carcase would be slightly less decontaminating, it is regarded as an attractive alternative from a cost viewpoint.

*SafePork 2005, proceedings book of 6 international symposia on the epidemiology and control of foodborne pathogens in pork, National Pork Board, USA