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Alan Doyle--Salmonella infections in pigs are largely asymptomatic.
Having successfully focused on the poultry sector, the European Food Standards Agency, EFSA, is now targeting pigs as it estimates 10% to 20% of human cases of Salmonellosis can be attributed to pig meat consumption.
Although Campylobacter is the most common bacterial food-borne pathogen, Salmonella remains high on the list of pathogens that most concern international food standards agencies, food retailers and consumers.
Salmonella positive pigs
Many pigs are Salmonella-positive at slaughter, as shown in EFSA’s baseline prevalence data (see Table 1), but at farm-level even greater numbers are Salmonella-positive (see Table 2 and Table 3)
In June 2010, an analysis of the costs and benefits of setting a target for the reduction of Salmonella in slaughter pigs was published. It came as little surprise to pig producers and veterinarians that the report concluded, “the cost-benefit analysis did not show an economic benefit from any of the intervention scenarios,” as they have been fighting an ongoing battle against Salmonella for many years.
The report also found that: “There is some economic rationale for a gradual introduction of Salmonella control measures, starting with the establishment of surveillance measures. Further interventions would be targeted according to the surveillance results.”
Surveillance measures are already in place across much of the EU, but judging from the EFSA prevalence data, these have done little to reduce the incidence of Salmonella in those countries. It could be argued, however, that this is because of insufficiently strong financial sanctions against “dirty” pig producers and insufficient financial incentives for “clean” farmers.
Contaminated pig feed
Salmonella infections in pigs are largely asymptomatic, with a high proportion of infected pigs becoming carriers and intermittent excretors of Salmonella in their feces. Preventing the contamination of farms with viable and persistent Salmonella burdens from outwardly ‘healthy’ pigs is difficult to achieve. For many farms, controlling the horizontal spread of infection (often in the grower phase of production) is a Sisyphean task.
What is becoming clear is that on-farm interventions will need to be tailored to suit each farm’s specific conditions.
As we have learned from the poultry industry, once breeding (and laying) stock were Salmonella-free and excellent biosecurity was maintained, the most effective way to keep Salmonella out was to ensure that animals received Salmonella-free feed*.
The link between feed, farm and food is well recognized and separate studies in a variety of domestic animals have demonstrated that Salmonella isolated from the feed-mill or the finished feed can be later isolated from the farm, meat processing plant, eggs or meat (Shirota et al., 2001, Liebanaet al., 2002, Österberg et al., 2006, Molla et al., 2010).
In countries with a low on-farm prevalence of Salmonella, human infection linked to pork consumption has been traced back to Salmonella-contaminated feedstuffs (Hald et al., 2006, Wierup et al., 2010). As other EU countries reduce their Salmonella incidence, they will have to become ever more vigilant in terms of producing Salmonella-free feedstuffs for their pigs.
Before looking at the various options for controlling Salmonella in feed, however, it is important to remember the aim should be to expose the pig to as few viable Salmonella organisms as possible and not to rely on that pig’s ability (or otherwise) to protect itself from the Salmonella it consumes.
An example of Salmonella typhimurium penetrating gut epithelium.
Pigs are capable of killing some of the Salmonella they ingest. However, the more they consume especially if concentrated in micro-colonies within the feed the higher the risk that their innate enteric defense mechanisms are unable to cope and they become infected.
Producing clean feed
Heating feed during the steam-conditioning process is often relied upon to produce Salmonella-free feed. However, given that to be confident of eliminating 100% of the Salmonella in feed (including heat-tolerant strains), the meal must be heated to more than 85°C, for at least four minutes and have a moisture content of between 14.5% and 15%. The vast majority of animal feed mills come nowhere close to achieving these conditions.
Importantly, even when meeting these standards, the “clean” feed still has to be cooled, stored and distributed. It is all too easy for Salmonella contamination to occur at these stages and for pigs to be exposed to Salmonella-contaminated feed.
Organic acids and their salts are also popular Salmonella-control products in pig feed. There are however significant limitations to their effectiveness in-feed:
Learning from the poultry industry
Again, looking at the poultry industry, where Salmonella control has largely been a success-story, we find that the most popular in-feed Salmonella control products include Termin-8. Containing a synergistic combination of anti-microbial substances, surfactant and propionic acid, this is said to provide rapid, persistent and non-pH-dependant anti-Salmonella activity.
Salmonella control in pigs will require legislative and economic pressure on feed-manufacturers, pig producers and meat processors to implement the increased surveillance and multiple interventions required, over the coming years.
As these interventions take effect, the need to ensure that pigs are fed Salmonella-free feed will become ever more important. As feed-manufacturers and pig producers seek the most effective ways to produce ‘this feed, they might be wise to look to the successes achieved by the poultry industry.
*“Salmonella-free” feed is taken to mean feed that contains insufficient numbers of viable Salmonella to pose an infective risk to animals consuming it.
References: Carrique-Mas J.J., Bedford S. and Davies R.H. (2006) Journal of Applied Microbiology ISSN 1364-5072 Hald T., Wingstrand A., Brǿndsterd T. and Lo Fo Wong DM (2006) Foodborne Pathog.Dis.. Winter;3(4):422-31 Liebana E., Crowley C.J., Garcia-Migura L., Breslin M.F., Corry J.E.L., Allen V.M. and Davies R.H. (2002) British Poultry Science, 43: 38-46 Molla B., Sterman A., Mathews J., Artuso-Ponte V., Abley M., Farmer W., Rajala-Schultz P., Morgan Morrow W.E. and Gebreyes W.A. (2010) Applied and Environmental Microbiology; 76(21): 7188-7193 Österberg J., Vågsholm I., Boqvist S. and Sternberg Lewerin S. (2006) Acta Veterinaria Scandinavica, 47, 13-22 Shirota K., Katoh H., Murase T., Ito T., and Otsuki K. (2001) Journal of Food Protection, Vol. 64, No.5: 734-737 Wierup M. and Häggblom P. (2010) Acta Veterinaria Scandinavica, Feb 17; 52:15
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Many pigs are Salmonella-positive at slaughter.
A high proportion of infected pigs become carriers and intermittent excretors of Salmonella in their feces.
Preventing the contamination of farms with viable and persistent Salmonella burdens from outwardly ‘healthy’ pigs is difficult to achieve.
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