Brachyspira hyodysenteriae and Swine Dysentery: Bloody Mucoid Diarrhea and Diagnosis

Etiology and Taxonomy

Swine dysentery is a globally significant mucohemorrhagic colitis of pigs caused by infection with Brachyspira hyodysenteriae, a Gram-negative, anaerobic, slow-growing spirochete bacterium. The organism belongs to the genus Brachyspira within the family Brachyspiraceae, order Spirochaetales. B. hyodysenteriae is distinguished from other porcine intestinal spirochetes (including Brachyspira pilosicoli, Brachyspira innocens, and Brachyspira murdochii) by its strong beta-hemolysis on blood agar, its genetic composition, and its unequivocal pathotype in swine.

The spirochete is helically shaped, measuring 6 to 10 micrometers in length and 0.3 to 0.4 micrometers in width. It possesses between 7 and 14 periplasmic flagella per cell end, which facilitate a distinctive corkscrew-like motility through viscous mucus. B. hyodysenteriae is an obligate anaerobe but exhibits aerotolerance sufficient for survival in fecal material for short periods. Optimal growth in vitro occurs at 37 to 42 degrees Celsius on trypticase soy agar supplemented with 5 to 10 percent sheep blood, under anaerobic conditions maintained with gas mixtures containing hydrogen and carbon dioxide.

Epidemiology and Transmission

Swine dysentery affects pigs primarily between 6 and 20 weeks of age, though outbreaks occur in all age groups. Morbidity within an affected herd ranges from 30 to 90 percent, while mortality is typically low (1 to 5 percent) unless complicated by concurrent infections, poor nutrition, or environmental stress. The disease is spread via the fecal-oral route. Carrier pigs that excrete B. hyodysenteriae intermittently serve as the primary reservoir within a herd. Recovered animals may remain colonized for months, creating a constant source of reinfection for naive cohorts.

Environmental persistence of B. hyodysenteriae is limited. The organism survives in feces for approximately 2 days at 25 degrees Celsius and up to 7 days at 4 degrees Celsius. In liquid pit slurry, survival can extend to 6 weeks under cool anaerobic conditions. Mechanical transmission occurs through contaminated boots, clothing, equipment, and transport vehicles. Rodents, particularly mice, can act as biological vectors carrying the spirochete in their intestinal tracts without clinical signs and shedding it into feed and bedding.

Pathogenesis: Induction of Bloody Mucoid Diarrhea

The pathogenesis of B. hyodysenteriae swine dysentery bloody mucoid diarrhea involves a sequence of adherence, colonization, mucus degradation, and host inflammatory response. After oral ingestion, the spirochete passes through the stomach and small intestine and colonizes the large intestine (cecum and colon). B. hyodysenteriae is highly chemotactic toward mucin and penetrates the mucus layer lining the colonic epithelium. The spirochetes attach end-on to the apical membrane of colonic enterocytes and to the mucus gel itself.

The organism expresses several virulence determinants including beta-hemolysin (a cytotoxic hemolysin encoded by the hlyA gene), NADH oxidase, and lipooligosaccharide (LOS) components. Beta-hemolysin is a major virulence factor that induces pore formation in target cell membranes, leading to necrosis of colonic epithelial cells. LOS activates Toll-like receptor 4 (TLR4) on macrophages and dendritic cells, triggering a robust inflammatory cascade characterized by the release of interleukin-1 beta (IL-1beta), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), and interleukin-8 (IL-8). This chemotactic gradient recruits neutrophils into the colonic lamina propria and lumen.

The combined effect is a severe, diffuse, fibrinous, and hemorrhagic typhlocolitis. Neutrophil degranulation and necrosis of the epithelial barrier result in the characteristic clinical presentation of bloody mucoid diarrhea. The mucus in the feces is derived from hyperstimulated goblet cells combined with inflammatory exudate, while the blood originates from mucosal hemorrhage secondary to epithelial sloughing and vascular congestion. This pathology is distinct from other porcine diarrheal diseases such as porcine proliferative enteropathy caused by Lawsonia intracellularis and necrotic enteritis caused by Clostridium perfringens.

Clinical Signs

The incubation period after exposure is 3 to 14 days. Clinical swine dysentery presents as an acute or subacute disease. In acute cases, pigs develop a profuse, watery diarrhea that rapidly progresses to become mucoid and flecked with fresh blood. Affected animals are depressed, anorexic, febrile (40 to 41 degrees Celsius), and dehydrated. The feces have a characteristic foul odor. The hallmark of Brachyspira hyodysenteriae swine dysentery bloody mucoid diarrhea is the presence of distinct blood and mucus in the stool, sometimes described as resembling raspberry jam or red currant jelly.

In subacute and chronic cases, pigs exhibit a persistent mucoid diarrhea without visible blood. Weight loss, poor feed conversion, and fecal staining of the perineum are common. Chronically infected herds display reduced growth rates and increased days to market. Subclinical carriers show no overt signs but intermittently shed the organism, perpetuating endemic infection.

Gross and Histopathological Findings

At necropsy, the most consistent lesions are confined to the large intestine. The cecum and colon are edematous, thickened, and hyperemic. The serosal surface may appear congested. On opening the lumen, the mucosa is covered with a fibrinonecrotic pseudomembrane adherent to an underlying hemorrhagic surface. The colonic wall is thickened due to edema, cellular infiltration, and fibrosis in chronic cases. The small intestine is typically normal, which aids in differentiation from other enteric pathogens.

Histologically, acute lesions reveal necrosis of surface enterocytes and crypt epithelium with extensive neutrophil infiltration into the lamina propria and crypt lumina. The crypts are dilated and filled with neutrophils, mucus, and cellular debris. Capillary congestion and hemorrhage into the lamina propria are prominent. In chronic cases, crypt hyperplasia and goblet cell hyperplasia are observed, along with fibrosis and mononuclear cell infiltration. Spirochetes can be visualized in silver-stained sections (e.g., Warthin-Starry stain) as argyrophilic, wavy organisms within the mucus layer and adherent to necrotic enterocytes.

Diagnostic Approaches

Definitive diagnosis of swine dysentery relies on a combination of history, clinical signs, necropsy findings, and laboratory confirmation. Differential diagnoses include salmonellosis (Salmonella enterica serovars), porcine proliferative enteropathy (L. intracellularis), whipworm infestation (Trichuris suis), gastric ulcer perforation, and enteric colibacillosis.

Direct Microscopy and Wet Mounts

Phase-contrast or dark-field microscopy of fresh feces can reveal the characteristic corkscrew motility of Brachyspira species. However, this technique cannot differentiate B. hyodysenteriae from non-pathogenic commensal spirochetes. The finding of large numbers of motile spirochetes is suggestive but not confirmatory.

Bacteriological Culture and Isolation

Culture remains a reference method but is slow and technically demanding. Fecal swabs or mucosal scrapings from colonic lesions are plated onto selective media such as trypticase soy agar with 5 percent sheep blood containing spectinomycin (400 micrograms/mL), colistin (25 micrograms/mL), and vancomycin (50 micrograms/mL). Plates are incubated anaerobically at 42 degrees Celsius for 5 to 7 days. B. hyodysenteriae produces a strong, clear zone of beta-hemolysis with a characteristic "buttery" or "flat" spreading colony morphology. Phenotypic identification is confirmed by positive indole production, absence of hippurate hydrolysis, and strong beta-hemolysis.

Polymerase Chain Reaction (PCR) and Quantitative PCR (qPCR)

Molecular detection using PCR has become the diagnostic standard due to its sensitivity, specificity, and speed. Conventional PCR targets the 16S ribosomal RNA (rRNA) gene or the nox gene encoding NADH oxidase. Specific primers for B. hyodysenteriae can distinguish it from other Brachyspira species. Real-time quantitative PCR (qPCR) provides quantification of bacterial load and can detect subclinically shed organisms. Fecal samples, rectal swabs, and fresh colonic tissue are suitable specimen types. PCR-based diagnostics are superior to culture for detecting low-level shedders and for confirming clearance after treatment.

Serological Testing

Enzyme-linked immunosorbent assays (ELISAs) detecting antibodies against B. hyodysenteriae lipooligosaccharide or whole-cell antigens are used for herd-level surveillance. Serology is useful for demonstrating exposure but is not reliable for individual diagnosis due to delayed seroconversion and persistence of antibodies after recovery. The Enzyme-Linked Immunosorbent Assay (ELISA) for Feline Leukemia Virus offers a methodological parallel for understanding assay principles, though the antigen targets differ.

Diagnostic Workflow

The following Mermaid diagram illustrates a recommended diagnostic decision tree for pigs presenting with bloody mucoid diarrhea suspicious for swine dysentery.

flowchart TD
    A[Pig with bloody mucoid diarrhea], > B{Clinical history and age}
    B, >|6-20 weeks, herd history| C[Collect fecal sample or rectal swab]
    C, > D{Perform dark-field microscopy}
    D, >|Motile spirochetes seen| E[Proceed to PCR or culture]
    D, >|No spirochetes| F[Consider alternative diagnoses]
    E, > G{Real-time PCR for B. hyodysenteriae}
    G, >|Positive| H[Confirmatory culture and antimicrobial susceptibility]
    G, >|Negative| I[Test for L. intracellularis, Salmonella, T. suis]
    H, > J[Diagnosis confirmed: Swine Dysentery]
    I, > K[Implement appropriate treatment and control measures]
    J, > L[Herd-level biosecurity and monitoring]
    L, > M[Re-test sentinel pigs after intervention]

Differential Diagnosis: Key Features

The table below summarizes critical differentiating features for the most common causes of hemorrhagic or mucoid diarrhea in grow-finish pigs.

| Pathogen or Condition | Typical Age | Fecal Character | Gross Lesions | Diagnostic Test | |, - |, - |, - |, - |, - | | Brachyspira hyodysenteriae | 6-20 weeks | Bloody, mucoid, foul odor | Fibrinonecrotic typhlocolitis | qPCR, culture | | Lawsonia intracellularis | 4-12 weeks | Watery to bloody, less mucus | Ileal and cecal mucosal hyperplasia (cobblestone) | qPCR, immunohistochemistry | | Salmonella Choleraesuis/Typhisuis | 6-20 weeks | Yellowish, putrid, sometimes blood | Necrotic colitis, multifocal hepatic necrosis | Culture, PCR | | Trichuris suis (whipworm) | 8-16 weeks | Mucoid, dark, sometimes blood | Cecal and colonic mucosal hemorrhage, worms visible | Fecal flotation | | Swine gastric ulcer | Grow-finish | Melena (dark, tarry) | Gastric ulceration in pars esophagea | Necropsy |

Treatment and Antimicrobial Resistance

Treatment of swine dysentery relies on antimicrobial agents active against anaerobic spirochetes. Historically, tiamulin, valnemulin, lincomycin, tylosin, and carbadox have been used with variable success. However, antimicrobial resistance in B. hyodysenteriae is well documented and geographically variable. Minimum inhibitory concentration (MIC) testing by agar dilution or broth microdilution under anaerobic conditions is recommended to guide therapy. Resistance to macrolides and lincosamides is frequently reported. Tiamulin and valnemulin remain among the most effective agents, but isolates with reduced susceptibility have been identified.

Treatment is most effective when administered via drinking water or feed at therapeutic doses for a full 7 to 14 day course. In-feed medication is used for metaphylaxis during outbreaks. Water-soluble tiamulin given at 8.8 milligrams per kilogram body weight for 5 consecutive days is a common regimen. Injectable formulations provide an alternative for severely anorexic pigs. Complete elimination of B. hyodysenteriae from a herd requires strict biosecurity, depopulation of carrier animals, and thorough cleaning and disinfection of facilities. Disinfectants with efficacy against spirochetes include phenolic compounds, quaternary ammonium compounds, and sodium hypochlorite.

Control and Prevention

Biosecurity is the cornerstone of swine dysentery control. Replacement stock should be sourced from herds confirmed free of B. hyodysenteriae by PCR testing. Quarantine and prophylactic testing of incoming animals for at least 30 days reduces introduction risk. All-in-all-out pig flow, strict separation of age groups, and cleaning of transport vehicles are essential management practices.

Rodent control programs are critical because mice can reintroduce infection after depopulation [1]. In herds undergoing elimination, a combination of partial depopulation (removal of known carriers), medication, and environmental sanitation can achieve eradication. Whole-herd medication for several weeks followed by sentinel testing has been described [2]. Vaccination is not widely practiced due to the antigenic diversity of B. hyodysenteriae strains and limited commercial vaccine availability. Autogenous bacterins are used on some farms but provide variable protection.

The article on Swine Gut Microbiota and Bacterial Pathogens: From Microbiome Dynamics to Acute Diarrhea Syndromes provides additional context on the role of the intestinal microbiome in susceptibility to Brachyspira colonization.

Conclusion

Brachyspira hyodysenteriae swine dysentery bloody mucoid diarrhea remains a major economic burden for pig producers worldwide. The disease is characterized by acute to chronic mucohemorrhagic colitis mediated by potent hemolysins and a dysregulated host inflammatory response. Rapid and species-specific molecular diagnostics, particularly qPCR, have replaced traditional culture for routine detection and surveillance. Antimicrobial resistance necessitates susceptibility testing for effective therapy. Multimodal control strategies combining biosecurity, rodent control, and strategic medication are required for herd-level elimination. Continued research into Brachyspira genomics and host-pathogen interactions will refine diagnostic tools and inform novel intervention strategies.

References

[1] Joens LA, Kinyon JM. Isolation of Treponema hyodysenteriae from experimentally infected mice. American Journal of Veterinary Research. 1982;43(1):147-149.

[2] Halbur PG, Bush E, et al. Comparison of eradication procedures for Brachyspira hyodysenteriae in a herd of pigs. Journal of Swine Health and Production. 2000;8(4):153-158.