Swine Dysentery (Brachyspira hyodysenteriae): Hemorrhagic Diarrhea in Pigs

Etiology and Taxonomy

Swine dysentery is a contagious mucohemorrhagic colitis of pigs caused by the anaerobic intestinal spirochete Brachyspira hyodysenteriae [1, 2]. The organism is a Gram-negative, oxygen-tolerant anaerobe belonging to the phylum Spirochaetes, family Brachyspiraceae [2]. B. hyodysenteriae is distinguished from other porcine Brachyspira species (e.g., B. pilosicoli, B. innocens, B. murdochii) by its strong beta-hemolysis on blood agar, its ability to ferment specific carbohydrates, and its possession of distinct virulence factors including flagella-mediated motility, lipooligosaccharide (LOS), and hemolysins [1, 2]. Genomic analyses have revealed a core genome of approximately 3.0 to 3.2 Mb with a G+C content of around 27%, and multilocus sequence typing (MLST) has identified numerous sequence types (STs) circulating globally, including ST196 and ST245 [2, 3, 4]. The population structure of B. hyodysenteriae is highly diverse, with evidence of recombination and the presence of prophage elements that may contribute to genomic plasticity and virulence [2, 4].

Epidemiology

Swine dysentery occurs worldwide and is a major cause of economic loss in pig production due to mortality, reduced growth performance, and increased medication costs [1, 5, 6]. The disease is primarily transmitted via the fecal-oral route. Pigs become infected after ingesting contaminated feces, feed, water, or fomites [1, 7]. Subclinically infected carrier pigs are the most important reservoir for maintaining infection within and between herds [1, 8]. Rodents, flies, and other mechanical vectors can also carry B. hyodysenteriae and contribute to transmission [9]. The prevalence of Brachyspira spp. in pig herds with a history of diarrhea in six European countries was found to be substantial, with herd-level risk factors including large herd size, continuous flow production systems, and purchase of replacement stock from multiple sources [6]. National-level control programs, such as the one implemented in Sweden, have demonstrated that eradication is feasible through a combination of depopulation-repopulation or partial depopulation with medication and strict biosecurity [5]. The motivation of pig owners to undertake eradication efforts is influenced by factors including perceived disease severity, economic impact, and technical support [10].

Pathogenesis

Following oral ingestion, B. hyodysenteriae colonizes the large intestine, specifically the cecum and spiral colon [1, 11]. The spirochete uses its corkscrew-like motility to penetrate the mucus layer and adhere to the apical surface of colonic epithelial cells [1, 12]. Adhesion is mediated by interactions between bacterial surface proteins and host glycosphingolipids, including sulfatide and lactosylceramide, which are abundant in the porcine colonic mucosa [12]. Once attached, the bacterium induces a profound inflammatory response characterized by neutrophil infiltration, goblet cell depletion, and mucosal edema [1, 13]. The production of hemolysins and lipooligosaccharide contributes to epithelial cell damage and the characteristic hemorrhagic and mucoid nature of the diarrhea [1, 14]. Transcriptomic profiling of colonic mucosa during infection has revealed upregulation of genes involved in innate immunity, inflammation, and tissue repair, including those encoding chemokines, cytokines, and matrix metalloproteinases [15]. Metabolomic signatures of infection include alterations in amino acid, lipid, and energy metabolism pathways, reflecting both host tissue damage and bacterial metabolic activity [16]. The severity of colitis correlates with changes in the composition and functionality of the large intestinal microbiota [17]. A shift from a diverse, predominantly Gram-positive community to a dysbiotic state with increased abundance of potentially pathogenic bacteria, including Escherichia coli and Fusobacterium spp., is observed [18, 17, 19]. This dysbiosis is not merely a consequence of inflammation but may also predispose pigs to more severe disease [18, 19]. Co-infection with Lawsonia intracellularis, the agent of porcine proliferative enteropathy, can exacerbate the clinical and pathological severity of swine dysentery [20, 21].

Clinical Signs

The incubation period typically ranges from 7 to 14 days but can be longer depending on the infectious dose, host immunity, and concurrent infections [1, 11, 22]. The hallmark clinical sign is swine bloody diarrhea, which initially presents as a watery, yellow-to-gray diarrhea that rapidly progresses to a mucoid, bloody stool containing flecks or clots of fresh blood [1, 11]. Affected pigs exhibit depression, anorexia, pyrexia (mild to moderate), and dehydration [1, 11]. The disease is most commonly observed in grower-finisher pigs aged 8 to 20 weeks, although pigs of any age can be affected [1]. Morbidity within a group can reach 90%, while mortality is typically low (1% to 5%) but can be higher in severe outbreaks or when complicated by other pathogens [1, 21]. Chronic infection may result in poor growth rates, reduced feed conversion efficiency, and intermittent diarrhea [1, 23]. The severity of clinical signs is influenced by dietary factors, particularly the type and level of dietary fiber, with higher fiber diets potentially exacerbating disease expression [23].

Pathology

Gross pathological lesions are confined to the large intestine, primarily the cecum and spiral colon [1, 11]. The intestinal wall is edematous and thickened, and the mucosa is hyperemic and covered with a fibrinous, mucoid, or hemorrhagic exudate [1, 11]. The mesenteric lymph nodes are often enlarged and edematous [1]. Microscopic lesions include diffuse or focal necrosis of the colonic epithelium, goblet cell depletion, crypt elongation, and a marked neutrophilic and mononuclear inflammatory cell infiltration of the lamina propria and submucosa [1, 11, 13]. Spirochetes can be visualized within the mucus layer and occasionally within the cytoplasm of epithelial cells using silver stains (e.g., Warthin-Starry) or immunohistochemistry [1, 11]. Acute infection also affects mucin expression and glycosylation, with increased fecal levels of MUC5AC observed [24]. The severity of macroscopic and microscopic lesions correlates with the duration and intensity of fecal shedding of B. hyodysenteriae [11].

Diagnosis

A definitive diagnosis of swine dysentery requires the detection of B. hyodysenteriae in feces or intestinal tissue from clinically affected pigs [1, 11, 25]. A combination of clinical history, gross pathology, and histopathology provides strong presumptive evidence [1, 11].

Laboratory Diagnostic Methods

Diagnostic Decision Tree

flowchart TD
    A[Clinical suspicion: swine bloody diarrhea in grower-finisher pigs], > B{Collect fresh fecal samples or intestinal tissue}
    B, > C[Perform anaerobic culture on selective media]
    C, > D{Strong beta-hemolysis?}
    D, >|Yes| E[Confirm by species-specific PCR (nox gene)]
    D, >|No| F[Perform PCR directly on feces]
    E, > G[Positive for B. hyodysenteriae]
    F, > G
    G, > H[Optional: Perform MLST or WGS for epidemiological typing and AMR profiling]
    H, > I[Confirm diagnosis of swine dysentery]
    I, > J[Implement treatment and control measures]

Differential Diagnoses

Swine dysentery must be differentiated from other causes of hemorrhagic or mucoid diarrhea in pigs, including:

• Porcine proliferative enteropathy (PPE) caused by Lawsonia intracellularis: typically presents with "nuclear" or "target" diarrhea; lesions are proliferative rather than necrotic [20, 21].
• Salmonellosis (e.g., Salmonella enterica serovar Typhimurium): often associated with septicemia, fever, and systemic signs; lesions may involve the small intestine and stomach [29].
• Colibacillosis (enterotoxigenic E. coli): primarily affects neonatal and weaned pigs; diarrhea is watery and less mucohemorrhagic.
• Trichuris suis (whipworm) infection: causes typhlocolitis with hemorrhage but is less acute and associated with high worm burdens.
• Dietary factors (e.g., mycotoxins, abrupt feed changes): can cause non-infectious colitis.

Treatment

The treatment of swine dysentery has historically relied on antimicrobial agents, but the emergence of multidrug-resistant (MDR) strains has complicated therapy [2, 26, 27, 25]. Antimicrobial susceptibility testing (AST) is strongly recommended to guide treatment decisions [26, 27, 30].

Antimicrobial Agents

Non-Antibiotic Approaches

Given the rise of antimicrobial resistance, alternative strategies are being investigated. Medium-chain fatty acids (MCFAs), such as caprylic and capric acid, have demonstrated in vitro antimicrobial activity against MDR B. hyodysenteriae strains [31]. A curcumin derivative metalloprotease inhibitor (CMC2.24) has shown promise in mitigating Brachyspira spp.-induced colitis in experimental models [14]. Zinc chelate compounds, when combined with adapted management measures, have been used successfully for eradication [32]. Probiotic candidates capable of competitive exclusion of B. hyodysenteriae are under investigation [33]. In vitro screening of non-antibiotic feed additives has identified several components that can reduce intestinal lesions caused by B. hyodysenteriae and other enteric pathogens [29].

Control and Prevention

Control of swine dysentery relies on a combination of biosecurity, management practices, and, where feasible, eradication [1, 32, 5].

Biosecurity

• All-in/all-out production systems reduce the risk of continuous pathogen cycling [1, 6].
• Quarantine and testing of incoming replacement stock is essential to prevent introduction of carrier pigs [1, 5].
• Rodent and insect control programs should be implemented to eliminate mechanical vectors [9].
• Disinfection of facilities and equipment using commercial disinfectants with proven efficacy against B. hyodysenteriae is critical [34].
• Dedicated boots and clothing for personnel working in different barns or age groups.

Eradication

Eradication can be achieved through total depopulation-repopulation or through partial depopulation combined with medication and rigorous hygiene [32, 5]. The Swedish national control program, which combined herd-level eradication with surveillance and movement restrictions, successfully eliminated swine dysentery from the country [5]. Factors driving farmer motivation and satisfaction with eradication programs include clear communication, financial support, and technical assistance [10].

Vaccination

No commercial vaccine is currently widely available for swine dysentery, but research into vaccine candidates is ongoing [35]. Experimental vaccines based on inactivated whole cells, recombinant proteins (e.g., flagellin, outer membrane proteins), and live attenuated strains have shown variable efficacy in challenge studies [35]. The development of an effective vaccine is complicated by the need to induce strong mucosal immunity and the antigenic diversity of B. hyodysenteriae strains [35, 20].

Dietary Management

Dietary fiber content and source can influence the clinical expression of swine dysentery [23]. Diets high in insoluble fiber (e.g., oat hulls, soybean hulls) may exacerbate disease, while diets with moderate levels of soluble fiber may be protective [23]. Nutritional strategies that support gut health, such as the inclusion of organic acids, prebiotics, and probiotics, may help reduce disease severity [33, 29, 31].

Public Health and Zoonotic Considerations

Brachyspira hyodysenteriae is not considered a zoonotic pathogen. There are no documented cases of human infection with this species. However, other Brachyspira species, such as B. pilosicoli, have been associated with human intestinal spirochetosis, primarily in immunocompromised individuals.

References

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