Salmonella contamination on pork carcasses: a study of critical points

The aim of the present study was to determine the frequency and distribution of Salmonella contaminations throughout the slaughtering process of pigs. The investigation was carried out in two slaughterhouses that were visited four times each on different days. Samples were taken from lairage areas, fecal contents, carcasses at different stages of processing and from the environment. Bacteriological analysis of the samples was carried out by a standard procedure consisting of a pre-enrichment and selective enrichment. Salmonella isolates were also serotyped. In total, 720 carcass swabs and 185 environmental samples were collected. Contamination was high in the lairage area (70.8% of the samples) and relatively low at the final step of carcass processing (3.7%). However, at intermediate phases, the frequency of contamination was higher, particularly after evisceration (6.2%). Global prevalence of fecal carriers was 12.5%. Salmonella was also isolated from hands of personnel in both abattoirs. Also in both cases, some samples taken in the slaughterline before the start of the operation were positive. The most spread serotype was Typhimurium; although other serotypes predominated at particular points. These results suggest that contamination is widespread in the slaughterhouses and that the patterns of spreading of the contamination are complex. Introduction Contamination of pig carcasses at the slaughterhouses is a critical point to allow the presence of Salmonella in pig meat. If the slaughtering process developed with absolute efficiency, carriage of Salmonella in lymph nodes or fecal contents would not contaminate the carcass. However, all studies done show that the process is less than perfect and contamination of pig carcasses is common (Swanenburg et al., 2001). Therefore, knowledge of the hazard points for contamination and evaluation of the risk of each step in the slaughterline is important to design corrective measures aimed to decrease the frequency of contaminated carcasses at the end of the slaughtering process. Beforehand, lairage areas, equipment and personnel are the most probable sources of external contamination while laceration of intestines of infected pigs is the other potential source. The present study was aimed to determine the frequency of Salmonella contaminations along the slaughtering process of pigs in two abattoirs of Catalonia, Spain. Materials and Methods Two commercial pig slaughterhouses of Catalonia were investigated. Slaughterhouse A slaughtered approximately 350 pigs per hour and slaughterhouse B processed approximately 650 pigs per hour. Slaughter operations and procedures were similar in both slaughterhouses except for the scalding. In slaughterhouse A, carcasses were scalded using water vapor while in slaughterhouse B there was a “scald tank” (60±1°C). Sampling was repeated four times at each slaughterhouse on different days. Samples were collected from carcasses (after evisceration, after polishing and before chilling), fecal contents, environment (lairage areas, scalding tank, evisceration drainages), equipment (knives, polishing brushes, rectal pistols, trimming devices) and from hands of the personnel. For each round of sampling, the same 30 pigs were marked and were followed up throughout the slaughterline. Carcasses were sampled by vigorous rubbing of ham, belly, jowl and back. Environmental sampling was carried out three times a day, at the beginning of the working day, one hour later and 3-4 hours after the beginning of the operation. Microbiological analysis were done using a pre-enrichment step in buffered peptonewater (37°C, 18h), selective enrichment in Rappaport-Vassiliadis broth (42°C, 48 h), and final plating in XLT4 agar. Suspect colonies were identified by means of API20E (Bio-Mérieux) and Salmonella isolates were further serotyped according to the Kauffman-White-LeMinor scheme. Results Contamination was more frequent in slaughterhouse B where 5.3% (19/360) of the carcasses swabs were positive compared to 1.4% (5/360) in slaughterhouse A (p<0.05). Regarding P O S T E R P R E S E N T A T IO N S 305 SafePork 2005 the evolution of carcasses contamination in each stage selected in both abattoirs, the presence of Salmonella increased from polishing to evisceration and decreased before chilling. This pattern was seen in both slaughterhouses although slaughterhouse B always had the highest frequencies of contamination. Also, the prevalence of carrier pigs was higher (p<0.05) in slaughterhouse B (28/120, 23.3%) than in slaughterhouse A (2/120, 1.6%). In contrast, the proportion of positive samples in lairage areas was equal in both cases (17/24, 70.8%). Regarding other environmental samples, polisher brushes, evisceration knives and knife-grinders, evisceration drain water and hands of the personnel were found to be contaminated at least once in slaughterhouse B. In slaughterhouse A, only rectal pistols and hands were found contaminated. Interestingly, some samples taken in the slaughterline before the start of the working activity were found to be positive (1.5% in slaughterhouse A and 4.1% in slaughterhouse B). However, the mean prevalence of Salmonella in environmental samples per time point did not seem to follow a pattern. Table 1 summarizes these results. Regarding the distribution of Salmonella serotypes, in lairage areas Salmonella Typhimurium and Rissen were common in both abattoirs while other serotypes could be also present (Wien, Manchester, Derby, Anatum, etc.). The most frequently isolated serotype in the other environmental samples in both slaughterhouses was Salmonella Typhimurium. In faecal samples, Typhimurium and Derby predominated and in carcasses 23/24 isolates corresponded to serotypes Typhimurium, Rissen and Derby. Discussion The results of this study show that Salmonella contamination is common at several points of sampling. In addition, frequency of isolations at each step and abattoir can differ between days. It is thought that the ultimate origin of Salmonella contaminations is the existence of carrier pigs (Berends et al., 1997). However, once the premises and the equipment is contaminated, the fact that one particular carcass become contaminated depends greatly on the hygiene and the efficiency of the slaughtering process (Botteldoorn et al., 2004) becoming thus a fact that depends on a series of probabilities. In the present case, and as reported by others (Swanenburg et al., 2001), it was seen that the presence of Salmonella on carcasses is similar or lower than the prevalence of carrier pigs. This fact clearly indicates that accurate procedures during the slaughter process are effective to reduce the level of contamination of carcasses; such is the case of scalding and singeing steps. Nevertheless, re-contamination may occur later in the slaughterline. For example, as shown by our results, no Salmonella was isolated on carcasses after scalding but then, there was a little increase in the frequency of contamination after evisceration. This has been also reported by others (Davies et al., 1999; Pearce et al., 2004). A proportion of these re-contaminations can be attributed to the environment. In our study, hands of the personnel working in the evisceration areas were found to be contaminated with Salmonella in both abattoirs. This fact suggests that strict application of good evisceration techniques and hygiene are necessary to reduce carcass contamination. Interestingly, most of carcasses that were found to be contaminated were not those of pigs that were fecal carriers. Botteldoorn et al. (2004) found that only between 4% and 25% of contaminated carcasses acquired contamination from their own fecal contents or lymph nodes. Conclusions Our results show that cross contamination is common and that if adequate hygiene practices and procedures during the slaughter process were strictly carried out, the incidence of Salmonella on carcasses could be reduced and therefore, food safety will improve. Acknowledgements Our grateful thanks for the cooperation of the abattoirs and we wish also thank to all people that helped in sampling.