since the serological status does not change after pigs leave the farm. Bacteriological tests determine
whether Salmonella bacteria are present in manure or in a tissue sample from a carcass. The
bacteriological testing of carcasses is the most important indicator for food safety.
The plant’s bacteriological prevalence level is related to farm-level serological prevalence
levels. In general, bacteriological prevalence levels are lower than serological prevalence levels
because (i) serological prevalence often indicates a past infection that is no longer active and (ii)
control measures taken at the plant level may influence bacteriological contamination on carcasses by
contributing to or minimizing cross-contamination.
The relation between herd serology and the prevalence of Salmonella bacteria measured at the
carcass surface is described for three Danish abbatoirs by a 1.4% increase in the probability of
Salmonella positive carcasses for a 10 percentage point increase in herd serological prevalence. The
Spearmann correlation between carcass prevalence and serological prevalence was 0.29 (Sorensen et
al., 2004). Swanenburg et al. (2001a) report also that the prevalence of Salmonella lower is in pork
samples of sero-negative herds than in samples of sero-positive herds
Salmonella contamination of carcasses after slaughter is partially caused by Salmonella-
infected herds that were slaughtered before, and partially by residential flora of the slaughterhouse.
Slaughter hygiene, including careful removal of the intestinal tract from the carcass and cleaning and
disinfection, can reduce cross-contamination by residential flora from the slaughterline. The carcass
splitter in the slaughter line is the most important contamination source for carcasses of pigs from both
sero-negative and sero-positive herds. Logistic slaughter, or separate slaughter of sero-negative pig
herds, can also be useful to decrease the Salmonella prevalence of pork after slaughter (Swanenburg et
al., 2001b).