Salmonella diagnosis in pig production: methodological problems in monitoring the prevalence in pigs and pork

Salmonellosis is an important foodborne infection in industrialized and developing countries. Especially for human Salmonellosis caused by Salmonella enterica serovar Typhimurium, pigs and pork are the major sources of infection. Mitigation and control strategies that result from surveillance programs attempt to reduce or even eradicate Salmonella in pork to lower consumers' risks. The methodology for Salmonella screening in pigs is generally based on antibody detection at slaughter with meat juice as the sample matrix. The instructions to most commercial enzyme-linked immunosorbent assay (ELISA) kits for the detection of Salmonella antibodies state that their product is suitable for antibody detection in meat juice and sera. In the present study, we show that it is essential to recalculate the percent optical density (OD%) data obtained from meat juice by the ELISA (IDEXX HerdCheck swine Salmonella) by the following regression equation: OD%sera = -70.5587 + 128.1490/ {1 + exp[(-18.8969 - OD%meatjuice)/27.6032]}(1.1771), r = 0.87, to compare results with those obtained from sera. By this regression equation, we were able to compare the Salmonella antibody levels (classified as <10, 10 to <20, 20 to <40, and > or =40 OD%) for sows, growers, and slaughter pigs. We identified significantly higher numbers of growers with lower OD% levels than for sows and slaughter pigs. Without a recalculation of the meat juice results, the higher fraction of samples with low OD% values led to an underestimation of the actual seroprevalence.     

Use of a serological approach for prediction of Salmonella status in an integrated pig production system

Relevance of a Salmonella serological detection technique was studied from complete results obtained from 9 pigs fattening units. Feces and overshoes were sampled at different periods after starting fattening (2, 3 and 4 months) while caecal contents were taken on the slaughter line. The bacteriological technique used was based on a Diasalm enrichment and a commercial test was used for serology on an average of ten animals per batch. The aim of this work was to establish a correlation between serological results obtained at slaughter (10 samples/batch) and bacteriological results. In this context, two types of logistic regression models were tested by considering alternatively serology and Salmonella detection in caecal contents as the dependent variables. Firstly, beside the fact that all logistic regression models show weak correlations, the first finding was that positive results in overshoes taken at 2 and 3 months are slightly correlated with serological status of herds (odds-ratios of 4.96 and 2.55). Secondly, when batches were characterized as positive on the basis of serological results, the probability of Salmonella recovery in caecal contents was higher than when the batches were considered as negative (odds-ratios comprised between 4.36 and 5.81). A major conclusion is that serology can be used to follow the improvement of an integrated pig production system, but is not the unique solution for assessing risk of Salmonella shedding from specific herds.     

Approaches for reducing Salmonella in pork production

Salmonellosis is an important disease in humans and is associated with contaminated food, including pork products. Salmonella infection is invasive in humans, but it usually remains latent within the swine population, creating reservoirs for carcass contamination. Although abattoirs implement stringent procedures during carcass processing, some raw pork products still have Salmonella contamination. To reduce the presence of Salmonella, a dynamic picture of the pork production chain is needed that includes management practices aimed at health and welfare of swine and practices within swine operations that affect the environment and community health. Swine practices indirectly influence the spread of zoonotic enteric pathogens. Pathogens in food animals can escape detection, and critical control points often are missed. Preharvest growth of swine by enhancement of normal gut flora and targeting intestinal pathogens through nonantibiotic approaches might improve food safety and reduce antibiotic residues. In light of the threat posed by multidrug-resistant pathogens, old dogma is being revisited with optimism for potential utility in promoting pre- and postharvest pork safety. This review includes possible approaches that can be implemented in swine operations and postslaughter during pork processing with simultaneous omission of subtherapeutic antibiotics to control Salmonella. We emphasize the vital roles of the veterinarians, pig producers, industry, food research scientists, and government guidelines for the strategic implementation of approaches to Salmonella control across the pork production and processing chains.     

Influence of Salmonella in pigs preharvest and during pork processing on human health costs and risks from pork

Salmonellosis in humans is a costly disease traditionally assumed to be associated with exposure to contaminated food. We have developed a farm-to-fork model that allows estimation of the human health costs and risks associated with Salmonella in pork. This analysis focuses on the stages of the pork production chain up to the point of producing a chilled pork carcass. The model predicts the number of human cases of salmonellosis associated with pork (mean, 99,430; 90% confidence interval, 20,970 to 245,560) and the corresponding social costs (mean, $81.53 million; 90% confidence interval, $18.75 million to $197.44 million). Sensitivity and scenario analyses suggest that changes in Salmonella status during processing are more important for human health risk and have a higher benefit:cost ratio when compared with on-farm strategies for Salmonella control. Specifically, benefit:cost ratios are less than 1 (indicating they are not likely to be profitable from a social economic perspective) for the on-farm strategies of vaccination and meal feeding, whereas rinsing carcasses at various temperatures with and without sanitizer all have benefit:cost ratios greater than 1 (indicating they are profitable from a social economic perspective). This type of modeling is useful for evaluation of the relative cost effectiveness of interventions at different points in the food chain when allocating limited food safety dollars and is best used for examining trends and alternative strategies rather than for providing definitive dollar value estimates of risk. The dollar value estimates must be considered in the context of the wide confidence intervals.     

Pre-harvest risk factors for Salmonella enterica in pork production

Salmonella is an important issue to the pork industry worldwide. Although Salmonella has been identified in all links of the pork production chain, there has been increasing focus on the pre-harvest phase (on-farm). Many investigations have been conducted to identify risk factors for Salmonella infections in pigs. In this review, we surveyed the literature to present a compilation of the scientific knowledge currently available about potential pre-harvest risk factors for Salmonella infection in swine populations, and discussed some of the potential fundamental issues associated with this type of on-farm studies. Because of the dynamic relationship between the host (the pig), the agent (Salmonella), and the environment, definitive statements regarding transmission, shedding, and carrier states are difficult. The number of potential sources of Salmonella infections for a swine population is endless, and the pattern of Salmonella transmission and shedding in swine populations is the result of the combination of a variety of factors resulting in a multitude of potential scenarios. Pigs may be infected with Salmonella during any of the production stages on-farm, and as shown in this review, a variety of risk factors may affect the probability of infection. Moreover, between the farm and the abattoir, additional factors can further increase the risk of Salmonella infections. Therefore, at the time of slaughter, the probability of Salmonella infections in pigs results from the occurrence of risk factors on-farm and between the farm and slaughter. Although a variety of risk factors has been reported, the lack of consistency, the methodological limitations, as well as the complex and dynamic epidemiology of Salmonella in swine populations prevent definitive conclusions. Based on the evidence available at the moment, we conclude that pre-harvest Salmonella control programs must be based essentially on strict measures of biosecurity and hygiene, which should minimize the risk of exposure of the pigs to potential infection sources, as well as the persistence of the bacteria in the herd. Moreover, particular attention must be given to the pre-slaughter process of transportation and lairage, where rigorous washing and disinfection programs should be applied.     

Effect of logistic slaughter on Salmonella contamination on pig carcasses

Previous studies have shown that infected pigs are the source of carcass and slaughterhouse environment contamination by Salmonella. The present study tried to evaluate the effect of a logistic slaughter, organised according to Salmonella seroprevalence, on Salmonella contamination on carcasses. The study was performed at the beginning of slaughtering during three consecutive days. Low risk herds (8 batches) were slaughtered on day I, high risk herds (6 batches) on day II, and finally, moderate risk herds (5 batches) were slaughtered on day III. Each slaughtering day, holding pens, five points of the slaughter line, and 80 carcasses were sampled. The number of positive carcasses on days I, II and III was 7 (8.8%), 5 (6.3%) and 19 (24.4%) respectively. The results evidenced no clear effect of the logistic slaughter on carcass contamination, with a three times higher risk of finding a positive carcass when moderate Salmonella risk batches were slaughtered. Carcass contamination in low risk herds was linked to the contamination of holding pens and the slaughter line activities. On the other hand, Salmonella was not detected in any of the sampled carcasses in three out of six high risk Salmonella batches, showing that proper slaughtering practices can prevent carcass contamination. The experience reported here, demonstrates that apart from an accurate batch separation according to their seroprevalence levels, strict measures for cleaning and disinfection in the lairage and the slaughterhouse facilities are needed when logistic slaughter is performed.     

An evaluation of the Norwegian Salmonella surveillance and control program in live pig and pork

Population data and apparent prevalence data from the Salmonella surveillance and control program in pigs (NSSCP) from 1998 and 1999 were used in a simulation model to evaluate the efficacy of the program. The model consists of three parts: modelling of individual prevalence at the abattoir (abattoir part), modelling of the number of sampled herds of different sizes when carcasses are randomly sampled at the abattoir (sampling strategy part) and finally, modelling of the within herd prevalence (within herd part). A total of 136,550 sows and 2,866,550 finishing pigs slaughtered, 4446 herds and 11 herds positive for Salmonella in 1994/1995-2000 were included in the abattoir part, sampling strategy part and the within herd part of the model, respectively. The abattoir part showed an average estimated prevalence of Salmonella in sows and finishing pigs of (median) 0.4% (5-95 percentiles = 0.03-2%) and 0.1% (0.04-0.2%) respectively. The estimated number of infected sow carcasses that entered the market was 502 (37-2157) while the estimated number of finishing pig carcasses was 2919 (1218-5771). The probability of being sampled for the 10% smallest herds was (mean) 1.9% (1.6-2.2), to 25% (24.7-26.5%) for the 10% largest herds. The within herd prevalence was estimated to be from 1% to 4% for Norwegian pig herds. The conclusions drawn from this evaluation are that the NSSCP does not have any significant consumer protection effect, and that the documentation could be done more effectively using the herd rather than the individual animal as the unit of sampling. Sampling should focus on the larger herds supplying most of the meat on the market and on herds that produce breeding sows and piglets and thus can contribute to the spread of Salmonella among herds.     

Diversity of Salmonella enterica serovar Derby isolated from pig, pork and humans in Germany

Salmonella enterica serovar Derby (S. Derby) is one of the most prevalent serovars in pigs in Europe and in the U.S. and ranks among the 10 most frequently isolated serovars in humans. Therefore, a set of 82 epidemiologically unrelated S. Derby strains isolated between 2006 and 2008 from pigs, pork and humans in Germany was selected and investigated in respect to the transmission of clonal groups of the serovar along the food chain. Various phenotypic and genotypic methods were applied and the pathogenicity and resistance gene repertoire was determined. Phenotypically 72% of the strains were susceptible to all 17 antimicrobials tested while the others were monoresistant to tetracycline or multi-resistant with different resistance profiles. Four major clonal groups were identified based on PFGE, sequence data of the virulence genes sopA, sopB and sopD, VNTR-locus STTR5 and MLST revealing also the new sequence type ST774. Thirty different PFGE profiles were detected resulting in four clusters representing the four groups. The pathogenicity gene repertoire of 32 representative S. Derby strains analyzed by microarray showed six types with differences in the Salmonella pathogenicity islands, pathogenicity genes on smaller islets or prophages and fimbriae coding genes. The pathogenicity gene repertoire of the predominant types PAT DE1 and DE2 were most similar to the ones of S. Paratyphi B (dT+, O5−) and to a minor degree to S. Infantis and S. Virchow PATs. Overall this study showed that in Germany currently one major S. Derby clone is frequently isolated from pigs and humans. Contaminated pork was identified as one vehicle and consequently is a risk for human health. To prevent this serovar from entering the food chain, control measurements should be applied at the farm level.     

Interventions to control Salmonella contamination during poultry,cattle and pig slaughter

The fundamental principle of controlling microbial contamination during slaughter is based on sanitary and hygienic processes. Both choosing abattoir technologies and conducting individual operations should be approached with the primary goal of minimizing microbial load on the final product. Nevertheless, even when best hygienic abattoir practices are applied, complete prevention of all microbial contamination of carcasses is unachievable under commercial conditions. Therefore, in some situations it may be considered necessary to further reduce the microbial loads on carcasses through application of additional control interventions, i.e. decontamination treatments. Treatments applied on poultry carcasses or parts include water, steam and chemical solutions (e.g., lactic or acetic acid, chlorine-based compounds, cetylpyridinium chloride, and trisodium phosphate) and result in overall microbial reductions of 0.6–3.8 log units; antimicrobial activity of some chemicals (e.g., chlorine compounds) is reduced in the presence of organic material. Decontamination treatments of hides (pre-skinning) and/or cattle carcasses reduce Salmonella by < 0.7–5.1 log units. Salmonella prevalence reductions achievable by decontamination of porcine carcasses seem to be at least two-fold. Overall Salmonella reductions on final carcasses and meat can be significantly improved when multiple decontamination treatments are applied sequentially during slaughter and dressing operations. It is important to note that decontamination interventions should be validated and considered as part of a hazard analysis critical control points (HACCP)-based food safety system which is subject to verification and auditing, and they should never be used as a substitute for good sanitation and proper hygiene practices.     

Bacterial contamination of recirculating brine used in the commercial production of moisture-enhanced pork

In a commercial process for the production of moisture-enhanced pork, boneless pork loins were conveyed through a recirculating injection apparatus, and brine (sodium phosphate, sodium chloride, and lemon juice solids) was pumped into the meat through banks of needles inserted automatically into the upper surfaces of cuts. Brine samples were collected at intervals during the production process and analyzed to determine the total plate count and the numbers of lactic acid bacteria, pseudomonads, Brochothrix thermosphacta, and Enterobacteriaceae. Listeria monocytogenes numbers in the brine were determined using a PCR with primers for the hemolysin gene in combination with a most probable numbers determination. Maximum numbers of bacteria (log CFU/ml) recovered from the brine after 2.5 h of recirculation were as follows: total plate count, 4.50; lactic acid bacteria, 2.99; pseudomonads, 3.95; B. thermosphacta, 2.79; and enterics, 3.01. There was an increase in the number of L. monocytogenes in the recirculating brine with time, reaching a maximum of 2.34 log CFU/100 ml after 2.5 h of moisture-enhanced pork production. Thus, recirculating brines can harbor large populations of spoilage bacteria and L. monocytogenes and are an important source of contamination for moisture-enhanced pork.     

Estimating the probability and level of contamination with Salmonella of feed for finishing pigs produced in Switzerland--the impact of the production pathway

Contaminated feed is a source of infection with Salmonella for livestock, including pigs. Because pigs rarely show clinical signs of salmonellosis, undetected carriers can enter the food production chain. In a “Farm to Fork” food safety concept, safe feed is the first step for ensuring safe food. Heat treatment or adding organic acids are process steps for reducing or eliminating a contamination with Salmonella. The aims of this study were (I) to estimate the probability and the level of Salmonella contamination in batches of feed for finishing pigs in Swiss mills and (II) to assess the efficacy of specific process steps for reducing the level of contamination with Salmonella. A quantitative release assessment was performed by gathering and combining data on the various parameters having an influence on the final contamination of feed. Fixed values and probability distributions attributed to these parameters were used as input values for a Monte Carlo simulation. The simulation showed that–depending on the production pathway–the probability that a batch of feed for finishing pigs contains Salmonella ranged from 34% (for feed on which no specific decontaminating step was applied) to 0% (for feed in which organic acids were added and a heat treatment was implemented). If contamination occurred, the level of contamination ranged from a few Salmonella kg⁻¹ feed to a maximum of 8E+04 Salmonella kg⁻¹ feed. Probability and levels of contamination were highest when no production process able to reduce or eliminate the pathogen was implemented. However, most of the Swiss production was shown to undergo some kind of decontaminating step. A heat treatment, in combination with the use of organic acids, was found as a solution of choice for the control of Salmonella in feed.     

Salmonella contamination during production of cantaloupe: a binational study

Six cantaloupe farms and packing plants in South Texas (950 cantaloupe, 140 water, and 45 environmental samples), including the Rio Grande Valley area, and three farms in Colima State, Mexico (300 cantaloupe, 45 water, and 15 environmental samples), were sampled to evaluate cantaloupe contamination with Salmonella and Escherichia coli during production and processing. Samples collected from external surfaces of cantaloupes, water, and the environments of packing sheds on cantaloupe farms were examined for the presence of Salmonella and E. coli. Of a total of 1,735 samples collected, 31 (1.8%) tested positive for Salmonella. Fifteen Salmonella serotypes were isolated from samples collected in Texas, and nine from samples collected in Colima. Two serotypes (Poona and Oranienburg) that have been associated with three large Salmonella outbreaks in the United States and Canada linked to the consumption of contaminated cantaloupe were found in water samples collected at four farms (three from the United States). Susceptibility of Salmonella isolates to 10 antimicrobials was evaluated by disk diffusion. Eighty-eight percent of the isolates from the United States and Mexico were pansusceptible to the antimicrobials tested; eight isolates from the United States demonstrated an intermediate susceptibility to streptomycin and only two isolates were resistant to the same antimicrobial. From Mexico, four isolates showed an intermediate susceptibility to streptomycin and one isolate was resistant to nalidixic acid and streptomycin. Repetitive sequence-based PCR analysis of Salmonella isolates helped to trace potential sources of Salmonella contamination in source water and in subsequent water samples obtained after the filtration systems of U.S. and Mexican cantaloupe farms. No differences could be seen between the levels of Salmonella contamination in melons from both countries.     

HACCP体系在冷却分割猪肉生产中的应用

探讨了HACCP原理在冷却分割猪肉生产及安全控制中的实施和应用。通过对冷却分割猪肉生产过程的危害分析,确定了生猪收购、电击昏、冷却、冲洗、分割、包装、冷藏、运输等8个关键控制点。并根据HACCP原理,分别制定了各个关键控制点的关键限值和监控措施。结果表明,通过在冷却分割猪肉中实施HACCP体系,可以有效的将危害降低到可接受水平,从而确保肉的品质与安全。     

Sulfamethazine water medication pharmacokinetics and contamination in a commercial pig production unit

Sulfamethazine is often used to treat disease in the swine industry. Sulfamethazine is available as water or feed medication and historically (over the past 40 years) has been associated with residue violations in both the United States and Europe. Despite sulfamethazine's approval for use as a water medication, little research on the pharmacokinetics of the water formulation is available. Therefore, a pilot study was performed to determine the plasma levels of an approved sulfamethazine water medication. Plasma levels in pigs treated with an oral bolus (250 mg/kg), which is equivalent to the total drug consumed within a 24-h period, achieved therapeutic concentrations (50 microg/ml). Noncompartmental-based pharmacokinetic model parameters for clearance, half-life, and volume of distribution were consistent with previously published values in swine. However, the above treatment resulted in exposure of pen mates to sulfamethazine at levels currently above tolerance (0.1 ppm). Using a physiologically based pharmacokinetic model, the treatment dose simulation was compared with observed plasma levels of treated pigs. Flexibility of the physiologically based pharmacokinetic model also allowed simulation of control-pig plasma levels to estimate contamination exposure. A simulated exposure to 0.15 mg/kg twice within approximately 8 h resulted in detectable levels of sulfamethazine in the control pigs. After initial exposure, a much lower dose of 0.059 mg/kg maintained the contamination levels above tolerance for at least 3 days. These results are of concern for producers and veterinarians, because in commercial farms, the entire barn is often treated,and environmental contamination could result in residues of an unknown duration.     

Carry-over and contamination of veterinary drugs in feed production lines for poultry and pigs

The current model of pigs and poultry production has increased stocking density and reduced downtime between lots. This scenario may favour a rise of infection and disease risk in animals. To avoid this problem, routine use of drugs has been made, intensifying the possibility of residues in animal products and antimicrobial resistance. The aim of this study was to measure carry-over and contaminants of veterinary drugs included in the feed production lines for poultry and pigs. Samples were taken from 25 production lines. LC-MS/MS was used for simultaneous analysis of 62 active ingredients. In the medicated feed, 80.4% of the samples had a concentration of active ingredients different from the stated doses. In 70% of the feed samples, there was heterogeneity in the mixture of active ingredients. In subsequent feeds, carry-over was greater than 1% of the stated dose in 63% of cases. Of the 25 analysed lines, only one showed no contamination with other active ingredients. No correlations were found between the nutritional composition of the feed and carry-over. The present study demonstrated that the current production model allows the occurrence of unwanted drugs in feed-in doses that can contaminate animal products and can select antimicrobial-resistant bacteria.     

Detection of Listeria monocytogenes in pigs and pork

In this study, we surveyed hogs (n = 300) as well as pork products (ground pork and raw chitterlings) for Listeria monocytogenes. Pig specimens collected before (tonsil swabs) and after slaughter (tonsils, lymph nodes, carcass swabs, and rectal contents) were examined for L. monocytogenes by enrichment with conventional enrichment broths followed by subculturing to selective agar. A multiplex polymerase chain reaction assay targeting the highly conserved 16S rRNA gene of the Listeria species as well as the hlyA gene unique to L. monocytogenes was used to screen aliquots of the enrichment (method I) as well as to confirm presumptive Listeria colonies from Columbia agar with 0.05% glucose supplemented with polymyxin B-acriflavine-lithium chloride-ceftazidime-aesculin-mannitol (PALCAM; method II). Subculturing to PALCAM agar was the more sensitive of the two methods on the basis of the overall detection of Listeria. For hog tissues, method I detected L. monocytogenes (0.87% positive) and no other Listeria spp. in all samples (n = 1,849). In contrast, method II detected significantly more (P < 0.05) L. monocytogenes (2.38%) and Listeria spp. (0.38%) in these tissues. For small intestines (n = 300 raw chitterlings), L. monocytogenes was identified in 8.3% of enrichments with University of Vermont modified Listeria enrichment broth; plating to PALCAM slightly improved recovery (9%). Overall, ground pork samples (n = 340) harbored L. monocytogenes (45% positive) and other Listeria species (1.5% positive), as determined by method I. Subculturing to PALCAM significantly (P < 0.05) improved the detection of L. monocytogenes (50.2%) but not that of other Listeria species (1.7%). L. monocytogenes isolates (n = 243) were assigned to serotype 1 (53.5%), serotype 4 (25%), and serotypes other than 1 and 4 (21.4%).     

生猪肉中沙门氏菌的分离与鉴定

目的建立一套高效的沙门氏菌分离和鉴定方法,同时对本地区生猪肉中沙门氏菌的污染情况做初步评估。方法分别从本地区屠宰场、农贸市场和超市各采集10份生猪肉样品,共计30份。然后对采集的样品进行选择性培养,初步筛选疑似菌株,通过对疑似菌株进一步进行生理生化鉴定、16S rDNA基因序列的比对并构建系统发育树,最终确认沙门氏菌。结果研究表明在采集的30份样品中共获得1株疑似菌株,并通过一系列试验鉴定为沙门氏菌,同时表明该地区生猪肉中沙门氏菌的检出率为3.3%。结论该研究为丰富沙门氏菌数据库资料和流行病学的研究提供一定的理论依据,同时为进一步对该地区肉及肉制品中沙门氏菌的风险评估奠定基础。     

济南市肉鸡生产链沙门菌污染监测及分析

了解肉鸡孵化、养殖、屠宰加工和配送分销各环节的沙门菌污染状况,确定易受污染的环节和样品类别,为预防和控制由沙门菌引起的食源性疾病提供科学依据。方法 按照随机抽样的原则,在济南市肉鸡孵化场、养殖场、屠宰场(点)和配送分销点设置16个监测点,按不同的季节和环节定期随机采集监测样本。检验方法按照《2012年全国食源性致病菌监测工作手册》执行。结果 监测17种样品共1204份,13种样品检出沙门菌202株,总检出率为16.78%。除1种不可分型外,其他分属4群28种血清型,以印第安纳血清型沙门菌最多(37.13%);4个环节中,以屠宰加工环节沙门菌检出率最高(23.14%);四个季度中,以第二季度沙门菌检出率最高(21.74%);17种样品中,以屠宰环节的褪毛后整禽样本沙门菌检出率最高(42.19%)。结论 济南市肉鸡相关行业的沙门菌交叉污染严重,应加强肉鸡生产链全过程的监管和监测,并针对各污染环节和工序,确定不同的关键控制点,制定相应的操作程序,以减少沙门菌的交叉污染。