First molecular detection of Brachyspira suanatina on pig farms in Poland

Swine dysentery (SD) is a severe mucohaemorrhagic diarrhoeal disease spreading via the faecal–oral route. The first experimental reproduction of SD was published in Indiana, USA in 1921 (11). Its causative agent, the bacterium Brachyspira hyodysenteriae (originally named Treponema hyodysenteriae) was identified almost simultaneously by two independent teams of researchers who successfully fulfilled Koch’s postulates in conventional pigs in the early 1970s (11, 35). Regardless of the substantial effect of SD on the pig sector worldwide and the undertaking of several attempts to elucidate the complex mechanisms underlying the disease, its exact pathogenesis remains incompletely understood.

Traditionally, the infection has been thought to be transmitted mainly by direct contact between pigs (11); nevertheless, numerous research works have revealed that other species can also be involved in the carriage of the pathogen. Among others, the carriers are rodents (2), insects (3), feral pigs (27), and some species of domestic and wild birds (10, 18, 19). Brachyspira hyodysenteriae is able to persist for a long period of time in the environment (up to 112 days in porcine faeces at 10°C) (4). The length of the incubation period is variable, but usually ranges between 10 and 14 days. Infection provides a varying degree of protection. Moreover, untreated pigs that have recovered from SD can shed the pathogen asymptomatically for 70 to 90 days.

Swine dysentery is characterised by high morbidity and variable mortality. Once ingested, the bacterial agent of SD survives in the highly acidic gastric environment, passes through the small intestine, and eventually colonises the colonic and caecal crypts, which results in the development of mucohaemorrhagic colitis (11). The clinical manifestation of SD varies from moderate mucoid to bloody diarrhoea depending on the virulence of the isolate (36), the composition of the offered diet (9, 33), and environmental conditions in the wide sense (11).

Gross pathological changes typical of the disease are (except for signs accompanying dehydration) limited to the large intestine, and include mesocolonic oedema and rugose mucosa covered diffusely or in patches by fibrin, mucus, and flecks of blood (38). Microscopically, depending on the stage and severity of the disease, the most pronounced pathological lesions are mostly restricted to the large intestine, i.e. the caecum and/or colon, and involve erosion of the luminal epithelium, crypt hyperplasia, goblet cell hyperplasia, and blood congestion (17).

Uncontrolled development of profuse SD diarrhoea in affected herds results in substantial financial losses as a consequence of acutely depressed feed conversion and variable mortality (11). Besides which, the disease may ignite animal welfare concerns. Despite extensive research conducted on orally administered attenuated vaccines (34), bacterins (37), and recombinant vaccines (6), effective commercial prophylaxis against SD is not available yet. Consequently, B. hyodysenteriae infection needs to be controlled by antibiotic treatment (11). The escalating costs of high antimicrobial usage, tight restrictions on selling animals to some locations, and a considerable risk of violations of rules governing antimicrobial treatments collectively impel attempts towards complete eradication of the disease. Accordingly, a wide variety of measures applied in SD eradication plans have been thoroughly described in scientific and trade journals. Nevertheless, all the strategies, which are based on total or partial depopulation, heavy medication followed by outbreak-specific management and internal biosecurity measures, or a combination of all of these still have widely variable success rates in the long run (25).

Amongst all the virulence factors of Brachyspira spp. which have been characterised until now, those with haemolytic activity seem to be of the utmost importance in SD pathogenesis (11). Hence, historically, what sufficed for diagnosis was an appropriate clinical manifestation and characteristic necropsy findings followed by a successful blood agar anaerobic culture of a strongly beta-haemolytic, i.e. ring-phenomenon-positive, spirochete from affected intestine tissue or faeces excreted by diseased animals. More recently, a broad range of modern molecular methods have been successfully incorporated into the laboratory diagnosis of SD: bacteria species identification with PCR assays targeting the nox gene, 16S rRNA or 23S rDNA; other DNA-based typing methods such as restriction fragment length polymorphism analysis; or matrix-assisted laser desorption/ionisation–time-of-flight mass spectrometry.

Prior to the 2000s, SD was considered to be caused only by B. hyodysenteriae, with an essential, yet obscure, contribution made by commensal intestinal anaerobes (23). Nowadays, it is known that the disease is caused by the colonisation of the caecum and colonic epithelium by three Gram-negative, motile, strongly beta-haemolytic and fastidious species of the anaerobic spirochaetal genus Brachyspira. The most vital enteropathogenic spirochete of these three, and the one of which the biology and devastating impact on the economy of global swine production have been thoroughly studied, is B. hyodysenteriae, and B. hampsonii and B. suanatina are the two newly emerged other species. The newer species are thought to cause disease clinically indistinguishable from cases of SD in growing pigs (5, 11, 29).

Brachyspira suanatina was originally identified and characterised in wild ducks (Anas platyrhynchos) sampled in southern and south-eastern Sweden in the early 2000s (18). In the same period of time, three isolates were recovered from material collected from one Danish piglet-producing herd and on three Swedish farms comprising two which fattened pigs and one which produced piglets. Their enteropathogenic potential was successfully proven using a conventional porcine model in 2007 (29). Based on a thorough investigation involving DNA–DNA hybridisation and whole genome comparisons, B. suanatina was described as a unique species in 2015 (24) and validly published one year later (26). The first isolation of B. suanatina outside Scandinavia took place in 2017 in Germany. The samples were collected from diarrhoeic pigs reared in outdoor climate barns (31). The very first report demonstrating an occurrence of the pathogen in pigs farmed outside continental Europe was published by the Animal and Plant Health Agency in the UK in February 2022. The B. suanatina-positive samples were collected in England from 10-week-old diarrhoeic pigs (1).

Similar diagnostic investigation of another novel strongly haemolytic Brachyspira sp., i.e. B. hampsonii, started in the mid-2000s in Canadian and US American pig herds suffering from mucohaemorrhagic diarrhoea. Isolation of high levels of Brachyspira spp. not related to B. hyodysenteriae eventually resulted in a molecular and phenotypic characterisation of B. hampsonii in 2012 (5); however, recent epidemiological exploration of archived Canadian samples pushed the date of its first appearance on North American swine farms back to 2002 (15). To date in Europe, B. hampsonii has been reported in Belgium (in pigs imported from the Czech Republic) (21) and in Germany (in pigs transported from Belgium) (30).

The wide distribution and sharply varying prevalence of B. hyodysenteriae has been described in several studies from most pig-rearing regions of the world (8) and extensive data has been compiled on avirulent or weakly virulent strains (13) and those presenting markedly lower antimicrobial susceptibility (14). Reports of the prevalence of newly emerged strongly beta-haemolytic Brachyspira spp. are contrastingly scarce. Indeed, peer-reviewed publications on infections with B. hampsonii and B. suanatina in pigs are exceptionally rare. Therefore, this study aimed to indicate the presence of B. suanatina and B. hampsonii in faecal samples collected from diarrhoeic finishing pigs reared under farm conditions on modern high-performing operations located in Poland.

Out of the total of nine samples examined, seven were detected to contain the genetic material of B. suanatina. This bacterium was in material collected from farms F1–F3, F5 and F7–F9 (Table 3). Brachyspira pilosicoli was detected in four samples from farms F2–F4 and F9. The co-occurrence of these two bacteria was confirmed in three samples, farms F2, F3 and F9 being their places of origin. Salmonella Choleraesuis was identified in one sample from farm F7 out of four S. enterica-positive samples from farms F3 and F6-F8. Non-pathogenic/questionably pathogenic Brachyspira spp. were found in six samples: B. murdochii and B. intermedia were respectively detected in five samples taken on farms F2, F5, F6, F8 and F9 and one sample brought from farm F1. All the collected samples were B. hyodysenteriae-, B. hampsonii-, PEDV- and TGEV-negative. Helminth eggs, encysted protozoa and Cryptosporidia were also not detected.

This publication is the very first report on B. suanatina detection on pig farms in Poland. To date, the isolation of the aforementioned bacterium from swine has been reported in samples collected in Sweden (18), Denmark (29), Germany (31), and the United Kingdom (1). The available research reported B. suanatina in waterfowl and indicated its role in the most proximate cases of infection reported in outdoor pigs reared in Germany (31); however, this particular type of link between free-living carriers and farm pigs can be entirely excluded from having had any significance in our study. There could have been no such transmission because the sampled animals originated from one integrated production system with strict biosecurity rules preventing all contact with other animals, including migratory birds, and/or contact with their natural habitats. Although in view of the strong environmental persistence of other pathogenic Brachyspira spp., transportation of B. suanatina from remote locations via other vectors could have been possible (4, 11), in the context of the biosecurity enforced on the farms sampled, both biological and mechanical carriage remain purely hypothetical.

Unfortunately, we are not able to definitely prove which route was the route of B. suanatina transmission to the Polish finisher farms; however, taking into account the clinical picture described and the history of B. hampsonii in North American swine herds (specified in an epidemiological study using archived samples) (15), B. suanatina was more likely to have entered the sampled locations with asymptomatic individuals. In other words, the pathogen was highly likely to have been circulating within some European pig herds and eventually spread to others as an undesirable effect of the globalisation of swine production. Even though asymptomatic passage reasonably explains its detection in Poland, further research aimed at identification of potential factors triggering disease outbreaks is of great importance.

Detailed assessment of the clinical relevance of B. suanatina infection in finishing pigs sampled during the investigation was beyond the scope of our study. Scientific reports describing its significance in disease development are infrequent and focused on experimental exposure only (29); neither the specific dose required in natural infection nor factors affecting its manifestation have been defined hitherto. With little knowledge of the disease course of B. suanatina infection contained in the literature and no determinants proved or disproved, it could be contended that some environmental and/or nutritional factors could have affected the clinical presentation, as this has already been described in B. hyodysenteriae infection (9, 11, 17, 28, 33). In a similar way, it might also be attributed to either the concurrence of infections with atypical strains or to abstruse interactions with other bacteria, both typical of Brachyspira spp. (11, 23). No peer-reviewed articles describing successful reproduction of clinical symptoms in gnotobiotic pigs after exposure to the three causative agents of SD separately have yet been published.

Furthermore, the diversity of the group comprising the bacterial genus Brachyspira, with its several unofficial and unrecognised species (13), may prompt an erroneous conclusion about the aetiology of the manifested disease. Even though a few weakly beta-haemolytic species colonising specialised niches in the digestive system of the pig have been identified so far (B. innocens, B. intermedia and B. murdochii) (11), none of those bacteria have been associated with the clinical presentation or pathological alterations typical of SD in the target species under field conditions (2, 12, 20).

The only weakly beta-haemolytic species detected in our study of which the role in the development of a disease in swine is certain is B. pilosicoli. This is an agent causing porcine intestinal spirochaetosis, a disease characterised by a milder colitis, loss of body condition, and low or no mortality (11). Nevertheless, the concurrence of the pathogens identified in each sample analysed in our study (including contagious agents which potentially could have exacerbated the outcomes of B. suanatina infection), is not diagnostically informative because the prevalence of diarrhoeic pigs and development of clinical symptoms were the same in all the sampled herds, including B. suanatina-positive and B. pilosicoli-negative, B. suanatina- and B. pilosicoli-positive, and B. suanatina-negative but B. pilosicoli-positive cohorts. It must be borne in mind that the presented data were established applying qualitative methodology. The potential role of concurring intestinal bacteria deserves deeper examination using quantitative analysis amongst affected individuals.