Control of Salmonella in the Pork Production Chain

Salmonellosis is a major food borne disease threat to public health. In Canada, Salmonella is the second most common bacterial food borne pathogen (after Campylobacter) for which swine serve as a reservoir. Salmonella has a complex epidemiology. Despite copious research, there has been little progress in identifying cost-effective interventions for Salmonella on-farm. Significant strides have been made at decreasing Salmonella contamination at slaughter and processing and it is expected that standards in abattoirs will become more stringent, creating pressure to reduce prevalence of Salmonella-positive swine through on-farm interventions.

Potential On-Farm Control Points

1. Humans and Other Animals as Vectors
Both entrance biosecurity practices for personnel and visitors and higher herd health status have been associated with decreased Salmonella risk for swine. Rodents, birds and invertebrate animals are all known to be potential carriers of Salmonella, but the actual risk posed by them to swine is unclear.

2. Environmental Contamination
Salmonella is capable of surviving at least six years in the environment. A recent study reported that high prevalence farms tended to have more residual contamination of feeders and equipment after barn cleaning than low prevalence farms, suggesting that more stringent cleaning is associated with decreased prevalence.

3. Pig Flow
Few studies have identified all-in/all-out pig flow as being associated with decreased Salmonella prevalence. In fact, Salmonella prevalence can be quite high on farms with all-in/all-out production. The limited and contradictory evidence in the literature warrants further investigation.

4. Feed
Animal feeds and feedstuffs can be contaminated with Salmonella. Appropriate decontamination steps are needed during feed processing to reduce contamination of feedstuffs in order to avoid dissemination of contaminated feed to herds. However, the role of contaminated feed in the epidemiology of Salmonella on swine farms is questionable. In a multi-country survey in Europe, Salmonella was isolated from feedstuffs in 17.6% of herds and 6.9% of all samples. Yet, the Salmonella serotypes isolated from the feeds were not the same serotypes isolated from pigs on those farms.

Many epidemiological studies have found that pigs fed pelleted rations were at increased risk of high Salmonella seroprevalence compared to those fed diets in meal form. Additionally, diets that are acidified, either as a result of the addition of whey, organic acids, or are fermented have been associated with reduced Salmonella prevalence. Conversely, wet, but not fermented, diets have been associated with increased Salmonella prevalence.

5. Vaccine
Vaccination is associated with reduced Salmonella prevalence in swine at slaughter. Unfortunately there are few quality published studies sufficient to assess the value of the intervention.

6. Thermal Environment
There is an association between season and/or environmental temperature and Salmonella prevalence in finishing swine. Recent work has demonstrated increased prevalence in winter and spring.

7. Antimicrobial Use
Field investigations have shown variable results with antimicrobials on controlling Salmonella prevalence on farms. The impact on shedding may be associated with the antimicrobial resistance profile.

8. Cost-Effectiveness
There is minimal data regarding the evaluation of cost-efficacy of different interventions on Salmonella control. In a Danish study, the variables with the maximum effect on Salmonella prevalence on carcasses were:

     * the number of herds with a high prevalence of Salmonella,
     * singeing efficiency,
     * contamination and cross-contamination at degutting and
     * cross-contamination during handling.

Interestingly, improvement in any one intervention had no effect, suggesting that both on-farm and in-abattoir interventions may be required to achieve reduced carcass contamination.

Presented by Julie Funk, DVM. MS. PhD, Michigan State University

Summarized by Janet Alsop - Vet Disease Prevention, Swine Veterinary Services/OMAFRA
June 2008 Pork News & Views - Government of Ontario Ministry of Agriculture, Food and Rural Affairs

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Control of Salmonella in the Pork Production Chain