Increasing incidence of non-O157 Shiga toxin-producing Escherichia coli (STEC) in Michigan and association with clinical illness.

Infection with Shiga toxin-producing Escherichia coli (STEC) by serotypes other than O157 (non-O157) have been increasingly reported in the United States. This increase in reporting is primarily due to the improvements in diagnostic tests. We analysed 1497 STEC cases reported in Michigan from 2001 to 2012. A significant increase in the number of non-O157 STEC cases was observed over time, and similar incidence rates were observed for O157 and non-O157 STEC cases in certain time periods. The odds of hospitalization was two times higher in O157 STEC cases relative to non-O157 STEC cases when adjusted for age and gender, suggesting that O157 STEC causes more severe clinical outcomes in all age groups. The use of population-based surveillance to better define trends and associations with disease severity are critical to enhance our understanding of STEC infections and improve upon current prevention and control efforts.

Shiga toxin-producing E. coli (STEC) in swine: prevalence over the finishing period and characteristics of the STEC isolates.

This descriptive longitudinal study was conducted to investigate the faecal shedding of Shiga toxin-producing E. coli (STEC) in finishing swine and to characterize the swine STEC isolates that were recovered. Three cohorts of finishing swine (n = 50/cohort; total 150 pigs) were included in the longitudinal study. Individual faecal samples were collected every 2 weeks (8 collections/pig) from the beginning (pig age 10 weeks) to the end (pig age 24 weeks) of the finishing period. STEC isolates were recovered in at least one sample from 65·3% (98/150) of the pigs, and the frequency distribution of first-time STEC detection during the finishing period resembled a point-source outbreak curve. Nineteen O:H serotypes were identified among the STEC isolates. Most STEC isolates (n = 148) belonged to serotype O59:H21 and carried the stx 2e gene. One O49:H21 STEC isolate carried the stx 2e and eae genes. High prevalence rates of STEC during the finishing period were observed, and STEC isolates in various non-O157 serogroups were recovered. These data enhance understanding of swine STEC epidemiology, and future research is needed to confirm whether or not swine STEC are of public health concern.

Risk factors associated with persistence of Salmonella shedding in finishing pigs.

The objective of this study was to identify risk factors associated with persistence of Salmonella shedding in finishing swine. A longitudinal study was conducted in 18 cohorts of pigs from three finishing sites of one swine production company. Among the 446 Salmonella isolates (isolated from 187 pigs), there were 18 distinct serovars. The six most common serovars were S. enterica serovar Derby (47.3%), S. Agona (27.4%), S. Johannesburg (10.5%), S. Schwarzengrund (2.7%), S. Litchfield (2.5%) and S. Mbandaka (2.2%). Survival analysis techniques, Kaplan-Meier methods and Log-rank test were used to estimate the duration of Salmonella shedding in days and to evaluate differences in shedding associated with risk factors at different organizational levels: isolate (serovar), pig, cohort and site. The risk factors at the pig-level were: sex, age and individual health status; and the risk factors at the cohort-level were: health risk, treatment and "at risk pigs" proportions, nursery and barn environment Salmonella status and prior exposure to the same serovar in the nursery or barn environment. Survival analysis using acceleration failure time models, with a log-normal distribution, was applied to investigate risk factors associated with Salmonella persistence (175 pigs) and serovar-specific persistence (151 pigs) during the study period. Pigs detected Salmonella positive for the first time at 10 weeks of age had a longer duration of shedding, than pigs first detected at an older age. The duration of shedding was shorter among pigs infected with S. Derby, S. Johannesburg and other serovars as compared to pigs infected with S. Agona. A significant difference was observed among sites. Cohorts with pig treatment proportions greater than the median were more likely to have a shorter duration of Salmonella shedding. Pigs from cohorts with nursery positive pools greater than the overall mean had a longer duration of Salmonella shedding as compared to pigs from cohorts with nursery pools less than or equal to the mean. These results suggest that the duration of Salmonella shedding may depend on Salmonella serovar, pig age at the time of infection, farm site and cohort-level risk factors. Identification of risk factors associated with the duration of shedding may allow more targeted interventions for the control Salmonella by evaluation of control measures not only for prevalence reduction, but also to decrease the duration of shedding. Such measures may decrease the risk of contamination of pork and subsequent risk of foodborne illness.

Longitudinal study to evaluate the association between thermal environment and Salmonella shedding in a midwestern US swine farm.

The objective of this study was to document the association between the thermal environment in the barn and Salmonella shedding in finishing pigs. For this purpose, individual fecal samples from 900 finishing pigs (8 collections per pig) were repeatedly collected from 18 cohorts (50 pigs per cohort) on 3 sites of a multi-site farrow-to-finish production system in a longitudinal study. Pen temperature and humidity were measured every 2 min during the study period. The thermal parameters of interest were: hourly average temperature, minimum and maximum temperature, hourly temperature variation, temperature humidity index (THI) and cumulative number of hours/degree above and below the thermal neutral zone at the pen level prior to fecal sampling for 6 time periods (12h, 24h, 48 h, 72 h, 1 week and 1 month). Additional potential risk factors at the individual (e.g., sex, health events), cohort (e.g., mortality, morbidity, Salmonella status of the nursery) and pen level (e.g., type of pen) were also evaluated. Multilevel logistic models using generalized linear models, with random intercepts at pig, pen and cohort levels to account for clustering (individual samples nested within pigs, pigs nested within pens, pens within cohorts) were constructed. Site (A, B, C) was considered as a fixed effect in order to control for clustering within site. The outcome variable was Salmonella fecal status of the individual sample. Cold exposure (temperatures below the thermal neutral zone) and exposure to a THI>72 were both positively associated with risk Salmonella shedding. Nursery Salmonella status was positively associated with Salmonella shedding and pig age was negatively associated with Salmonella shedding. In the multilevel intercept-only model the largest proportion of model variance was associated with the individual fecal sample (44.8%) followed by cohort (24.5%), pen (20.5%) and pig (10.2%). The present study allowed the investigation of the association of time-variant thermal factors and Salmonella shedding. Interventions that target the thermal environment may have an effect on reducing Salmonella shedding in swine and also improve pig well-being and production efficiency. Alternatively, thermal parameters may be used to identify groups of pigs at high risk for Salmonella shedding. Future studies should be performed to investigate the cost-efficacy of interventions to improve the thermal environment of swine.

Longitudinal study of Salmonella shedding in naturally infected finishing pigs.

A 3-year longitudinal study was conducted on a multi-site farrow-to-finish production system. For each of 18 cohorts at three finishing sites, 50 pigs were randomly selected. Faecal samples were collected every 2 weeks for 16 weeks. Salmonella was cultured from 453 (6·6%) of 6836 faecal samples. The pig-level incidence of Salmonella was 20·8% (187/899 pigs). Salmonella prevalence varied between cohorts and within pigs. The adjusted Salmonella prevalence decreased over the finishing period from 6·4% to 0·8%. Intermittent detection of Salmonella was found in more than 50% of pigs that were positive at more than one collection. The finding that the majority of pigs shed intermittently has implications for surveillance and research study design when determining Salmonella status. The variability in shedding over time, as well as between and within cohorts and pigs suggests that there may be time-variant risk factors for Salmonella shedding in swine.

Quantification of campylobacter in swine before, during, and after the slaughter process.

Campylobacter has been implicated as a major cause of foodborne illness worldwide. Pigs can be subclinically infected, and fecal contamination of meat during slaughter is a food safety risk. The objective of this study was to determine the association between the concentration of Campylobacter pre- and periharvest with postharvest contamination in swine. Samples were collected from 100 individually identified swine during the pre-, peri-, and postharvest periods. For each animal, the following phases were sampled: on farm (fecal sample), in lairage (hide swab), post-stunning and exsanguination (rectal contents), prechilling (carcass swab), and final product (rib meat) sample. The proportions of samples that were Campylobacter positive were 90, 95, 76, 100, and 49% for fecal, rectal content, hide, carcass, and rib meat samples, respectively. The mean Campylobacter concentrations for each sample were fecal sample, 1.7 × 10(6) CFU/g; rectal content, 1.2 × 10(7) CFU/g; hide swab, 1.4 CFU/cm(2); carcass swab, 1.7 × 10(3) CFU per half carcass; and rib meat, 18 CFU/g. There was a positive correlation between Campylobacter concentrations in fecal samples (R = 0.20, P = 0.065) and concentration of Campylobacter on rib meat, and between rectal content sample concentration (R = 0.20, P = 0.068) and the concentration on rib meat. There was no association between the isolation of Campylobacter on rib meat and the isolation of Campylobacter at any pre- or periharvest stage. This could indicate that the risk of a meat product being contaminated is associated with pigs that shed higher concentrations of Campylobacter before slaughter.

Comparison of fecal culture and Danish Mix-ELISA for determination of Salmonella enterica subsp. enterica prevalence in growing swine.

In the USA, control of food-borne salmonellosis associated with meat consumption has been predominantly focused at slaughter and processing. It is expected that standards at slaughter and processing will become more stringent, creating pressure to reduce prevalence of Salmonella-positive food animals through on-farm interventions. The aim of this study was to compare traditional fecal culture and the Danish Mix-ELISA (DME) for determination of Salmonella prevalence pre-harvest in swine. In Trial 1, five cohorts of individually identified pigs were longitudinally sampled during the growing period to compare the kinetics of prevalence as estimated by fecal culture and the DME. In Trial 2, the correlation between fecal prevalence and seroprevalence was estimated pre-marketing in 49 groups of pigs. In Trial 1, fecal prevalence and seroprevalence showed similar kinetics, with a tendency of a higher OD% cut-off to more closely approximate fecal prevalence. In Trial 2, correlations between fecal culture and the DME were 0.40, 0.36, 0.43, and 0.43 (p<0.001) for OD% cut-offs > or =10, 20, 30, and 40, respectively. Based on these results, a higher OD% cut-off would be recommended if more approximate estimation of fecal prevalence is desired and longitudinal sampling would be suggested for evaluating the impact of on-farm interventions for Salmonella reduction whether utilizing fecal culture or the DME. Further evaluation of the impact of Salmonella serovar present on farms on seroprevalence and the relationship of on-farm seroprevalence with food safety risk are needed prior to utilizing the DME for pre-harvest Salmonella diagnostics in the US swine herd.

Risk factors associated with Salmonella enterica prevalence in three-site swine production systems in North Carolina, USA.

The goal of this study was to identify risk factors associated with increased fecal shedding of Salmonella enterica (SE) in groups of market swine reared in large three-site production units. We conducted an intensive, long-term investigation of potential management and environmental risk factors operating during the growing phase of pig production. Data regarding finisher site characteristics, biosecurity protocols, group growth performance, medication usage, and environmental temperature were collected. Results indicate that SE infection is common. Risk factors were identified at both the finisher site and group level. Biosecurity and hygiene practices (absence of a toilet, more than 2 people present at a finisher site daily, and other domestic species at the site), environmental temperature (winter and spring seasons, increased temperature variability, and below median high temperature the day of sampling), and production performance (above median feed conversion) were associated with elevated SE prevalence. In addition, an association between the floor space allowances per pig at the time of sampling (a measure of the number of pigs sold prior to sampling) was identified, with greater space allowance associated with decreased prevalence. The results of this study identify potential management practices for evaluation for SE control and suggest caution in interpretation of fecal culture results when sampling from different marketing groups in swine production systems.

Longitudinal study of Salmonella enterica in growing pigs reared in multiple-site swine production systems.

Intensive longitudinal investigations of breeding and growing pig populations in two multiple-site swine production systems were conducted in NC, USA. Five cohorts of sows and individually identified growing pigs from their litters were serially sampled in order to determine the prevalence and serotypes of Salmonella enterica in each stage of production based on fecal culture. In addition to fecal samples, feed and environmental samples were obtained. Fifteen different serotypes were isolated from the two systems, the most frequently isolated serotypes were S. typhimurium var Mbandaka and S. typhimurium var Copenhagen. Pig prevalence estimates ranged from 0 to 48.1%. Environmental contamination was frequently encountered despite cleaning and disinfection. Feed was rarely (2/800, 0.25%) identified as S. enterica positive. We observed highly variable patterns of S. enterica prevalence and serotype profiles within cohorts over time and among cohorts within systems. These observations indicate that point estimates of S. enterica prevalence and serotypes cannot be considered as reliable indicators of the S. enterica status of farms, and that uncontrolled studies of interventions to control S. enterica may yield misleading results. These findings are critical to the design of epidemiological studies of S. enterica on swine farms and may suggest that cohort level, as opposed to farm or company level events or management practices, may be important as potential risk factors for S. enterica fecal shedding in market age pigs.

Antimicrobial resistance of Salmonella isolates from swine.

We examined the antimicrobial resistance of 1,257 isolates of 30 serovars of Salmonella enterica subsp. enterica isolated from swine. Serovars Typhimurium and Typhimurium var. Copenhagen were widespread and were frequently multidrug resistant, with distinct resistance to ampicillin, kanamycin, streptomycin, sulfamethoxazole, and tetracycline and to ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline, respectively.

The effect of fecal sample weight on detection of Salmonella enterica in swine feces.

The effect of different fecal sample weights on the detection of Salmonella enterica in swine feces was examined. Sample weights evaluated were rectal swabs and fecal samples weighing 1 g, 10 g, and 25 g. Comparisons were made on matched fecal samples obtained from individual pigs housed on 2 commercial swine farms in North Carolina. Relative sensitivity (number of positive pigs per fecal weight category/number positive in all weight categories) increased (P < 0.001) with fecal sample weight, and ranged from 9% for rectal swabs to 78% for 25-g samples. Stomaching of fecal samples did not affect detection of S. enterica. These observations demonstrate that fecal sample weight can markedly influence estimates of prevalence of S. enterica in epidemiologic studies. Failure to consider the imperfect sensitivity of bacterial culture in the design and interpretation of epidemiologic studies will lead to underestimation of prevalence and reduced power to detect the presence of S. enterica-infected herds.

Comparison of methods for isolating Salmonella bacteria from faeces of naturally infected pigs.

A series of experiments was conducted using faecal samples collected from commercial swine farms to evaluate the effects of variation in methods used for the detection of Salmonella bacteria. The primary objective of the studies was to compare the protocols routinely used in two laboratories in the USA. The studies included five experiments comparing the enrichment protocols used routinely in the respective laboratories (Method 1: 10 g faeces--buffered peptone water (BPW) pre-enrichment--selective enrichment in Rappaport/Vassiliadis (RV) broth; Method 2: approximately 1g faeces--primary enrichments in tetrathionate and Hajna GN broths--secondary enrichment in RV broth). The effects of enrichment temperatures (37 vs 42 degrees C) using RV broth (two experiments) and delayed secondary enrichment (four experiments) were also evaluated. Direct comparison of Method 1 and Method 2 indicated comparable results. However, when compared using faecal samples of equal weight, the Method 2 enrichment protocol was more sensitive for detecting Salmonella bacteria than the Method 1 protocol. Enrichment in RV at 42 degrees C was superior to 37 degrees C, particularly for samples that were pre-enriched in BPW. Delayed secondary enrichment increased detection of Salmonella bacteria in swine faeces. These results highlight the imperfect sensitivity of culture methods, and the need for researchers to consider the sensitivity of bacteriological methods in the design and interpretation of the results of epidemiologic studies based on faecal culture.

In vitro susceptibility of Yersinia enterocolitica isolated from the oral cavity of swine.

Antimicrobial susceptibility of 181 (107 ail-harboring isolates and 74 non-ail-harboring) Yersinia enterocolitica isolates obtained from the oral cavity of swine was determined against 24 antimicrobial agents. All Y. enterocolitica isolates were susceptible to sulfamethoxazole/trimethoprim, enrofloxacin, aminoglycosides, and nitrofurantoin. Susceptibility to tetracycline appeared to vary by lot of origin. Isolates were resistant to sulfonamides (other than sulfamethoxazole/trimethoprim), penicillin, ampicillin, ticarcillin, cephalothin, macrolides, and tiamulin.

Prevalence of pathogenic Yersinia enterocolitica in groups of swine at slaughter.

In order to estimate the prevalence of swine herds infected with pathogenic Yersinia enterocolitica, 103 lots of market swine were randomly selected at slaughter during six 1-month intervals. Pigs within each lot were sampled by swabbing the oral-pharyngeal surface, poststunning and postexsanguination but prescalding. Ninety-five lots (92.2%) contained at least one pig infected with Y. enterocolitica. Pathogenic strains were defined as those harboring the ail gene which has been identified in Y. enterocolitica that causes human clinical disease. Identification of those strains harboring the ail gene was accomplished using a polymerase chain reaction technique. Twenty-nine lots (28.2%) contained at least one pig from which ail-containing (pathogenic) Y. enterocolitica were isolated. Of the 107 pathogenic Y. enterocolitica isolates identified, 89.7% were serotype O:5 and 3.7% were serotype O:3. The results from this study will aid in the design of future epidemiological investigations concerning on-farm prevalence and associated risk factors for pathogenic Y. enterocolitica. Additionally, the results support the hypothesis that swine are a significant potential reservoir for human infections by Y. enterocolitica.

Isolation of Salmonella serotypes from feces of pigs raised in a multiple-site production system.

OBJECTIVE: To determine the prevalence and serotypes of Salmonella organisms in feces of pigs raised in a modern, multiple-site production system. DESIGN: Cross-sectional study of prevalence. SAMPLE POPULATION: Swine housed on 7 farms (1 gilt development farm, 2 breeding farms, 1 nursery farm, and 3 finishing farms) that formed a multiplesite production system. PROCEDURE: Fecal samples were obtained from 792 pigs (96 to 202/farm) and submitted for bacteriolgic culture of Salmonella organisms. RESULTS: Salmonellae were isolated from pigs on all 7 farms and from 95 of 792 (12%) fecal samples. Prevalence ranged from 3.4% at the gilt development farm to 18 and 22% at the breeding farms. Serotypes identified were Salmonella derby, S typhimurium var. copenhagen, S heidelberg, S typhimurium, S mbandaka, S worthington, and S tennessee. No single serotype was not isolated from all the farms of the production system and the most prevalent serotypes at the 3 finishing farms (S typhimurium or S typhimurium var. copenhagen) were not isolated from the breeding or nursery farms. CLINICAL IMPLICATIONS: Upstream infection (pigs infected before arriving at finishing farms) appears to be an unimportant source of Salmonella infection of finished hogs in multiple-site systems. High prevalence of Salmonella shedding in breeding animals suggests that food products derived from culled breeding livestock may be an important source of foodborne disease.