Swine Dysentery: Etiology, Clinical Signs, and Management of Bloody Diarrhea in Pigs

Introduction

Swine dysentery (SD) is a globally distributed, highly contagious mucohemorrhagic enterocolitis of pigs, primarily caused by the anaerobic spirochete Brachyspira hyodysenteriae [1, 2]. The disease is characterized clinically by the passage of bloody, mucoid feces, often referred to as swine bloody diarrhea, and results in significant economic losses due to mortality, reduced weight gain, feed conversion inefficiency, and costs associated with treatment and control [3]. Although first described in the 1920s, SD remains a major challenge in modern swine production systems, particularly in continuous-flow or high-density operations. This article provides a comprehensive, clinically oriented review of the etiology, epidemiology, pathogenesis, clinical presentation, pathology, diagnostic approaches, therapeutic management, and biosecurity-based control of swine dysentery.

The condition is distinct from other enteric diseases such as porcine proliferative enteropathy (caused by Lawsonia intracellularis), porcine colonic spirochetosis (caused by Brachyspira pilosicoli), and salmonellosis [2, 4]. Accurate diagnosis and targeted management are essential to mitigate herd-level impacts and reduce antimicrobial use.

Etiology

Swine dysentery is primarily caused by Brachyspira hyodysenteriae, a Gram-negative, oxygen-tolerant, anaerobic spirochete [1, 3]. The organism is 6 to 10 µm in length, 0.3 to 0.4 µm in width, and possesses axial flagella that confer characteristic serpentine motility, often described as serpentine or cork-screw movement on dark-field microscopy [2]. B. hyodysenteriae produces beta-hemolysis on blood agar, a feature that aids in its preliminary identification [3].

Other Brachyspira species, such as B. hampsonii and B. suanatina, have been reported to cause SD-like disease in pigs [5]. These emerging pathogens are increasingly recognized in some geographic regions and may produce clinical signs indistinguishable from classical SD [5, 6]. The primary virulence factor of B. hyodysenteriae is a beta-hemolysin (a hemolytic glycolipid) that damages colonic epithelial cells and disrupts tight junctions, leading to inflammation and hemorrhage [1, 3]. Additionally, flagella-mediated motility and the ability to colonize the deep colonic crypts are critical for pathogenesis [2].

Epidemiology

Swine dysentery occurs worldwide, with a higher prevalence in conventional, continuous-flow finishing units compared to all-in/all-out systems [1, 4]. Transmission occurs primarily through the fecal-oral route. Contaminated feces, feed, water, equipment, and boots can serve as fomites [3]. Recovered carrier pigs frequently shed the organism intermittently, serving as reservoirs for naive pennates [2].

B. hyodysenteriae can survive in feces for up to 48 days at 4 degrees Celsius and for at least 7 days in lagoon water [1, 3]. The organism is susceptible to desiccation, direct sunlight, and high temperatures; therefore, environmental persistence is limited in well-dried, cleaned facilities [4]. Rodents and birds can act as mechanical vectors [2].

Age susceptibility is variable. Clinical disease most commonly occurs in grower-finisher pigs (10 to 70 kg body weight) but can affect weaners and adults under high stress or heavy challenge [1, 4]. Morbidity within affected groups can reach 90%, but mortality is typically low (1% to 5%) unless complicated by concurrent infections (e.g., Salmonella spp., Lawsonia intracellularis) or poor husbandry [3].

Pathogenesis

Following ingestion, B. hyodysenteriae colonizes the colonic and cecal mucosa. The spirochetes penetrate the mucus layer and localize within the crypts of Lieberkühn, attaching to the surface of absorptive enterocytes but not invading beyond the epithelium [1, 2]. Beta-hemolysin production induces detachment of epithelial cells, loss of tight junction integrity, and an intense neutrophilic inflammatory response [3].

The resultant pathology includes mucosal erosion, capillary thrombosis, and extravasation of erythrocytes, producing the characteristic bloody, mucoid feces [2]. The mucus itself results from goblet cell hyperplasia and hypersecretion in response to epithelial irritation and inflammation [4]. The infection is confined to the large intestine (cecum and colon) with no involvement of the small intestine, a key distinction from many viral enteritides [1].

Clinical Signs

The incubation period ranges from 3 to 21 days depending on challenge dose and host immunity [1, 2]. The hallmark clinical sign is swine bloody diarrhea: initially, feces may be pasty and yellowish to gray, but rapidly progress to watery, mucoid, and blood-tinged feces [3]. Fresh blood and strands of fibrin may be evident. Affected pigs exhibit anorexia, pyrexia (up to 40.5 degrees Celsius), dehydration, and hunched posture [4]. Tenesmus is common. Subacute and chronic cases show progressive weight loss, rough hair coat, and stunted growth [2]. In herd outbreaks, peracute deaths can occur in naive populations, though mortality is generally low (<5%) if uncomplicated [1].

A subset of infected pigs become asymptomatic carriers and intermittently shed the organism, especially during periods of stress such as regrouping, transport, or nutritional changes [3].

Pathology

Gross lesions are confined to the cecum and spiral colon. The serosal surface may appear edematous and congested [1]. On opening, the mucosa is thickened, hyperemic, and covered by a fibrinous, mucoid, bloody pseudomembrane [2]. Overlying the lesions is a variable amount of tenacious mucus mixed with flecks of blood. In chronic cases, the mucosa appears turgid and corrugated due to fibrotic changes [4].

Histologic examination reveals necrosis of superficial epithelium, crypt elongation, goblet cell hyperplasia, and intense infiltration of neutrophils and macrophages in the lamina propria. The spirochetes are visible in silver-stained sections (e.g., Warthin-Starry) as thin, wavy organisms clustered within the crypt lumina [2, 3]. The inflammation is primarily neutrophilic, distinguishing SD from the proliferative lesions of L. intracellularis [1].

Diagnostics

Accurate diagnosis of swine dysentery relies on the integration of clinical history, gross pathology, and laboratory confirmation.

Direct Microscopy

Dark-field examination of fresh mucosal smears from affected feces or colonic scrapings can reveal the characteristic serpentine motility of Brachyspira spp. [1]. This method is rapid but cannot differentiate B. hyodysenteriae from other non-pathogenic spirochetes [3].

Culture

Isolation of B. hyodysenteriae requires selective media (e.g., trypticase soy agar with spectinomycin, vancomycin, and colistin) incubated under anaerobic conditions at 37 to 42 degrees Celsius for 3 to 7 days [2]. Beta-hemolysis on blood agar is a key distinguishing feature. Biochemical profiling (indole production, hippurate hydrolysis) and antimicrobial susceptibility testing can be performed on isolates [4].

Molecular Diagnostics

Polymerase chain reaction (PCR) assays targeting the nox gene or the 16S rRNA gene provide rapid, species-specific detection directly from feces or tissue [3]. Real-time PCR and multiplex panels have been developed to differentiate B. hyodysenteriae from B. pilosicoli and B. hampsonii [5, 6]. Molecular methods are the current gold standard for confirmation due to their high sensitivity and specificity [1].

Serology

Indirect fluorescent antibody tests and enzyme-linked immunosorbent assays (ELISAs) can detect antibodies in serum, but are more useful for herd-level surveillance than individual diagnosis because seroconversion occurs late in the course of infection [2].

Differential Diagnosis

Several enteric pathogens can mimic SD. A structured diagnostic workup is mandatory.

flowchart TD
    A[Clinical suspicion of swine dysentery based on bloody mucoid diarrhea in grower-finisher pigs], > B[Collect fresh fecal samples or colonic scrapings]
    B, > C[Dark-field microscopy for motile spirochetes]
    C, > D{Positive?}
    D, >|Yes| E[Confirm with species-specific PCR for B. hyodysenteriae]
    D, >|No| F[Culture on selective media under anaerobic conditions]
    F, > G{Beta-hemolytic colonies?}
    G, >|Yes| E
    G, >|No| H[Consider other causes: \n- B. pilosicoli (PCS)\n- Lawsonia intracellularis (PPE)\n- Salmonella spp.\n- Escherichia coli\n- Rotavirus or coronavirus]
    H, > I[Perform additional targeted PCR or histopathology]
    E, > J[Confirm SD; institute treatment and control measures]

Table 1 summarizes key differential diagnoses for swine dysentery.

For further reading enteric diseases, see the article on Swine Enteric and Systemic Diseases: Dysentery, Fever, and Bloody Diarrhea in Pigs and the more focused piece on Brachyspira hyodysenteriae and Swine Dysentery: Bloody Mucoid Diarrhea and Diagnosis.

Treatment

Antimicrobial therapy is the mainstay of clinical case management. Historically, tiamulin, valnemulin, lincomycin, tylosin, and carbadox have demonstrated efficacy against B. hyodysenteriae [1, 3]. However, antimicrobial resistance, particularly to macrolides and lincosamides, has emerged in many regions, necessitating susceptibility testing of isolates [2]. Tiamulin (water-soluble or in-feed formulations) remains a drug of choice in many countries for treatment and control [4].

Treatment regimens should comply with local veterinary regulations and withdrawal periods. Supportive therapy includes fluid and electrolyte replacement for dehydrated pigs. In acute outbreaks, mass medication via drinking water is preferred for rapid intake, followed by in-feed medication for prevention of relapses [3].

Control and Prevention

Effective control of swine dysentery requires an integrated approach combining biosecurity, herd management, and targeted antimicrobial use.

Biosecurity

Breaking the fecal-oral cycle is critical. Depopulation followed by thorough cleaning, disinfection, and an empty period of at least 4 weeks can eliminate the pathogen from facilities [1, 2]. Partial depopulation (e.g., removing farrow-to-finish) and establishing negative breeding stock are proven eradication strategies [4]. All-in/all-out production, especially in grow-finish barns, reduces the risk of transmission between batches [3].

Cleaning protocols must include detergent and degreaser, followed by a disinfectant effective against spirochetes (e.g., peroxygen compounds, glutaraldehyde, or chlorocresol) [2]. Fumigation may be added. Equipment sharing should be avoided; if unavoidable, strict sanitation between groups is mandatory.

Vaccination

To date, no commercial vaccine for SD is widely available. Autogenous bacterins have been used in some herds, but evidence of efficacy is variable and primarily limited to reducing clinical severity rather than preventing colonization [1, 3].

Surveillance and Monitoring

Regular monitoring of suspect cases using PCR on fecal pools can detect re-emergence of B. hyodysenteriae in a herd. Serologic monitoring may be used in eradication programs to confirm negative status following depopulation [2].

For related information, refer to the article on Swine Dysentery: Age Susceptibility, Causative Agent, and Clinical Management in Pigs and the historical perspective in Treponema hyodysenteriae (Swine Dysentery): Serpentine Colonization and Diagnostic Approaches.

Conclusion

Swine dysentery remains a significant enteric disease of pigs, characterized by the classic presentation of swine bloody diarrhea due to Brachyspira hyodysenteriae infection. Accurate diagnosis via molecular methods, coupled with prudent antimicrobial use and rigorous biosecurity protocols, forms the foundation of effective control. Emerging resistance patterns and the recognition of novel Brachyspira species underscore the need for continuous surveillance and research into alternative control strategies, including vaccine development and gut health modulation.

References

[1] Merck Veterinary Manual. Swine Dysentery. Kenilworth, NJ: Merck & Co., Inc.

[2] Straw BE, Zimmerman JJ, D'Allaire S, Taylor DJ, editors. Diseases of Swine. 10th ed. Ames, IA: Wiley-Blackwell; 2012.

[3] Quinn PJ, Markey BK, Leonard FC, Hartigan P, Fanning S, Fitzpatrick ES. Veterinary Microbiology and Microbial Disease. 2nd ed. Oxford: Wiley-Blackwell; 2011.

[4] Gyles CL, Prescott JF, Songer JG, Thoen CO, editors. Pathogenesis of Bacterial Infections in Animals. 4th ed. Ames, IA: Wiley-Blackwell; 2010.

[5] Rubin JE, Costa MO, DeLay J, Boerlin P. Detection of Brachyspira hampsonii in an Ontario swine herd with dysentery. Can Vet J. 2013;54(9):867-870.

[6] Rohde J, Habighorst-Blome K, Seehusen F, Rautenschlein S. A case of porcine dysentery associated with Brachyspira suanatina in a German pig farm. Berl Munch Tierarztl Wochenschr. 2016;129(9-10):418-421. *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.