Comparison of Antimicrobial Resistance Profiles in Salmonella spp. from Swine Upon Arrival and Postslaughter at the Abattoir.

Antimicrobial resistance (AMR) developed by Salmonella within animals used for food products is a major global issue. Monitoring AMR in animals destined for slaughter is, therefore, critical. Abattoirs may serve as potential candidate checkpoints for monitoring resistance patterns on farms. A complicating factor, however, is the impact of lairage on Salmonella detected in pigs at slaughter. This study sought to compare AMR patterns in Salmonella spp. in swine collected upon arrival (fecal samples) at the abattoir with those at postslaughter (cecal samples) and evaluate the feasibility of using slaughterhouse samples for surveillance of prevailing AMR Salmonella on farms. Eighty-four Salmonella isolates were recovered from a large, midwestern U.S. abattoir between September and November 2013. Isolates were tested for phenotypic AMR to 12 antimicrobials using the broth microdilution assay. Whole-genome sequencing identified the AMR genes harbored by the strains. Significant differences were observed in the isolate phenotypes and genotypes; however, no significant difference was observed in genotypic resistance patterns. Hence, the AMR profiles of Salmonella spp. postslaughter cannot be predicted from preslaughter samples. Further research considering the genetic diversity of isolates and statistical power of the genotypic analysis is warranted to improve the performance of WGS-inferred antimicrobial susceptibility.

Salmonella enterica in swine production: assessing the association between amplified fragment length polymorphism and epidemiological units of concern.

The aims of this study were to determine the ability of amplified fragment length polymorphism (AFLP) to differentiate Salmonella isolates from different units of swine production and to demonstrate the relatedness of Salmonella between farms and abattoirs by AFLP. Twenty-four farms in the midwestern United States were visited four times from 2006 to 2009. At each farm or abattoir visit, 30 fecal samples or 30 mesenteric lymph nodes were collected, respectively. A total of 220 Salmonella isolates were obtained, serotyped, and genotyped by multilocus sequence typing (MLST) and AFLP. These 220 isolates clustered into 21 serotypes, 18 MLST types, and 14 predominant AFLP clusters based on a genetic similarity threshold level of 60%. To assess genetic differentiation between farms, harvest cohorts, and pigs, analysis of molecular variance was conducted using AFLP data. The results showed 65.62% of overall genetic variation was attributed to variance among pigs, 27.21% to farms, and 7.17% to harvest cohorts. Variance components at the farm (P = 0.003) and pig (P = 0.001) levels were significant, but not at the harvest cohort level (P = 0.079). A second analysis, a permutation test using AFLP data, indicated that on-farm and at-abattoir Salmonella from pigs of the same farms were more related than from different farms. Therefore, among the three subtyping methods, serotyping, MLST, and AFLP, AFLP was the method that was able to differentiate among Salmonella isolates from different farms and link contamination at the abattoir to the farm of origin.

Sub-iliac lymph nodes at slaughter lack ability to predict Salmonella enterica prevalence for swine farms.

The aim of this study was to assess the value of deep systemic sub-iliac lymph nodes collected at slaughter as predictors of Salmonella prevalence in live hogs. An observational study was conducted on 24 farms from September 2006 to February 2009. At least one cohort of market-weight pigs was visited for each farm. Within each cohort, 30 farm fecal samples on farm and 30 sub-iliac lymph nodes from matched pigs at slaughter were collected. Samples were cultured for Salmonella enterica and serotyped by conventional methods. Overall, 3.4% (51 of 1490) of farm feces and 0.06% (1 of 1739) of sub-iliac lymph nodes were Salmonella positive; 71.4% (15 of 21) of farms had at least one positive fecal sample, and 4.2% (1 of 24) had at least one positive sub-iliac lymph node. The median within-farm prevalence of Salmonella in farm fecal samples was 1.7%, ranging from 0% to 38.3%; for sub-iliac lymph nodes the median was 0%, ranging from 0% to 1.1%. The median within-cohort prevalence in farm fecal samples was 0%, ranging from 0% to 43.3%; for sub-iliac lymph nodes the median was 0%, ranging from 0% to 4%. The predominant serotype detected was Derby, followed by Anatum and Typhimurium (Copenhagen). Salmonella Braenderup was recovered from the sub-iliac lymph node. The low detection rate of Salmonella in sub-iliac lymph nodes (0.06%) limits its usefulness as a dependable predictor of Salmonella contamination originating on farm (3.4%).

Split marketing as a risk factor for Salmonella enterica infection in swine.

On-farm reduction of Salmonella carriage prevalence in pigs requires the identification of risk factors to direct interventions development. This study was designed to determine if split marketing of finishing pigs constitutes a risk factor for Salmonella infections, by comparing Salmonella prevalence in the first group of pigs selected for harvest ("first pull") versus the prevalence in the last group of pigs selected for harvest ("close out") from multiple commercial finishing lots. Nine paired samplings were conducted consisting in matched groups of pigs from individual barns as the first pull and the close out with a 4-week interval between groups. From each group, fecal and meat samples were collected, on-farm and at harvest, respectively. Fecal samples were selectively enriched, and analyzed for the presence of Salmonella, whereas meat juice samples were analyzed for the presence of antibodies against Salmonella. In 7/9 (77.8%) of the studied barns, an increase in Salmonella prevalence was observed, based on both bacteriologic and serologic analysis. Overall, there was an increase of 9.2% (p < 0.05) in bacteriologic prevalence, and 31.3% (p < 0.05) in serologic prevalence from first pull to close out groups. This study demonstrates that a significant increase in Salmonella prevalence occurs between the first and the last group of pigs harvested from finishing lots, with close out groups of market pigs posing a higher risk for Salmonella contaminations.

Low prevalence of Listeria monocytogenes in cull sows and pork.

The goal of this study was to determine the prevalence of Listeria monocytogenes in sows slaughtered at a single Midwestern plant on two occasions (trial 1, n = 179 sows; trial 2, n = 160 sows). Fecal samples collected antemortem (trial 1) as well as animal tissues, and carcass swabs collected at the abattoir (trials 1 and 2) were analyzed. Eight isolates of L. monocytogenes were recovered from five samples that represented 0.18% of the total samples (n = 2,775). In trial 1, L. monocytogenes was detected in a tonsil sample (0.6%; 1 positive of 181 tonsils), in a carcass (0.6%; 1 positive of 179 carcasses), which was sampled prior to the organic rinse, and in two chopped meat block samples (1.2%; 2 positive of 165 samples). In trial 2, L. monocytogenes was only detected in a single chopped meat block sample (0.15%; 1 positive of 688 total samples). These data indicate the low prevalence of L. monocytogenes in the cull sow.

Efficacy of vaccination to reduce Salmonella prevalence in live and slaughtered swine: a systematic review of literature from 1979 to 2007.

A systematic review was conducted to evaluate the efficacy of vaccination to reduce Salmonella prevalence in market weight finisher swine. A search of online databases and selected conference proceedings was conducted to identify relevant studies. The review process followed relevance screening, methodological quality assessment, and data extraction. Although multiple outcomes were frequently reported, only outcomes describing culture of Salmonella were extracted. Five clinical trials and 23 challenge studies were considered likely relevant to the review as they described vaccination to reduce Salmonella in swine. Five clinical trials reported vaccination was associated with reduced isolation of Salmonella in market weight pigs, however, information required to assess the internal validity of the study was often not described in the manuscripts. All challenge studies assessed vaccine efficacy in pigs aged <15 weeks reducing the relevance of results to the review which focused on market weight pigs. Only five of the 23 challenge studies reported the majority of information necessary to evaluate the quality of vaccine studies. Given large variability in population type, sample size, type of vaccine, dose and dosing regimens, and type of outcomes observed, pooled data analysis was not possible, and therefore, a qualitative synthesis of the studies was conducted. Available evidence suggests that vaccination is associated with reduced Salmonella prevalence in swine at or near harvest; however, this conclusion is based on studies with design and reporting deficiencies that could potentially indicate biases with the outcome.

Prevalence of exposure to Salmonella spp in finishing swine marketed in Iowa.

OBJECTIVE: To describe the prevalence of antibodies against Salmonella spp in swine marketed in Iowa. ANIMALS: Swine marketed by 1,044 low-volume producers and 45 high-volume producers. PROCEDURE: Samples of diaphragm muscle collected from swine carcasses were tested by an indirect ELISA based on lipopolysaccharides from Salmonella spp, in particular Salmonella serovar Typhimurium. Prevalence of positive results for antibodies against Salmonella spp for carcasses, lots, and swine for each producer was determined. Producer-level seroprevalence was used to classify swine from producers as having negligible, low, moderate, or widespread evidence of previous or historical exposure to Salmonella spp. RESULTS: From low-volume producers, 23,609 of 25,478 (92.7%; 95% confidence interval [CI], 92.4% to 92.9%) samples had negative results, and 1,863 (7.3%; 95% CI, 7.05% to 7.56%) had antibodies against Salmonella spp. Of the 6,299 lots of swine tested, 1,191 (18.9%) contained at least 1 sample with positive results. From high-volume producers, 203 of 2,486 (8.1%; 95% CI, 6.8% to 9.3%) samples had antibodies against Salmonella spp, and 124 of 629 lots had at least 1 sample with positive results for antibodies against Salmonella spp. CONCLUSIONS AND CLINICAL RELEVANCE: Less than 10% of pigs marketed in Iowa are apparently exposed to Salmonella spp. Most swine marketed by low-volume producers had negligible or little evidence of exposure to Salmonella spp, whereas a higher percentage of swine marketed by high-volume producers had positive results when tested to detect antibodies against Salmonella spp.

Culture methods differ on the isolation of Salmonella enterica serotypes from naturally contaminated swine fecal samples.

Four culture methods (A, B, C, and D) were comparatively evaluated for their ability to isolate Salmonella enterica from pooled swine fecal samples (n = 100). None of the methods was able to isolate Salmonella from all positive samples. The relative sensitivity of the culture methods evaluated was 82%, 94%, 95%, and 78% for methods A, B, C, and D, respectively. The comparison of sensitivities showed that methods B and C performed significantly better (P < 0.05) than methods A and D. Although relative sensitivities of methods B and C were equal, from the 89 positive samples concomitantly detected by both, 35 (39.3%) had different serotypes (no match) isolated by each method. On the basis of the results of this study, it was concluded that culture methods differ on the isolation of S. enterica serotypes from naturally contaminated swine fecal samples. Depending on the objective(s) of investigations on the ecology and epidemiology of S. enterica in swine populations, a method or a combination of methods should be considered for more reliable results.

Resting pigs on transport trailers as an intervention strategy to reduce Salmonella enterica prevalence at slaughter.

Recent research has shown that much preharvest Salmonella enterica infection in pigs occurs immediately before slaughter during this rest period in the contaminated abattoir holding pens. The objective of this study was to evaluate a potential intervention strategy to reduce the prevalence of S. enterica-positive pigs at slaughter, which consisted of resting pigs prior to slaughter on their transport vehicle, instead of in the abattoir holding pen. Additionally, the effect of transportation of pigs from farm to the abattoir on S. enterica prevalence was investigated. A total of 120 animals were included in the experiment, divided in four replicates (n = 30 pigs per replicate). Fecal samples were collected from each animal at the farm and at the abattoir, where 15 randomly chosen pigs were unloaded and moved to a holding pen, while the remaining 15 pigs stayed in the transport trailer. After approximately 1.5 h of resting, both groups were slaughtered. Samples collected included distal ileum portion, cecal contents, and ileocecal lymph node. The overall S. enterica prevalence (pigs positive in at least one of the samples collected at slaughter) was higher for pigs held in the abattoir pens (40.7% versus 13.3%, P < 0.05). There was no difference (P > 0.05) for the S. enterica prevalence before and after transportation from farm to abattoir (5.8% versus 0.8%, respectively). This study demonstrates that resting pigs on the transport vehicle has the potential to decrease S. enterica levels entering the abattoir.

The association between cleaning and disinfection of lairage pens and the prevalence of Salmonella enterica in swine at harvest.

A series of four field trials were conducted to evaluate the ability of a cleaning and disinfection procedure in swine lairage pens to reduce the prevalence of Salmonella enterica in slaughtered pigs. A cleaning and disinfection procedure was applied to lairage pens at a large Midwest abattoir. Each trial consisted of a cleaned (alkaline chloride detergent) and disinfected (H2O2 plus peracetic acid sanitizer) pen (treated) and a control pen, each holding 90 to 95 pigs for 2 to 3 h before slaughter. Ileocecal lymph nodes, cecal contents, and rectal contents were collected from 45 pigs from each study pen at harvest and cultured for S. enterica. In all trials, cleaning and disinfection reduced the prevalence of S. enterica-positive floor swabs in the treated pen (P < 0.05). However, the postharvest prevalence of S. enterica-positive pigs varied between trials. In trial 1, there was no significant difference in the prevalence of S. enterica in pigs between treatment and control groups. In trials 2 and 3, the prevalence of S. enterica was higher in pigs from treated pens versus pigs from control pens (91% versus 40%, P < 0.0001, and 91% versus 24%, P < 0.0001, respectively). In trial 4, the prevalence of S. enterica was lower in pigs from treated pens compared with pigs from control pens (5% versus 42%, P < 0.0001). This study indicates that cleaning and disinfection effectively reduces the amount of culturable S. enterica in lairage pens, but the ability of cleaned and disinfected pens to reduce the prevalence of S. enterica in market-weight pigs remains inconclusive.