Salmonella is a leading cause of food-borne illness in humans, and the world's food safety agencies are constantly searching for ways to reducing the incidence and severity of human cases through intensive monitoring and increasingly, through regulation. Significant progress has been made in recent years in the poultry industry, and the introduction of Salmonella vaccination for laying hens has made a significant contribution to cutting food poisoning from this source in countries where vaccination is routinely practised.

International focus is now turning to the pig industry, where there has been less research into various aspects of Salmonella sources and control than for poultry.

However, an important report has been published recently, investigating on farm-strategies against Salmonella on pig farms. Written by R.M. Friendship, A. Mounchili and S. McEwen of the Department of Population Medicine at the University of Guelph and A. Rajiæ of the Public Health Agency of Canada, it takes the form of a review of published research on the topic. With a refence list running to almost 200 papers, the authors looking at four particular areas:

• effectiveness of on-farm interventions against Salmonella
• evaluation of existing diagnostic tests for Salmonella
• review of studies on the cost-benefits of single interventions or Salmonella control programmes, and
• summary of existing scientific evidence associated with antimicrobial resistance and Salmonella in pigs.

Literature Review of Interventions

Friendship and co-authors found that of the 15 references on vaccination, the majority showed a beneficial effect of vaccine in reducing Salmonella shedding or other beneficial outcomes. Three papers referring to sodium chlorate also reported generally improved results, as did studies evaluating the efficacy of off-site weaning to control Salmonella.

Under the sub-category 'type of feed' (meal and pelleted feed), there were 14 related studies. Five of eight experiments assessing the effect of meal over pelleted feed reported favourable results. Liquid feed and coarse feed also tended to demonstrate positive outcomes.

Another intervention delivering generally positive effects against Salmonella was acidification of feed or water although the authors noted that the result were inconsistent.

The results for probiotics and competitive exclusion products were contradictory and inconsistent, they said.

The authors assessed 11 references on antibiotics, from which they concluded, "In general, antibiotics tend not to be useful and can be detrimental. The use of antibiotics likely promotes the selection for resistant serovars and under certain circumstances might lead to a more severe infection of S. Typhimurium."

Two studies suggested that depopulation/repopulation initially reduced Salmonella prevalence on the repopulated farm but eradication is unlikely as Salmonella can persist in the environment and be carried by other animals such as rodents.

The authors recorded that a few studies on novel strategies, e.g. egg-yolk immunoglobulin, spray-dried porcine plasma, prebiotics, essential oils and bacteriophages, appeared to be successful on a laboratory scale but they had doubts that the potential beneficial effects would be achieved under field conditions.

Evaluation of Diagnostic Tests for Salmonella

Traditional culture methods for isolating Salmonella take three to five days, wrote the Canadian group, but this method remains standard for detecting this organism. Indeed, it is the only method that allows the identification of definitive serovars and antimicrobial resistance profiles of isolated Salmonella. The sensitivity of the culture methods may be affected by the phase of the Salmonella infection: in acute cases, large numbers of Salmonella may be excreted in the faeces but in chronic cases and carrier pig, low numbers of Salmonella are excreted and only intermittently. A direct culture is only appropriate for clinical cases, while enrichment will be required for other cases.

Two selective enrichment methods dominate most epidemiological investigations for Salmonella, according to Friendship and co-authors. Culturing is the most specific diagnostic technique and can lead to further information including serotype, phage type, antimicrobial resistance pattern and molecular determinants. However, it may not detect Salmonella in the faeces of non-shedding carriers, and it is both labour-intensive and costly.

The Canadian group says that enzyme-linked immunosorbent assays (ELISA) can be used to detect either the organism or a humoral response to the organism. The latter is used in pigs to demonstrate previous and current exposure to this organism. As culture and ELISA tests measure different stages of Salmonella infection, these methods cannot be compared directly. However, the report states that the correlation between serology and Salmonella shedding has been demonstrated using experimental trials and in field studies.

Molecular diagnostics based on the polymerase chain reaction assays and genotyping have been developed recently to diagnose both clinical salmonellosis and sub-clinical infections in pigs. The report says that promising results have been obtained in laboratory studies.

Review of Studies on the Cost-Benefits of Single Interventions or Salmonella Control Programmes

* "The costs of on-farm intervention procedures were borne by the producers, and any benefits went to the consumers."

Turning their attention to cost/benefit aspects, the author comment on the overall lack of data on these aspects for the implementation of single or multiple interventions or control programmes against Salmonella in pigs, and say that more primary research is needed.

In one study evaluating the implementation of farm-to-abattoir interventions in the United States, it was concluded that interventions at the plant level are cheaper than those at the farm level.

Another study performed a cost-benefit analysis of Salmonella control strategies in Danish pork production and concluded that only hot-water decontamination at the abattoir was 'socio-economically profitable'. Those researchers drew attention to the fact that the costs of on-farm intervention procedures were borne by the producers, and any benefits went to the consumers.

Researchers from the Netherlands estimated that an acid mixture costs €2.49 per pig, while the pump and pipelines adds a further €0.41.

Finally, a Danish study estimates that the cost of a comprehensive integrated farm-to-abattoir control programme in Denmark originally cost the industry and government US$14 million per year. A recent revision to the programme lowered the annual cost to US$8.5 million but this was borne solely for the industry.

Summary of the Work Associated with Antimicrobial Resistance

Friendship et al. write: "It has been suggested that the use of antimicrobials in food animals may be associated with the increased faecal excretion of gram-negative enteric pathogens such as Salmonella, however, a systematic review of several challenge trials conducted in pigs between 1976 and 1988 did not find significant differences in Salmonella shedding between groups fed or treated with or without antimicrobials."

However, they say that the emergence and spread of Salmonella strains resistant to one or more antimicrobials are of increasing threat to human health and presumably to animal health. An infection with a multi-resistant strain of S. Typhimurium PT104 is of particular concern, and this and other resistant serovar have been found in pigs.

It is desirable from a food safety perspective to keep levels of resistance in the intestinal flora of food animals low. The report authors suggest on-going monitoring of antimicrobial use and antimicrobial resistance is necessary to evaluate the overall resistance trends and to detect emerging resistance phenotypes in swine and other livestock populations.

Recommendations

* "No single approach or combination of interventions can be expected to result in 100 per cent success."

Friendship and his co-authors found that some practices have shown benefits in reducing the prevalence and/or level of salmonella shedding. These include: vaccination, feeding coarse mash feeds or fermented liquid feeding, acidification of feed or water, strategic movement of pigs to off-site (or separate) weaning or finishing units along with good general hygiene. These should be considered for use in on-farm Salmonella control programmes.

There are other areas that may have potential but require research to support claims, say the authors, and there is a lack of studies on some potential risk factors for Salmonella infections in swine. Linked to general biosecurity, these include limiting visitors to the farm, changing clothes and boots for visitors, the use of foot-baths where necessary, and the control of pests, e.g. rodents, wild birds and other wildlife species. Purchasing replacement animals from fewer suppliers, and stocking density are also among this group of factors, which merit further investigations.

There is also limited information on on-going monitoring of antimicrobial use and antimicrobial resistance in pig production, and this area requires continuing support, say the report authors.

Friendship and co-authors highlight that there is a lack of data on the costs and benefits associated with the implementation of the interventions against Salmonella in swine at the farm level. More studies are needed that include formal economic analyses of interventions or control programmes for reducing Salmonella in pork or swine. The researchers point out that all economic models that evaluate interventions that prevent Salmonella shedding but have no impact on pig growth performance are unlikely to show an economic benefit unless the cost is shared by other sectors of the industry in addition to the pork producers.

Finally, the report says there is no guarantee of success with the application of any particular intervention on an individual farm. The impact of wide-scale adoption of an intervention strategy is also unpredictable in that no single approach or combination of interventions can be expected to result in 100 per cent success.

Further Reading

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You can view the full report by clicking here.

July 2009