Salmonellosis in pigs

Host adaptation of Salmonella serovars is of greatest importance in determining the clinical features and outcome of the infection. Apart from host-restricted serovars that are able to cause a typhoid-like disease in a single host species (e.g. S. Typhi and S. Paratyphi A in humans, and S. Typhisuis in pigs), other serovars, referred to as host-adapted serovars, are associated with one host species, but are also able to cause illness in other hosts (e.g. S. Choleraesuis in pigs and S. Dublin in cattle) [Reference Uzzau7]. S. Choleraesuis and S. Dublin generally cause severe systemic disease in pigs and cattle, respectively, but occasionally can be responsible for disease in other mammalian hosts, including humans [Reference Uzzau7]. In contrast, ubiquitous Salmonella serovars are the unrestricted serovars that are capable of causing systemic disease in a wide range of host animals, but more frequently cause a self-limiting gastroenteritis in a broad range of species. Examples of unrestricted serovars include S. Typhimurium and S. Enteritidis [Reference Uzzau7].

Infections with S. Typhisuis and S. Choleraesuis serovars in pigs usually result in swine paratyphoid, a severe systemic disease that is often fatal and characterised by fever, shivering, diarrhoea, respiratory distress and depression [Reference Wilcock, Schwartz, Leman, Straw, Mengeling, D'Allaire and Taylor8, Reference Gray9]. Infected pigs exhibit clinical signs within 36–48 h after infection, and shed S. Choleraesuis in their faeces within 24 h of exposure. After experimental inoculation, shedding of the microorganism can vary between 10³ and 10⁶ colony-forming unit (CFU)/g of faeces. However, in natural infections, pigs might be exposed to lower levels of S. Choleraesuis (as low as 4 × 10² CFU/g of faeces) that can still be responsible for high morbidity and severe outbreaks of swine paratyphoid within a relatively short period. Transmission of S. Choleraesuis can thus be very efficient on the farm, during transport and in lairage before slaughter [Reference Gray9]. S. Choleraesuis, including variant Kunzendorf, was the predominant serovar isolated from pigs worldwide, over the past century (especially during the 1950s and 1960s). However, during the late 1990s and early 2000s, it was rarely detected in the EU countries [Reference Fedorka-Cray, Gray, Wray, Wray and Wray10, Reference Davies11]. S. Choleraesuis var. Kunzendorf reappeared in Danish herds from 2012 to 2013, causing multiple outbreaks that were characterised by high mortality (20–30%) among 7–50 kg pigs [Reference Pedersen12].

Although infections in pigs by ubiquitous Salmonella serovars could result in enteric and even fatal disease, infected animals frequently and asymptomatically carry these serovars in the tonsils, gut and gut-associated lymphoid tissue [Reference Fedorka-Cray, Gray, Wray, Wray and Wray10]. In EU countries, non-typhoidal Salmonella infections in pigs are related mostly to S. Typhimurium and its monophasic variant (antigenic formula 1,4,[5],12:i:-; 4,[5],12:i:-; or 4,12:i:-), S. Derby and S. Infantis [3, Reference Nollet13–Reference Nathues15]. Other frequently reported serovars in pigs include Rissen, Panama, Goldcoast, Agona, Brandenburg, London, Anatum, Manhattan, Enteritidis and Bovismorbificans [3, Reference Wales16, Reference García-Fierro17].

Transmission of Salmonella between pigs occurs mainly via the faecal–oral route [Reference Boyen18], although some studies have demonstrated that the upper respiratory tract and lungs could be portals of entry as well [Reference Fedorka-Cray19]. A comparison between the prevalence of infection in pigs on the farm and in the abattoir revealed that prevalence on the farm often seems lower, partly because of the existence of latent, undetectable Salmonella carriers [Reference Hurd20]. These latent carriers may begin to shed Salmonella only after leaving the farm, a process that might be triggered by stress factors linked to group housing, transportation and holding pens at the slaughterhouse, as the physiological changes associated with stress could promote recrudescence in latent carriers, or increase the susceptibility of non-carriers to new infections [Reference Hurd21].

Salmonella in pig farms (pre-harvest stage)

The ‘pre-harvest stage’ refers to that part of the food chain that includes the holding period of pigs on the farm until their departure and loading for transportation to the slaughterhouse [22]. During this period, pigs could become infected with Salmonella and show clinical signs, or they could become asymptomatic carriers and excrete the pathogen in faeces, or harbour it in several tissues, such as the digestive tract, closely associated lymph nodes or the tonsils [Reference Côté23].

Among the clinically affected pigs, most are of the weaning or post-weaning age, and the fattening pigs (body weight from 65 to 100 kg, and over) are generally asymptomatic carriers of unrestricted Salmonella serovars. The carrier status of pigs could be determined by the analysis of faecal cultures, obtained for example, via the collection of faeces with rectal swabs from randomly selected pigs during the fattening period [Reference Visscher24]. The most frequently detected serovars in pigs on farms in the EU are S. Typhimurium monophasic variant, S. Typhimurium and S. Derby [Reference Nollet13, Reference Visscher24, Reference Merialdi25–Reference Denis27]. In 2014, the most common Salmonella serovars isolated from pigs in the EU were S. Typhimurium (50·3%), S. Derby (17·5%), S. enterica 1, 4,[5],12:i:- (8·4%), S. Typhimurium var. Copenhagen (4·4%), S. London (2·1%), S. Infantis (1·7%), S. Muenchen (1·6%), S. Rissen (1·5%), and S. Livingstone (1·2%) [3]. S. Typhimurium has been the predominant serotype detected within the last few years, accounting for as much as 72·8% of isolates in 2012. Since S. Typhimurium had been reported in 9 out of 10 member states in 2014, and was commonly reported in the baseline surveys of slaughter pigs and breeding pigs in 2006/2007 and 2009, respectively, its distribution across the EU can be assumed to be relatively wide [28, 29]. The prevalence of isolates of monophasic strains of S. Typhimurium has shown no considerable change over the last few years, ranging between 8·4% of all isolates in 2014, and 14% in 2013. Within recent years, Poland, Malta, the UK and Italy accounted for a large proportion of pig isolates of the monophasic S. Typhimurium (reported antigenic formulae 1,4,5,12:i:- and 1,4,1,2:i:-) [3].

With respect to the prevalence on positive farms, the EU baseline study performed in 2008 reported an average of 33·3% herds that tested positive for Salmonella, with a wide range among member states (0·0–55·7%). The most recent report on zoonoses and zoonotic agents in the EU, reported a 10·1% herd prevalence of Salmonella among nine countries in 2014 [3]. However, a comparison between the on-farm and abattoir prevalence of infection in pigs revealed that the on-farm prevalence is often underestimated. In Germany, the on-farm prevalence of infection in pigs, based on faecal analysis, was 5·58%; however, the caecal contents of 24·9% of slaughtered pigs were found to be Salmonella positive [Reference Visscher24]. In a study by Hurd et al. [Reference Hurd20], the abattoir prevalence of Salmonella in pigs was 39·9%, as opposed to an on-farm prevalence of 5·3%, a disparity that was probably due to the sample type (1 g faecal swabs on farm vs. 10 g caecal contents after necropsy). The sensitivity of faecal samples collected on farm was particularly poor for pigs infected in the lymph nodes (0% on farm, 12·2% at abattoir). The sensitivity of faecal culture increases only twofold with a 10 g sample, as compared with a 1 g sample; thus, low faecal volume might partially explain the low sensitivity of samples collected from live pigs on farm [Reference Funk, Davies and Nichols30]. Indeed, other factors might contribute to this discrepancy, such as the incidence of recent infections during transportation or slaughter [Reference Hurd21]. Otherwise, the presence of latent undetectable carriers among infected pigs is a common characteristic in the epidemiology of Salmonella [Reference Hurd20]. Moreover, intermittent shedding by pigs is a common feature that can interfere with monitoring and research programmes on Salmonella infection and the determination of health status in animals [Reference Pires, Funk and Bolin31].

Prevalence can vary at the farm level, depending on various factors, such as feeding practices, including the degree to which the feed is ground, and the pH and type of feed; management procedures, such as continuous or all-in/all-out production system; different types of herds (farrow-to-finish herds or fattening herds); size of the herd; as well the level of hygiene and general health status of the pigs. The provision of safe feed is the first step in ensuring safe food, especially in a ‘farm-to-fork’ concept [Reference Sauli32]. Therefore, pig feed should be Salmonella-free to guarantee a safer pork chain.

Wet feed has been demonstrated to reduce the risk of Salmonella infection, in comparison to pelleted feed, probably because of a fermentation step and consequent growth of lactic acid bacteria and yeasts [Reference Prohászka33]. The large amounts of organic acids produced, thus exert a protective effect, in a similar manner to the addition of organic acids to water [Reference Van der Wolf34] and feed [Reference Van Winsen35]. In contrast, pelleted feed is considered one of the most consistently reported risk factors for Salmonella shedding in pigs [Reference Wilkins36]. In addition, feed particle size can affect the prevalence of Salmonella isolated from the gastrointestinal tract of pigs. In comparison to finely ground feed (<0·20–1·00 mm), coarsely ground feed (2·00– >3·15 mm) reduces the prevalence of Salmonella, by enhancing the fermentation of starch in the gut [Reference Visscher37].

Regarding the influence of pH, values of 4·5 or lower effectively inhibit Salmonella both in the feed and in the gastrointestinal tract of pigs. Fermented liquid feed can yield such values, and improves the performance of sucklings, weaners and grower-finishers. Feeding pigs fermented liquid feed prevents the growth and proliferation of pathogens, such as Escherichia coli and Salmonella, in the gut by reducing the pH in the stomach [Reference Missotten38]. This is particularly important, because Salmonella has been detected in pig feed. In 2014, for example, some member states of the EU reported various serovars, including S. Typhimurium, S. Enteritidis, S. Give, S. Agona, S. Anatum and S. Mbandaka in the feed of pigs [3]. However, despite several studies on various interventions in feeding practices, there remains a lack of strong evidence of the effects between any association (acidification of liquid or pelletised feed vs. mash; coarse vs. finely ground feed; wet vs. dry feed, etc.) in reducing the prevalence of Salmonella on farm. Indeed, researchers do not consistently support the association between non-pelleted feed and a reduction in the prevalence of Salmonella in pigs [Reference O'Connor39].

The all-in/all-out management system reportedly has a protective effect against Salmonella infection [Reference Lo Fo Wong40], particularly when practised by the entire barn more than by single rooms [Reference García-Feliz41]. The protective effect has also been observed when animals from different age groups are housed separately [Reference Gotter42]. Contact with other species, as dogs and cats, can introduce the microorganism on farms, thereby increasing the risk of Salmonella infection [Reference Gotter42]. Management practices also encompass the design of pen walls (solid, spindles or combination) and type of floor (fully slatted floors vs. <50% slatted floors can significantly reduce the prevalence of Salmonella) [Reference Nollet13]. Nose-to-nose contact between pens is an important risk factor, as pens that allow direct contact among pigs are more likely to be Salmonella positive than those without such contact [Reference Wilkins36]. Poor pen cleaning and disinfection, and poor biosecurity measures are also important risk factors for the persistence of Salmonella at the farm level [Reference Fosse, Seegers and Magras43]. Cleaning by pressure washing with water, disinfection with chemicals and effective rodent control programmes are effective hygienic measures that should be adopted on farms [Reference García-Feliz41]. Nevertheless, the resistance of Salmonella to some disinfectants, such as glutaraldehyde, formaldehyde and hydroxide peroxide at a concentration of 1·0%, make their use ineffective in field conditions, when it is protected by the development of a biofilm [Reference Marin, Hernandiz and Lainez44].

The prevalence of Salmonella shedding in pigs is higher in finishing farms (open farms) than in farrow-to-finish farms (closed farms) [Reference Rajić45]. A Belgian study demonstrated that finishing farms were two times more likely to have Salmonella shedders than farrow-to-finish farms, with positive rates of 10·3% and 5·4%, respectively (mean within-herd prevalence of 7·8%) [Reference Rasschaert46]. Other authors have observed similar findings [Reference Rajić45]. One possible explanation is that pigs raised in fattening farms originate from the piggery units of other farms. Upon arrival, they are frequently mixed with piglets of different origin, thus sharing the various health conditions of each (including Salmonella infection). Furthermore, transportation from piggeries to the finishing farms could be a stressful event that promotes the shedding of Salmonella by carrier animals, and the spread of infection throughout the barns [Reference Rasschaert46].

In farrow-to-finish herds, where sows maintain the infection and excrete Salmonella particularly after weaning [Reference Nollet47], prevalence might be influenced by herd size. Dors et al. [Reference Dors48] observed that prevalence in herds with more than 200 shedding sows was higher than in smaller herds. Furthermore, the size of finishing herds could increase the risk of development of the Salmonella carrier status in pigs, as this is usually higher in units that slaughter more than 3500 pigs per year [Reference García-Feliz41].

Salmonella during transportation and holding (the harvest stage)

The harvest stage refers to the part of the food chain that includes transportation of the animals from the farm, the lairage period, the slaughtering process and the cooling of carcasses [22].

During this stage, asymptomatic pigs could begin to shed Salmonella after having left the farm, owing to stress factors that are linked to group housing, transportation and holding at the slaughterhouse. Transportation significantly increases Salmonella shedding, thus, shedders become an important source of Salmonella to other pigs that are being transported [Reference Hurd21, Reference Rostagno, Eicher and Lay49]. Stress can be caused by rough handling of the pigs at the time of loading and unloading, high stocking density during transport, long duration of transport, poor driver skills, adverse weather conditions and feed withdrawal. A relatively long feed withdrawal period, which is usually 12–18 h before transport, could be associated with changes in the gut microbiota, and elevated levels of Salmonella in the faeces [Reference Martín-Peláez50].

Another factor that could influence the prevalence of infection among pigs at slaughter is the lairage duration. A positive relationship exists between the time spent at lairage and the frequency of Salmonella detection in the lymph nodes, probably due to increased opportunity for invasion of the mesenteric lymph nodes (MLN) under conditions of prolonged stress. Pigs held for 12 h or more showed a greater chance of acquiring Salmonella from the lairage environment (16·7%), as compared with pigs held for 1–3 h (11·1%) [Reference Bonardi51].

Transportation and lairage conditions are thus important steps in the pork production chain that can increase the number of infected animals that are slaughtered. However, some studies suggest that external sources of infection might have a greater impact than stress, in increasing the detection of Salmonella at slaughter. For example, in a comparison of on-farm and slaughtered pigs, Hurd et al. [Reference Hurd21] reported a sevenfold increase, and detected a variety of Salmonella serovars in necropsied animals, that had not been isolated from pigs on the farm. Such infections could be acquired before slaughter from various shedder pigs in transport trucks or the lairage environment, as infection of the gastrointestinal tract and infiltration of the associated lymph nodes can occur in as little as 2 h [Reference Hurd, Gailey and Rostagno52]. Rapid infection during transportation, and particularly during holding of pigs, is a major cause of increased Salmonella prevalence. Generally, the holding pen could be an important control point for Salmonella in the pork production chain [Reference Hurd21]. Reduced exposure in trucks and holding pens is more likely to reduce Salmonella prevalence, than attempts to minimise stress, which is inevitable during transportation and lairage [Reference Dickson, Hurd and Rostagno53]. To facilitate the movement of pigs, stress at lairage can be kept under control by using well-designed infrastructure, well-lit corridors and minimal and careful handling of pigs, as specified by Regulation (EC) 1069/2009 [54], and discussed by some authors [Reference De Busser55]. In addition, showering pigs when the temperature rises to >10°C [Reference Schütte56] improves animal welfare at lairage. However, these measures can successfully reduce infections in pigs, only if the lairage environment is not already contaminated with Salmonella, thus posing a challenge for all slaughterhouses [Reference De Busser55]. Contamination of the lairage might be responsible for oral infections in holding pigs, as well as skin contamination, which is directly related to carcass contamination during the slaughter process. In a comparison of carcasses with contaminated skin and those without, the probability of surface contamination of the carcass was reduced from 59% to 35%, respectively [Reference Rossel, Jouffe and Belceil57]. Furthermore, failure to dehair the carcass can significantly increase the number of contaminated carcasses [Reference Bonardi58].

Salmonella at slaughter (the harvest stage)

The prevalence of Salmonella contamination in pork carcasses has been extensively studied in most European countries. Detection rates vary among studies, but all underline that Salmonella can be frequently isolated from MLN and faecal samples of pigs. Indeed, detection of Salmonella in the lymph nodes is frequently considered the ‘gold standard’ for definition of the carrier state at slaughter [22]. Alternatively, caecal material or faeces can be tested for carriage of Salmonella in pigs [22]. According to recent studies, the prevalence of Salmonella in MLN ranges from 7·4% to 26·0% [Reference Argüello14, Reference Visscher24, Reference Bonardi51, Reference Gomes-Neves59–Reference De Busser62] in EU countries (Table 1). In 2014, very low levels of prevalence were detected in Sweden (0·0%) and Finland (0·03%), where control and eradication programmes reported the presence of Salmonella in the lymph nodes of finishing pigs [3]. Different surveys have reported prevalence at around 20–30% in faecal contents [Reference Davies11, Reference Visscher24, Reference Powell63, Reference Bonardi64] (Table 1). Furthermore, monitoring programmes based in the analysis of pig faeces in Denmark and Estonia reported rates of 21·6% and 27·3%, respectively [3]. Other data on the prevalence of Salmonella in pigs at slaughter are also available based on examination of the tonsils [Reference Visscher24, Reference Vieira-Pinto, Temudo and Martins61, Reference Powell63] (Table 1), mandibular lymph nodes [Reference Vieira-Pinto, Temudo and Martins61], gall [Reference Visscher24], heart and tongue [Reference Powell63].

Table 1.

Prevalence of Salmonella in MLN, faeces, tonsils and carcasses of pigs at slaughter in different EU countries

EU, European Union; MLN, mesenteric lymph nodes; n.s., not shown.

Routes of contamination might be related to the pig or the slaughter environment. Contamination from the faeces of pigs that have been slaughtered on the same day might occur, with a typical distribution of Salmonella to the distal and medial surfaces of carcasses [Reference Smid65]. Contamination of carcasses with Salmonella on the skin of pigs has been demonstrated [Reference Bonardi58], but is probably less significant than faecal contamination [Reference Smid65]. In addition to the pig, the slaughter environment, in which microflora pose a potential risk for carcass contamination, is a major source of Salmonella [Reference Argüello66]. Equipment, such as carcass splitters and belly openers, might be contaminated with Salmonella from fluids dripping from the carcasses onto the machines. Consequently, Salmonella on contaminated equipment could be transferred to other carcasses that are subsequently slaughtered [Reference Smid65]. Salmonella can also be spread by workers at the abattoir, as the hands of meat handlers can be frequently contaminated [Reference Gomes-Neves59].

A marked reduction has been observed in the prevalence of Salmonella and the number of contaminated carcasses as the slaughtering process progresses [Reference Duggan67], because of the steps taken to reduce bacterial flora on the skin of pigs. In one study examining a relatively large number of Salmonella-positive carcasses, the prevalence of Salmonella contamination was 96·6% at exsanguination and 35·9% after slaughter. During the slaughter process, skin contamination was reduced from 96·6% to 16·2%, but cross-contamination via equipment was responsible for the final number of Salmonella-positive carcasses reported [Reference van Hoek68]. Several authors have outlined that the main means of contamination is probably the result of a continuous cycle between pigs, the environment and the carcasses [Reference De Busser62, Reference Botteldoorn69].

Different levels of prevalence have been detected in pig carcasses in EU countries, ranging from 3·2% to 16% [Reference Davies11, Reference Argüello14, Reference Gomes-Neves59–Reference Vieira-Pinto, Temudo and Martins61, Reference Bonardi64, Reference Bolton, Ivory and McDowell70] (Table 1). Data from several countries in 2014, reported by the EFSA and ECDC are shown in Table 2. The differences observed could be attributed to several factors, such as the number of carrier pigs introduced to the slaughter line; implementation of effective steps for decontamination including dehairing, polishing and flaming; maintenance of good hygienic standards at slaughter; cross-contamination between carcasses and equipment; cross-contamination among carcasses; presence of resident slaughterhouse microflora and passive transmission via the hands of workers. Regulation (EC) 2015/1474 [71] regarding the use of recycled hot water to remove microbiological surface contamination from carcasses has recently offered increased opportunity to reduce the prevalence of Salmonella.

Table 2.

Prevalence of Salmonella in pig carcasses at slaughter in 2014

Sampling unit: single pig [3].

Because the pathogen is not only introduced to the slaughter line by the pigs, but could persist in the slaughterhouse environment, or be acquired during transportation and holding, the serovars isolated from on-farm samples can vary widely from those isolated after slaughter [Reference Visscher24, Reference Merialdi25, Reference Rostagno, Eicher and Lay49, Reference Gebreyes72]. The most commonly reported Salmonella serovars isolated from carcasses at slaughter are Derby, Typhimurium, Typhimurium monophasic variant, Rissen, Brandenburg, London, Manhattan, Muenchen and Stanley [Reference Bonardi51, Reference Smid65]. The monophasic variant of S. Typhimurium (but not the biphasic S. Typhimurium) has been identified in pigs at slaughter [Reference Bonardi51]. It is the most commonly isolated serovar in some countries, such as the UK, where it accounted for 32·9% of the serovars isolated from pigs in 2013 [73]. The rise in incidence of the monophasic variant of S. Typhimurium might be related to a novel clonal group that is characterised by the tetra-resistant pattern ASSuT (ampicillin, streptomycin, sulphonamides, tetracycline), which emerged during the 2000s in some EU countries, and has become particularly common in some member states, such as Italy, Denmark, the UK and Germany. In this clonal group, multidrug resistance is conferred by a new genomic island and the pattern ASSuT can be used for provisional identification of the isolates [Reference Hauser74, Reference Lucarelli75].

Salmonella in the post-harvest stage: the EU Regulation (EC) No 2073/2005 criteria

The criteria for Salmonella in foodstuffs, laid down by Regulation (EC) No 2073/2005 [76], have been in force since 1 January 2006. In member states of the EU, most national monitoring programmes for Salmonella in pork and pork products are based on the collection of swab samples of the carcass at the slaughterhouse, and/or meat samples at the processing plants. Regulation (EC) No 217/2014 [77] is a revision of Regulation 2073/2005 [76] and serves to reduce the acceptable number of Salmonella-positive pig carcasses from 5 out of 50 (10%) to 3 out of 50 (6%). Therefore, food business operators have to implement appropriate interventions to reduce the number of contaminated carcasses.

The studies on pork and ready-to-eat pork products have not been uniformly conducted among various EU countries, and show differences in sampling procedures, types of end-products and detection methods. Thus, consideration was given only to the most recent data provided by EFSA, which reported an overall Salmonella prevalence of 0·5% in fresh pork and 0·7% in ready-to-eat minced meat, meat preparations and meat products. Despite these relatively low numbers, pork and pork products, especially if consumed raw or undercooked, frequently represent a source of non-typhoidal Salmonella strains to humans [3].