A survey of pre-slaughter conditions, halothane gene frequency, and carcass and meat quality in five Spanish pig commercial abattoirs

A total of 116 deliveries, comprising 15,695 commercial pigs delivered to five abattoirs, were surveyed during winter and summer. Information about on-farm fasting, transport duration and stocking density, and lairage time was collected. Cortisol, creatine phospho-kinase (CPK), and lactate, and DNA for halothane genotype were analysed in a subsample of pigs at exsanguination in every journey. Electrical conductivity (PQM) in semimembranosus muscle (SM) and carcass characteristics (Fat-o-Meater and skin damage) were measured in each carcass. pHu of SM was analysed in the laboratory in a subsample in every journey. Carcasses were identified as PSE or DFD based on PQM and pHu, respectively. The n gene frequency ranged among abattoirs from 54 to 8%. Mean lean content was 58.9% for nn, 57.3% for Nn, and 55.8% for NN pigs, though a difference of 2.5% lean was observed between two abattoirs with the same n gene frequency. A straight relationship of the incidence of serious PSE carcasses and n gene frequency was found. The overall incidence of serious PSE and DFD carcasses was 6.5 and 12.5%, respectively. A higher incidence of PSE carcasses was found in summer; in deliveries with <12 h on-farm fasting; with transport stocking densities >0.40 m(2)/100 kg pig; and in transports of <2 h duration. A higher incidence of DFD carcasses was found in winter; with transport stocking densities <0.40 m(2)/100 kg pig; transports of >2 h duration; and lairage times >9 h. Cortisol level in blood increased in winter and decreased after 12-18 h fasting time. A rise in the lactate concentration was observed in pigs transported in high stocking density (<0.40 m(2)/100 kg pig) and for a longer time (>2 h). All blood stress indices increase as increasing lairage time. Carcasses with more skin damage had higher levels of cortisol, CPK and lactate, and higher incidence of DFD meat, compared with non and low skin damage carcasses.     

Risk assessment of PSE condition due to pre-slaughter conditions and RYR1 gene in pigs

A polychotomous logistic regression model was used to identify and assess the risk factors for pork meat becoming pale, soft and exudative (PSE). A total of 116 deliveries, comprising 15,695 commercial pigs delivered from different commercial farms to five Spanish pig commercial abattoirs were surveyed. The PSE condition was described as an ordinal response variable (normal, prone to PSE and PSE) based on measurements of electrical conductivity in the Semimembranosus muscle 1-2 h post-mortem. The RYR1 genotype, the abattoir, the season, the gender, the floor surface in the lorry, the loading system, and the stocking density during transportation influenced the risk of PSE condition, as well as on-farm fasting time, loading time, transportation and lairage times. The effect of the RYR1 gene, determined in a subsample of 1331 pigs, was due to nn stress-susceptible pigs, in which the risk of PSE meat (PQM>6 μs) increased fourfold. Abattoirs should be especially careful in summer, when the risk of PSE meat was found to be double that of winter. The risk of PSE meat decreased with the time of transportation, though its effect depended on the stocking density. Thus, our results indicate that, for transits longer than 3 h, the risk of PSE increases with stocking density during transport, while the opposite occurs for shorter transits.     

The effect of stocking density in transit on the carcass quality and welfare of slaughter pigs: 2. Results from the analysis of blood and meat samples

In a trial involving 2496 pigs, the influences on blood profile and pork quality of stocking densities ranging from 201 to 321 kg m⁻² were examined. The pigs came from four different farms and were killed in 16 weekly batches. They were transported for on average 3 hr and held in lairage for 1 hr. Higher stocking densities resulted in more physical stress to the pigs based on the activity of the enzyme CPK in the blood. Stocking density did not apparently affect psychological stress and high densities did not result in dehydration. The colour, water holding capacity and instrumentally-determined texture of the pork from the carcasses of the pigs were not affected by stocking density. The experimental design and precision of the measurements were sufficient to detect numerous differences in blood profile and pork quality between pigs from the four source farms. These differences probably related to different degrees of stress-susceptibility. The results suggest that the highest stocking density examined (321 kg m⁻²) is unacceptable for the transport of pigs. The second highest density (281 kg m⁻²) produced relatively little evidence of an adverse effect on the welfare of pigs. It may be scientifically acceptable for short journeys (≤3 hr) but not for longer ones where pigs need more space to lie down.     

Combined effects of pre-slaughter treatments and lairage time on carcass and meat quality in pigs of different halothane genotype

The effect of two different lairage times on meat quality was investigated in pigs with different halothane genotype previously submitted to controlled pre-slaughter treatments. One hundred and ninety nine Italian heavy pigs were loaded by ramp or lift and transported unmixed for 1 h to the abattoir at a stocking density of either <0.4 or >0.6 m(2) per 100 kg pigs. After unloading, an equal number of animals within each previous treatment was held in lairage for 2 h or overnight (22 h) before slaughter. Carcass and meat quality and incidence of skin damage were evaluated. Longer lairage did not negatively affect carcass traits and reduced the incidence of PSE meat without increased DFD occurrence. It also seriously increased the amount of skin damage due to fighting, resulting in long fasting. Pre-slaughter treatments and halothane genotype showed a negligible effect on the response to the resting time.     

Risk assessment of skin damage due to pre-slaughter conditions and RYR1 gene in pigs

A polychotomous logistic regression was used to assess the risk factors for skin damage prior to slaughter. A total of 116 deliveries (15,695 pigs) from commercial farms to five Spanish pig abattoirs were surveyed. The skin damage condition was described as an ordinal response (1: no damage; 2: very slight damage; and 3: slight or more damage). The abattoir by the season (p < 0.01), the floor surface of the lorry (p = 0.02), and the mixing of unfamiliar pigs at loading (p = 0.01) influenced the occurrence of skin blemishes. Skin damage increased with on-farm fasting time (p < 0.01), loading time (p < 0.01), lairage time (p < 0.01), as well as with carcass weight (p < 0.01). In winter transports the risk of skin damage decreased (p < 0.01) at higher space allowance in the lorry. The risk of developing PSE and DFD pork increased with the skin damage score (p < 0.01). The nn genotype for the RYR1 gene was less prone to skin damage but at the same time it is more sensitive to skin damage in relation to PSE risk. It is concluded that the skin damage score can be used as a rapid indicator of animal welfare and pork quality.     

Salmonella enterica serovars from pigs on farms and after slaughter and validity of using bacteriologic data to define herd Salmonella status

The primary objective of this study was to evaluate the validity of using data obtained from slaughtered pigs for farm-level epidemiologic studies of Salmonella. The study involved groups of pigs from five farms. Salmonella isolates were obtained from on-farm samples, and a total of 370 on-farm and an additional 486 isolates from samples collected after commercial slaughter were subsequently tested. Preharvest samples included feces of individual animals from defined groups of nursery and finishing pigs on commercial farms and swabs from trucks. Postslaughter samples were cecal contents and mesenteric lymph node samples. The concordance between Salmonella serovars isolated from on-farm samples and those serovars isolated after slaughter varied widely among farms. Results of paired lymph node and cecal cultures were strongly associated (odds ratio, 7.0), but the agreement between on-farm and postslaughter results at the pig level was poor (kappa = 0.34). The results support recent findings that risk of exposure to Salmonella during transport and lairage remains a concern under contemporary industry conditions. The findings further imply that slaughter plant studies based on phenotyping of Salmonella alone (such as serovars) may not reliably indicate the Salmonella status of commercial swine farms.     

Effect of different stocking densities during transport on welfare and meat quality in Danish slaughter pigs

Four stocking densities (0.35, 0.39, 0.42 and 0.50 m² per 100 kg pig) were investigated under Danish early Autumn conditions (16–24 ° C) in a study of the effect of transport conditions on pig welfare and meat quality. Two transports were carried out for each stocking density, the two producers concerned delivering pigs to the upper and lower tiers, respectively, of a specialist pig transport vehicle. Total average journey time was 2 hr 39 min and total average transport time 1 hr 47 min. After arrival at the factory pigs were lairaged in groups of 15 for about hr and then slaughtered using minimal stress with group CO₂-stunning. Pig behaviour was monitored during transport in the front compartment of the lower tier. Blood samples were taken at slaughter for analysis of creatine phosphokinase, lactate and cortisol and carcasses were evaluated for skin damage and meat quality. Stocking density, as such, had relatively little effect on blood profile or meat quality. Creatine phosphokinase levels were lowest with a stocking density of 0.50 m² and the incidence of unacceptable skin damage was generally lowest with a stocking density of 0.35 m² and highest with 0.42 m². The main source of variation in this experiment was day of transport/slaughter within stocking density so that factors other than stocking density have affected the results obtained. Stocking density had a direct effect on pig behaviour during transport. Giving pigs more space during the short transport did not result in them lying down. On the contrary there was continuous disturbance from other pigs and at 0.42 and 0.50 m² pigs had difficulty maintaining balance, when the vehicle negotiated bends or poor road surfaces. Variations in pig behaviour during transport could account for the observed differences in skin damage.     

Influence of lairage environmental conditions and resting time on meat quality in pigs

This study investigated the influence of lairage environmental conditions and resting time on pig carcasses and meat quality. The experimental material consisted of 1001 cross Pietrain-Duroc-Hampshire × Belgium-LR-LW pigs, held in lairage for either ≈30 min (direct slaughter) or between 2-3 h under 12 °C/90% relative humidity (RH), 20 °C/80% or 90% RH and 35 °C/50% or 85% RH. Prior to arrival at the lairage plant they were transported for about 45-60 min and subjected to a fasting period of 36 h before loading. Unloading operation and the driving of pigs to the point of stunning were carried out according to the practices used in the plant (sticks and electrical goads were used). Batches of 20-30 mixed pigs were used in each trial, held at a stocking density of approximately 0.55 m(2)/pig (≈100 Kg live weight). Lairage environmental conditions (LC), significantly affected almost all measurements, but not pH(1), in Semi-membranosus (SM) and Longissimus dorsi (LD) muscles and the carcass damage score. The influence of resting time (RT) was basically exerted on pH(u), deep ham temperature and in pH(1), of SM, the internal muscle reflectance being mostly unaffected. There were also significant batch (B) effects in a large range of parameters. Factors greatly interacted their influence on carcass and meat quality, denoting LC × B, LC × RT × B and LC × RT the most significant effects. RT × B only showed two low significant interactions for rigor value and pH(1), in SM, suggesting that, conversely to the lairage environmental conditions the influence of resting time is practically unaffected by the day of slaughter. The increase of lairage temperature decreased the frequency of normal carcasses, followed by an expressive higher incidence of PSE status. The influence of lairage relative humidity on the PSE/DFD muscle incidence depended on the associated temperature, but the most important detrimental effects were noticed in experiments carried out at 35 °C. In respect to lairage resting time, the influence on meat quality is strictly related to environmental conditions, mainly the temperature. Nevertheless, and excepting the assays at 35 °C/85% RH, direct slaughter of pigs (= 30 min in pens) generally produced less carcasses of normal quality than resting periods up to 2-3 h.     

The effect of preslaughter lairage on serum thyroxine and cortisol levels at slaughter,and meat quality of boars,hogs and gilts

Commercial crossbred hogs and gilts, and purebred Landrace boars and gilts were slaughtered after a lairage period of either 2 or 24 h. The meat quality of the carcasses was assessed by measuring the pH of the Longissimus dorsi and adductor muscles at 45 min and 20 h post mortem. Serum cortisol and thyroid parameters were measured in blood taken at slaughter. The incidence of the pale, soft and exudative (PSE) condition was slightly higher in purebred Landrace pigs killed after a short lairage period than in those held in lairage overnight. The PSE condition tended to be more prevalent in Landrace gilts than boars. The percentage of carcasses classified as dark, firm and dry (DFD) was much greater in both commercial crossbred and purebred Landrace pigs killed after overnight lairage. The incidence of the DFD condition was greater in Landrace boars than gilts, particularly after overnight lairage. Serum cortisol levels were higher and serum total thyroxine (T₄) and free thyroxine indices (FTI) lower in pigs killed after a short lairage. Serum total T₄ and FTI values at slaughter were lower in boars than gilts.     

Salmonella in slaughter pigs: the effect of logistic slaughter procedures of pigs on the prevalence of Salmonella in pork

A substantial part of the finishing pigs in the Netherlands is infected with Salmonella. Infection of pigs with Salmonella can occur already on the farm. Pigs can also get infected or contaminated during transport, lairage or slaughter. The aim of this study was to evaluate the effect of separating pigs from Salmonella-infected farms from pigs from Salmonella-free farms during transport, lairage and slaughter on the prevalence of Salmonella on pork after slaughter. Two experiments were carried out. In the first experiment, farms were selected to participate, based on serology of the pigs (Dutch Salmonella ELISA). The pigs were slaughtered at the beginning of the day: firstly, sero-negative herds, secondly, sero-positive herds and thirdly, again sero-negative herds. The latter were slaughtered to investigate the effect of a contaminated slaughterline due to a previously slaughtered positive herd. In the second experiment, farms were selected to participate, based on both serology and bacteriology of the pigs on the farm. Two hundred pigs from Salmonella-free farms were slaughtered after 200 pigs from Salmonella-infected farms. Results showed that the prevalence of Salmonella in pork samples of sero-negative herds was lower than in samples of sero-positive herds. Results also showed that Salmonella contamination of carcasses after slaughter was partially caused by Salmonella-infected herds that were slaughtered before, and partially by residential flora of the slaughterhouse. It is concluded that separate slaughter of sero-negative pig herds can be useful to decrease the prevalence of Salmonella-contaminated pork after slaughter. To avoid cross-contamination by residential flora from trucks, lairage and slaughterline, cleaning and disinfection have to be improved.     

Effects of transportation time,distance, stocking density,temperature and lairage time on incidences of pale soft exudative (PSE) and the physico-chemical characteristics of pork

The study determined the effects of transportation time, distance, stocking density, temperature and lairage time on incidences of PSE and pork quality. Frequencies of PSE cases in stocking density categories within transport duration classes were determined. General linear models, regression and the principal component (PC) analysis were used to analyse the data. Highest incidences of PSE were recorded in autumn season while lowest incidences were recorded in the spring season. Transportation time and stocking density significantly affected pH_u and ultimately PSE incidences although there were no interactive effects. Highest risks of PSE occurrence were observed with more space allowance. The highest incidences of PSE were observed for animals that had travelled for two hours while the PSE cases were lower in animals that travelled for longer times. Distance travelled and transportation time had significant effects (P < 0.05) on thawing loss (TL) % of pork. No relationships were reported between the other pre-slaughter variables and pork quality attributes. With the exception of transportation time and distance travelled which had a positive relationship with TL%, variation in other pre-slaughter variables did not affect meat quality variables. The risks of PSE occurrence were dependent on stocking density and transportation time.     

EFFECT OF ANTEMORTEM JOURNEY AND LAIRAGE AT ABATTOIR ON RABBIT MEAT QUALITY

A study was conducted to determine the effects of journey duration (1 versus 3 h) and lairage time at abattoir (0 versus 5 h) on rabbit meat quality traits. Rabbits transported for 3 h produced meat with significantly higher pH values ( P <  0.01), darker (lower L* values, P <  0.01) and less yellow ( P <  0.05) color, as well as lower losses during cooking ( P <  0.01), than those transported for 1 h. Moreover, animals laired for 5 h yielded meat with more ( P <  0.05) yellow color, cooking losses ( P <  0.05) and higher shear values ( P <  0.01) than rabbits not laired before slaughtering. The nuclear magnetic resonance analysis revealed that these differences ( P <  0.05) are mainly related to changes occurring in the distribution of myofibrillar (T₂₁) and extra-myofibrillar (T₂₂) water. In conclusion, this study indicated that journey time and lairage at abattoir may play an important role in determining rabbit meat quality traits.

PRACTICAL APPLICATIONS

In many previous studies that considered the effect of journey and lairage on rabbit meat quality, animals were stress-induced by extreme conditions of journey and/or lairage that were unlikely to occur during commercial operations. The aim of this study is to establish whether meat quality characteristics are still influenced when journey and lairage times match current commercial production practices. Because processing-plant efficiency depends on product uniformity, it will be interesting to test the potential variability effect of rabbit preslaughter practices on meat quality properties.     

Influence of loading method and stocking density during transport on meat and dry-cured ham quality in pigs with different halothane genotypes

The effect of loading method and stocking density in transit on meat and dry-cured ham quality was investigated in pigs with different halothane genotypes. A total of 507 Italian heavy pigs, supplied by two farms, were loaded by ramp or lift and transported unmixed for 35–55 min to the abattoir at a stocking density of either <0.4 or >0.6m² per 100 kg pigs. After overnight lairage in separate pens with free access to water, the pigs were slaughtered. Halothane genotype was assessed post mortem. Four hundred and thirty-nine pigs had a homozygous dominant (NN) genotype and 68 pigs were heterozygous (Nn). Carcass skin damage, meat quality traits and ham curing parameters were evaluated. Loading method and stocking density showed a negligible effect on meat and dry-cured ham quality while the predominant factor affecting these was the halothane genotype. Nn pigs produced meat with a faster rate of pH fall and lower water holding capacity as well as ham with higher weight losses in salting and greater incidence of defects in the dry-cured product. There were insignificant interactions between halothane genotype and loading method or stocking density. Overall, irrespective of pre-slaughter treatment, the Nn pigs were less suitable for the production of high quality products such as dry-cured ham.     

The colour of the adductor muscle as a predictor of pork quality in the loin

The relation between measurements of colour made in the m. adductor (AD) at 45 min or 20 h post mortem and the quality, assessed subjectively in terms of colour and waterholding capacity, of the m. longissimus (LD) in the loin was examined. The study used data from 100 pig carcasses exhibiting a wide range of meat quality from extreme PSE (pale, soft and exudative) to extreme DFD (dark, firm and dry). The subjective assessments were confirmed by objective measures of paleness (reflectance) and waterholding capacity (drip loss in storage) in the LD. Lightness (L^*) measured at 20 h post mortem in the AD was the best potential predictor of loin muscle quality, explaining 59% of the variation in subjective and objective quality measures. Comparable measurements at 45 min post mortem explained between 21% and 44% of the variation. The equation that described the relation between AD Lightness (L^*) and subjectively assessed LD quality was derived. This could be used to transpose the AD L^* values from a population of slaughtered pigs into nominal subjective scores for the LD, allowing the frequency of the five subjective quality groups (extremely DFD, slightly DFD, normal, slightly PSE, and extremely PSE) in the population of carcasses to be defined.     

A REVIEW OF THE CAUSES OF VARIATION IN MUSCLE GLYCOGEN CONTENT AND ULTIMATE pH IN PIGS

Muscle glycogen at slaughter causes variation in pork technological qualities, since it affects the ultimate pH (24 h postmortem). This review addresses various factors influencing muscle glycogen storage and the ultimate pH of pig meat. Results discussed in this paper indicate that breed and preslaughter handling, i.e., waiting time in lairage, mixing, and to a lesser extent fasting, significantly influence ultimate pH. Furthermore, interactions between handling procedures and breed could be highly significant, as with PSE meat. Since the recent discovery of the major gene rn⁺/RN⁻ which is assumed to influence glycogen concentration in the muscle of live pigs, genotype is considered an important source of variation in ultimate pH. All factors likely to influence glycogen concentration must be considered relative to the metabolic and contractile properties of muscle. These play an important role in the amount of glycogen at rest and in its depletion during chronic stress or muscular exercise.     

Unveiling contamination sources and dissemination routes of Salmonella sp. in pigs at a Portuguese slaughterhouse through macrorestriction profiling by pulsed-field gel electrophoresis

Sixty-nine isolates of Salmonella sp. isolated from the ileum, tonsils, carcass and mandibular and ileocolic lymph nodes of individual pigs slaughtered for consumption in one abattoir were analyzed using serotyping and macrorestriction profiling by Pulsed-Field Gel Electrophoresis (RFLP-PFGE), in order to identify clonal relationships. XbaI macrorestriction was able to distinguish 18 genotypes among the eight identified serotypes: Salmonella Typhimurium (4 genotypes), Salmonella Rissen (3), Salmonella Tennessee (2), Salmonella Enteritidis (2), Salmonella 4,[5],12:i:- (4), Salmonella Give (1), Salmonella Anatum (1), and Salmonella Derby (1). Except for one sample, the serotype and the genotype identified in the samples from the same pork were always the same, allowing to unravel possible dissemination routes of Salmonella sp. through these pork tissues and equate presumptive sources of contamination or infection. Highly significant associations (p < 0.001) were observed for the presence of Salmonella sp. in the ileum and in the ileocolic lymph nodes, as well as between the carcass contamination and the presence of Salmonella sp. in others samples of the correspondent slaughtered pig, such as the ileum, the ileocolic and mandibular lymph nodes and the tonsils. Moreover, 80% of the pigs with ileum and ileocolic lymph nodes positive samples also presented the same salmonella genotype in the correspondent tonsils and, among pigs with positive tonsils, 70% also carried the same genotype in the corresponding mandibular lymph nodes. The occurrence of cross-contamination was also detected, since a genotype identified in other pigs slaughtered in the same day was found in 31% of positive carcasses. The global analysis of the genotypes suggested three different sources of pig infection: the farm of origin, the transportation and the lairage. A particular attention should be paid to the last one, since the majority of the isolates from pig samples were related to infection in the lairage. Since the presence of Salmonella sp. in the ileum of pigs and faeces ingestion promotes tonsils infection and internal dissemination of the agent through the mandibular lymph nodes, as well as drainage to the ileocolic lymph nodes, a potential risk exists at slaughter for Salmonella sp. contamination in the carcasses during pork processing. This risk may be increased by incorrect evisceration techniques and by hygienically inappropriate meat inspection procedures, especially those concerned to the mandibular lymph nodes incisions.     

Influence of Pre-Slaughter Process on Pork Quality: An Overview

Handling practices prior to slaughter have significant influence in the stress level and animal welfare of pigs, and consequently, in the final meat quality. This paper reviews current knowledge about ante mortem factors (fasting period, farm handling, mixing, loading, unloading, transport, lairage conditions, and driving to the stunner) related to the live animal that influence pork quality with special emphasis on technological quality attributes. The development in slaughter technologies and their practical applications in pig slaughterhouses are described. In spite of genetic effects and slaughter techniques, poor pre-slaughter operations lead frequently to an increase of physiological and physical stress in pigs. Ante mortem factors, such as lairage time or moving pigs into the slaughter rooms, and inadequate design of slaughterhouse facilities have an important effect on pig stress, affecting meat quality after slaughter. Some recommendations to guarantee animal welfare and obtain the best meat quality are reviewed.     

A practical method to prevent dark-cutting (DFD) in beef

A method for the prevention of dark-cutting in beef, based on recognition of social relationships within groups of bulls, was tested on 2234 bulls slaughtered under commercial conditions. When the influence of time between loading the animals and slaughter was eliminated statistically, the bulls of the socially stablized group showed the lowest pH₂₄ values, whereas those of the socially unstable group showed the highest ones. When the type of social group was eliminated statistically, the earlier slaughtered animals had lower pH₂₄ values than those slaughtered later. When the two major factors were integrated, there was no statistically significant difference in the pH₂₄ values in bulls of the socially stabilized group whether they were slaughtered immediately after transport or during the following day. In contrast, in bulls from the socially unstable group, the pH₂₄ values increased substantially after overnight lairage at the abattoir. In conclusion, for longer transportion, bulls from loose housing with stable social relationships should be used. It is necessary to keep the same social groups from loading to slaughter, strictly avoiding any mixing of strange bulls. Bulls from tethered stalls should be transported and slaughtered within as short a time after regrouping as possible.     

The incidence of dark cutting beef in Sweden

The incidence of DFD (Dark, Firm, Dry) in Swedish cattle was studied at four abattoirs and the influence of some parameters of special interest for the development of DFD were evaluated. The pH was measured in a total of 2686 carcasses of different categories. Carcasses with a pH(24) ≥ 6·2 were classified as being DFD. Only for electrically stimulated carcasses could the pH(24) be considered to be the ultimate pH value. DFD incidences of 3·4% and 13·2% were recorded as overall means for electrically stimulated and non-stimulated carcasses, respectively. The highest incidence was found in young bull carcasses having almost twice as high an incidence of DFD compared with the other categories. Considering seasonal variations, the highest incidence of DFD was found in the period May to August but significant differences in pH were found only for short lairage times. Overnight lairaging increased the incidence of DFD. However, the influence of the lairage time is closely related to the lairage conditions and needs further study.     

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.