Swine Hemorrhagic Dysentery: Diagnostic and Therapeutic Approaches for Bloody Diarrhea

Etiology and Pathogenesis of Swine Hemorrhagic Dysentery

Swine hemorrhagic dysentery, commonly referred to as swine dysentery (SD), is a severe mucohemorrhagic colitis affecting grower and finisher pigs, leading to substantial economic losses in swine production systems worldwide [1, 2, 30]. The primary etiological agent is the anaerobic intestinal spirochete Brachyspira hyodysenteriae, a strongly beta-hemolytic bacterium that colonizes the large intestine [3, 1, 2]. Additional species, including Brachyspira hampsonii and Brachyspira suanatina, have also been identified as causative agents of clinically indistinguishable disease in North America and Europe [4, 5, 29]. The clinical presentation of swine hemorrhagic dysentery is characterized by fever, anorexia, and profuse mucohemorrhagic diarrhea (swine bloody diarrhea), resulting in dehydration, weight loss, and potentially death if untreated [1, 6, 2].

The pathogenesis of SD begins with oral ingestion of B. hyodysenteriae and subsequent colonization of the colonic and cecal mucosa [3, 30]. Histological and ultrastructural examination reveals that the earliest changes in colonocytes include destruction of microvilli, which serves as a morphological expression of membrane indigestion and malabsorption [3]. The damaging effect of the pathogen leads to desquamation of colonocytes, exposure of bare stroma, circulatory abnormalities, and proliferation of cellular elements [3]. The resulting inflammation is classified as catarrhal hemorrhagic inflammation with focal necrosis of the epithelium and edema of the lamina propria [3]. In connective tissue stroma, mucoid and fibrinoid swelling of collagen fibers, fragmentation and coarsening of elastic and reticular fibers, and proliferation of fibroblasts and histiocytes are observed [3].

The host-pathogen interaction involves complex interference with colonic immune and epithelial repair mechanisms [5]. Pathogenic strains of Brachyspira impair the host's ability to activate humoral immune responses through IL-4/CCR6/KLHL6 interactions, disrupt epithelial wound repair mechanisms via LSECtin impairment of macrophages, and induce mitochondrial dysfunction linked to MDR1 expression [5]. Furthermore, pathogenic strains upregulate stress-associated genes and alter the colonic microbiota composition, promoting dysbiosis that favors disease progression [7, 5, 8].

The clinical signs of swine dysentery typically appear 4 to 9 days after exposure [29]. Affected pigs exhibit high fever (above 40 degrees Celsius), anorexia, and watery diarrhea that progresses to mucohemorrhagic feces, often described as gray to brown in color with flecks of blood and mucus [6, 2]. Weight loss is common, and mortality rates can reach 40 percent or higher in naive populations without therapeutic intervention [9, 10, 11]. Acute SD is characterized by fibrinonecrotic and hemorrhagic typhlocolitis, while subacute forms may present with diphtheritic-necrotizing colitis [12, 26].

The colonic innate immune response is significantly altered during acute SD, with marked changes in the expression of antimicrobial peptides and inflammatory cytokines [7]. The disruption of the colonic epithelial barrier allows for bacterial translocation and exacerbates the inflammatory response, contributing to the characteristic hemorrhagic lesions [7, 13].

Clinical Presentation and Differential Diagnosis

Swine dysentery must be differentiated from other causes of swine bloody diarrhea, including proliferative hemorrhagic enteropathy (caused by Lawsonia intracellularis), salmonellosis (caused by Salmonella enterica serovar Typhimurium), and clostridial enteritis (caused by Clostridium perfringens type C) [1, 2, 31]. Co-infection with L. intracellularis and B. hyodysenteriae has been documented, complicating both diagnosis and therapeutic management [31]. Porcine circovirus-like virus has also been identified in pigs with hemorrhagic dysentery and diarrhea, though its role as a primary pathogen remains under investigation [14].

The gross lesions of SD are limited to the large intestine and include a fibrinous, blood-flecked membrane covering the colonic and cecal mucosa, with visible swelling and hemorrhage of the colonic wall and mesenteric lymph nodes [6, 26]. Histologically, mucin expression is altered in goblet cells, and the mucosa exhibits acute inflammatory infiltrates with edema and hemorrhage [3, 13, 26].

The differential diagnosis also includes non-infectious causes of colitis such as dietary factors, including the type and fermentability of dietary fiber. Highly fermentable fiber has been shown to reduce the incidence of clinical SD in challenged pigs, suggesting that nutritional management is an important consideration in disease control [15].

Diagnostic Approaches

Accurate diagnosis of swine hemorrhagic dysentery is critical for implementing effective treatment and control measures. A combination of clinical observation, pathological examination, microbiological culture, and molecular detection methods is recommended for definitive diagnosis [12, 6, 2].

Histological and Staining Methods

Histological examination of colonic sections using special stains can aid in the visualization of spirochetes. The use of Victoria blue 4-R stain allows for clear and consistent observation of spirochetes in the lumen of mucosal glands in histologic sections of the colon from pigs with hemorrhagic diarrhea [16]. Direct fluorescent antibody tests using hyperimmunized swine serum have also been developed for the detection of large spirochetes in fecal samples [17]. Gram-stained fecal smears from diarrheic pigs often reveal large numbers of spirochetes, though this method lacks specificity for species-level identification [29].

Microbiological Culture

Isolation of B. hyodysenteriae requires anaerobic culture conditions using selective media such as tryptic soy agar supplemented with 5 to 10 percent sheep blood and antimicrobials [3, 29]. The characteristic strong beta-hemolysis around colonies is a key phenotypic feature for presumptive identification [18]. However, weakly hemolytic strains have been identified in healthy pigs, and these strains may be avirulent or exhibit reduced pathogenicity [18, 33, 35]. Such weakly hemolytic strains can be used as live vaccines to protect pigs from developing SD without causing clinical disease [19, 35].

Molecular Diagnostics

Polymerase chain reaction (PCR) is the gold standard for molecular detection of Brachyspira species [1, 12, 6]. PCR targeting the nox gene (NADH oxidase) is widely used for genus-level detection, while species-specific PCR assays differentiate between B. hyodysenteriae, B. hampsonii, and other Brachyspira spp. [20, 29, 30]. Quantitative PCR (qPCR) provides additional information on pathogen load and is valuable for monitoring fecal shedding during infection and treatment [29].

In a recent outbreak investigation in Ukraine, PCR detected B. hyodysenteriae in 100 percent of fecal samples from imported pigs and 70 percent of samples from the farm's own herd, confirming the utility of molecular diagnostics for outbreak confirmation and epidemiological tracking [12]. Similarly, a survey in Northern Vietnam identified B. hyodysenteriae in 11.6 percent of samples tested, with higher prevalence in post-weaning pigs and farms with poor hygiene practices [6].

Advanced Strain Typing

Multilocus sequence typing (MLST) and whole genome sequencing (WGS) provide high-resolution discrimination of B. hyodysenteriae isolates for epidemiological studies [18, 20, 21]. MLST of Australian isolates revealed diverse and distinct sequence types (STs) that were different from those reported in other countries, with evidence of ongoing strain evolution [20]. Italian isolates from 2003 to 2012 also showed diverse STs, and pleuromutilin susceptibility varied among sequence types [21]. Analysis of virulence-associated plasmid genes (a block of six genes) has shown that loss of these genes may correlate with milder disease presentation and may account for the occurrence of subclinical infections [18, 20].

The following diagnostic algorithm summarizes the recommended workflow for cases of swine bloody diarrhea.

flowchart TD
    A[Clinical Case: Swine Bloody Diarrhea], > B{Detailed Clinical Examination}
    B, > C[Differential: Other Enteric Pathogens]
    B, > D[Fecal Sample Collection]
    D, > E[Gram Stain / Dark Field Microscopy]
    E, > F{Spirochetes Present}
    F, Yes, > G[Anaerobic Culture on Blood Agar]
    F, No, > H[Alternative Diagnosis: Lawsonia, Salmonella, etc.]
    G, > I{Strong Beta-Hemolysis}
    I, Yes, > J[PCR: nox Gene / Species-Specific Assay]
    I, Weak, > K[Consider Atypical or Avirulent Strain]
    J, > L[B. hyodysenteriae / B. hampsonii Confirmed]
    L, > M[Treat with Susceptible Antimicrobial]
    L, > N[Further Typing: MLST, WGS for Epidemiology]

Fecal Microbiota Analysis

The colonic microbiota is profoundly altered during SD, with significant differences in richness and beta diversity between diseased and healthy pigs [7, 8]. Luminal and mucosa-associated microbial profiles differ, and the evaluation of the mucosal microbiome may be of higher value in elucidating bacterial mechanisms underlying the development of SD [8]. Brachyspirales, Campylobacterales, Desulfovibrionales, and Enterobacteriales are more abundant in pigs with dysentery, while Lactobacillus and Bifidobacterium spp. are more abundant in pigs without disease [8]. These compositional changes can be leveraged for diagnostic purposes, though standardized protocols are not yet widely adopted.

Therapeutic Approaches

Therapeutic management of swine dysentery relies heavily on antimicrobial administration, as no commercial vaccines are currently available [5, 2]. However, the emergence of antimicrobial resistance (AMR) is an increasing concern, and therapeutic decisions should be guided by susceptibility testing where possible [22, 20, 21].

Antimicrobial Agents

Several antimicrobial classes have been evaluated for the treatment and prevention of SD.

Pleuromutilins

Tiamulin is a pleuromutilin antibiotic widely used for SD treatment and control [12, 22, 27]. Administration via drinking water or feed has been shown to be effective in reducing mortality and clinical signs [12]. However, resistance to pleuromutilins has been reported globally [22, 20, 21]. A newly identified resistance gene, tva(A) (tiamulin valnemulin antibiotic resistance), encodes a predicted ABC-F transporter that confers reduced susceptibility to pleuromutilins in B. hyodysenteriae [22]. This gene does not lead to clinical resistance on its own but broadens the mutant selection window, facilitating the development of higher-level resistance through mutations in chromosomal genes such as the 23S rRNA gene, rplC (encoding L3 ribosomal protein), and fusA (encoding Elongation Factor G) [22]. Valnemulin, another pleuromutilin, has also been evaluated for SD treatment, often in combination with zinc chelate, though efficacy may vary [27].

Lincosamides and Macrolides

Lincomycin, a lincosamide antibiotic, has been used for SD treatment and prevention [9, 12]. However, resistance to lincomycin has been documented in Australian isolates from more recent years compared to historical strains [20]. Tylosin, a macrolide antibiotic, was historically used for the treatment of vibrionic swine dysentery but has been associated with treatment failures and resistance development [23, 9]. A combination of lincomycin and spectinomycin (an aminocyclitol) at concentrations of 44 and 77 mg/kg in feed prevented experimentally induced SD in swine, and disease did not recur after medication withdrawal [9].

Nitroimidazoles

Ronidazole, a nitroimidazole compound, administered in drinking water at concentrations of 0.003 to 0.012 percent, was effective for treatment of experimentally induced SD [10, 24]. Pigs treated with 0.003 percent ronidazole had no deaths, and diarrhea receded during or after treatment [24]. Lower concentrations (0.0015 and 0.00075 percent) aided in treatment and allowed for development of immunity to the disease, though nonhemorrhagic diarrhea was more common in these groups [24]. Ipronidazole, another nitroimidazole, at concentrations of 50 and 100 mg/L in drinking water also effectively treated SD, with reduced days of diarrhea and improved growth performance [11].

Organoarsenicals

Sodium arsanilate, an organoarsenical compound, was evaluated for SD prevention but was found to be ineffective, and pigs that developed hemorrhagic diarrhea while on this medication had a more severe form of the disease than nonmedicated controls [9].

Antimicrobial Resistance Monitoring

The emergence of multi-drug resistant strains of B. hyodysenteriae is a serious threat to disease control [22, 20]. Four distinct multi-drug resistant strains were identified in five Australian herds, with reduced susceptibility to lincomycin and tiamulin compared to historical isolates [20]. In Italy, pleuromutilin susceptibility of B. hyodysenteriae isolates from 2003 to 2012 showed variability among different sequence types, highlighting the need for ongoing surveillance [21]. Antimicrobial susceptibility testing using broth dilution or agar dilution methods should be performed to guide therapeutic choices.

Dietary and Management Interventions

Dietary manipulation may serve as an adjunct to antimicrobial therapy or as a preventive measure. Replacing lowly fermentable fiber (e.g., corn distillers dried grains with solubles, DDGS) with highly fermentable fiber (e.g., beet pulp and resistant starch) significantly reduced the incidence of clinical SD in challenged pigs [15]. In a 42-day challenge study, only 15 percent of pigs fed a highly fermentable fiber diet developed SD compared to 85 percent of pigs fed a lowly fermentable fiber diet [15]. The highly fermentable fiber diet also improved average daily gain and modulated the fecal microbiota, increasing beneficial bacteria such as Shuttleworthia and Lactobacillus while reducing Prevotellaceae [15].

Vaccination Strategies

Despite decades of research, no commercial vaccine for SD is currently available [5, 19]. Experimental vaccines using inactivated adjuvanted S. hyodysenteriae (the former name for B. hyodysenteriae) have shown limited efficacy and, in some cases, exacerbated disease onset [25]. Vaccinated swine that died had higher antibody titers before challenge, suggesting an immune-mediated component to disease exacerbation [25].

More promising results have been obtained using weakly hemolytic, avirulent strains of B. hyodysenteriae as live vaccines [19, 35]. Colonization by such strains did not cause clinical disease and induced local IgA production in feces, which correlated with a later onset of disease upon challenge with a virulent strain [19]. A weakly hemolytic strain from a healthy German multiplier herd was shown to be avirulent and could protect pigs from developing SD [18, 35]. This strain had multiple amino acid substitutions in hemolysin proteins, a disrupted promoter site in the hlyA gene, and lacked certain plasmid genes associated with colonization [18].

Control and Biosecurity

Control of SD on farms relies on a combination of biosecurity measures, early detection, and effective treatment. The introduction of replacement breeding stock is the main risk factor for the introduction of B. hyodysenteriae into previously uninfected herds [12, 20]. In a Ukrainian outbreak, the cumulative incidence reached 57 percent after the introduction of replacement gilts from breeding farms, with 100 percent morbidity in imported animals and a 15 percent mortality rate [12].

Biosecurity measures include quarantine and testing of incoming animals, strict hygiene protocols, and prevention of indirect transmission via fomites, equipment, and personnel [30]. B. hyodysenteriae can survive in the environment for extended periods, and rodents, birds, and other vectors may play a role in between-herd transmission [30]. In herds with endemic SD, elimination strategies may involve depopulation, cleaning and disinfection, and repopulation with SD-free stock, or a combination of medication and management changes to reduce pathogen load and break the cycle of infection [2, 30].

The use of antimicrobials for metaphylaxis (mass medication of exposed groups) and prophylaxis (medication of naive animals at risk) remains common but should be rationalized to minimize selection pressure for AMR [22]. The identification of resistance markers such as tva(A) provides valuable information for designing treatment regimens that reduce the risk of resistance development [22].

Conclusion

Swine hemorrhagic dysentery (swine bloody diarrhea) caused by Brachyspira hyodysenteriae and related species remains a significant challenge to global swine production. Advances in molecular diagnostics, including PCR, qPCR, and whole genome sequencing, have improved the accuracy of species identification and epidemiological tracking. Understanding the complex pathogenesis involving colonic immune disruption, epithelial barrier dysfunction, and microbiota alterations is essential for developing novel control strategies. Antimicrobial therapy, particularly with pleuromutilins such as tiamulin, remains the mainstay of treatment, but the emergence of resistance necessitates judicious use and routine susceptibility testing. Dietary manipulation with highly fermentable fiber and the development of avirulent live vaccines represent promising avenues for reducing antimicrobial reliance and improving disease management.

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