December 2007 No. 6

QLIF Training and Exchange workshop

Concept for QLIF
workshops at 2nd ISOFAR conference

QLIF research in
focus of JSFA

Handbook of
Organic Food Safety
and Quality

Tutorial for
uploading papers
to Organic Eprints

QLIF research
articles on:

Low salmonella
in outdoor pigs

Measures influencing
udder health in
organic dairy farms

Effect of agronomic management
on lettuce quality

Importance of international
and exchange



Low level of Salmonella shedding in pigs from outdoor production systems

by Marianne Bonde and Jan Tind Sørensen, Danish Institute of Agricultural Sciences

Pigs from low input systems may be more resistant to Salmonella, or may encounter the infection earlier in life so they have cleaned themselves from infection at time of slaughter. This is the main conclusion of a recent study on the effect of pig management on the risk of transfer of Salmonella to the food chain.


Pork and pork products are one of the major sources of human salmonellosis. Pigs in outdoor production systems benefit from a low animal density, and access to outdoor area, and organic pig production furthermore differs from conventional production in terms of feeding, weaning age, and use of preventive medication (Bonde and Sørensen 2004). It is therefore likely that the risk of Salmonella is different in organic, outdoor, and indoor pig production, respectively.

The objective of this survey was to investigate the effect of different pig production systems with indoor or outdoor rearing, and the effect of transport duration, on the potential pathogen transfer risk into the food chain from Salmonella in pig faeces. Further we evaluate seropositivity as a predictor of Salmonella shedding at pig level. Seropositivity means that antibodies against Salmonella are present and indicates that the pig has been exposed to challenge by the enteric pathogen at some stage of its development. All methods used are further described in Box 1 (click here).

Low prevalence of Salmonella in outdoor pigs

The prevalence of Salmonella in the different production systems is illustrated in Figure 1. The overall prevalence of Salmonella in 1609 faecal samples from pigs on-farm (black bars) was 0.87 percent, with the lowest level of salmonella in the two outdoor production systems.


Figure 1. Prevalence of Salmonella shedding on-farm, and at slaughter, and prevalence of Salmonella antibodies in meat juice.

The prevalence of Salmonella shedding was 2,2 % in 1556 of these pigs at slaughter (grey bars), again with the lowest level at the two outdoor production sytems.

Prevalence of Salmonella antibodies in meat juice (hatched bars) showed that overall 8,5 percent of the pigs were seropositive with no significant differences between systems.

Predicting risk at slaughter from on-farm data

Analysis of each production system separately showed that Salmonella shedding at slaughter in conventional outdoor pigs was predicted by seropositivity (P<0,01). In indoor pigs it was predicted by on-farm shedding (P<0,0001), as well as seropositivity (P<0,10). Contrary to this, Salmonella shedding in organic pigs was not predicted by seropositivity.

Neither of the clinical parameters, e.g. diarrhoea, constipation or poor body condition, acted as significant predictors of Salmonella shedding.

Role of transport duration

A number of stress factors related to the routine management in a pig herd may increase faecal shedding of pathogens. Further, transport of pigs to the abattoir causes significant stress to the animals, which can trigger an increase in shedding and duration of transport and lairage may also affect the level of Salmonella shedding at slaughter. It is therefore essential to compare the faecal shedding before and after transport to the abattoir, when assessing the risk of pathogen transfer into the food chain.

In the present study, information about duration of transport to slaughter was collected from 155 batches of pigs (50 organic, 58 conventional outdoor, and 47 indoor batches), and differences between systems were analysed. The mean durations of transport to slaughter were 175,3 min (organic pigs), 128,6 min (conventional outdoor pigs) and 96,8 min (indoor pigs) (P<0,0001). These differences in transport did not affect the Salmonella shedding at slaughter.


The prevalence of Salmonella shedding in pigs from outdoor systems was less than in indoor herds. The low levels of Salmonella shedding in organic and outdoor pigs suggest that pigs from low input systems may be more resistant to the pathogen, or may encounter the infection earlier in life so they have cleaned themselves from infection at time of slaughter.

The present data further indicate that seropositivity as a means to identify individual pigs that are more likely to shed Salmonella might be better suited to conventional than organic herds.


Bonde M., Sørensen J.T. (2004). Herd health management in organic pig production using a quality assurance system based on Hazard Analysis and Critical Control Points. NJAS-Wageningen Journal of Life Sciences 52: 133-143.

An elaborated account of the present study is available in the archive Organic Eprints at http://orgprints.org/10281. Acknowledgements and disclaimer are posted here.