BRACHYSPIRA HYODYSENTERIAE (SWINE DYSENTERY)
LEVELS: Highly unlikely: No controls necessary; Highly unlikely: No evidence of non-foodborne zoonotic transmission; Highly effective: Routine on-farm biosecurity measures are effective in preventing farm-to-farm transmission; Easy: Distinct clinical signs and/or existing test(s) available at local/regional laboratory(s); Substantial: Unsustainable acute or chronic losses related to severe clinical signs in a high prevalence of animals; Temporary disruption: Measureable negative effect on demand for less than a month when disease occurs on one or more farms; High risk: Resistance to antibacterial or antiviral treatments is, or can be expected to be a common problem; High risk: Antibacterial or antiviral treatments commonly used on affected groups, or for prophylaxis; Available but with uncertain efficacy: Limited treatments available in US or are only effective in some situations; No availability: Effective vaccines not currently available in the US (or have not been developed); Possible: Eradication possible but likely to require major changes into logistic systems, regulatory environment, infrastructure, and producer behaviors
OVERVIEW
Brachyspira hyodysenteriae is an anaerobic, gram-negative spirochete that causes swine dysentery (SD), a severe mucohemorrhagic colitis recognized since the 1920s. SD is characterized by bloody diarrhea (dysentery) with mucus, fibrin, and blood in the feces, affecting primarily grower and finisher pigs. B. hyodysenteriae remains the most commonly recognized agent of SD worldwide, though B. hampsonii and B. suanatina can also cause indistinguishable disease. The spirochetes are 5-11 µm in length with periplasmic flagella that confer corkscrew-like motility, enabling penetration through viscous digesta and mucus to colonize colonic epithelium. Key virulence factors include lipooligosaccharide (LOS), hemolysins (including TlyA and TlyC), dermonecrotic toxin activity, and chemotaxis toward mucin. Disease expression requires both B. hyodysenteriae and other anaerobic gut bacteria—gnotobiotic pigs colonized with B. hyodysenteriae alone do not develop SD. Strongly hemolytic strains typically cause disease, though weakly hemolytic avirulent variants have been identified. SD causes considerable economic loss through mortality (up to 30% in outbreaks), reduced growth rates, poor feed conversion, treatment costs, and slaughterhouse condemnations. The emergence of multidrug-resistant strains worldwide has complicated control. Herd-level eradication is possible but challenging, requiring depopulation or intensive medication combined with strict biosecurity.
FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Highly unlikely: No controls necessary
B. hyodysenteriae does not infect humans. The chapter explicitly states: "None of the agents of SD are known to infect humans." There is no evidence of foodborne transmission risk to humans from consumption of pork products or any other route.
NON-FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Highly unlikely: No evidence of non-foodborne zoonotic transmission
Human infection with B. hyodysenteriae has not been documented. The organism is adapted specifically to the porcine large intestine and related hosts. Unlike B. pilosicoli (which can infect immunocompromised humans), B. hyodysenteriae poses no recognized zoonotic risk through occupational exposure or any other route.
EFFECTIVENESS OF ON-FARM BIOSECURITY IN PREVENTING FARM-TO-FARM TRANSMISSION
Level: Highly effective: Routine on-farm biosecurity measures are effective in preventing farm-to-farm transmission
Multiple routes allow B. hyodysenteriae to bypass standard biosecurity: (1) Carrier pigs: "Outbreaks of SD typically occur following introduction of asymptomatic carrier pigs"; recovered pigs "may transmit infection to susceptible pigs for at least 70 days"; (2) Environmental persistence: "It survives in feces diluted in water for 48 days at temperatures from 0 to 10°C"; "112 days in pure pig feces"; "78 days in soil in the presence of 10% pig feces"; (3) Wildlife vectors: "Mice and rats can serve as potential reservoirs"; "B. hyodysenteriae...isolated from rodents, birds, raccoons, opossums"; waterfowl are "a significant reservoir"; (4) Fomites: "Infectious materials may also be carried into a herd by fomites such as workers' boots, farm implements, feed or animal trucks"; (5) Contaminated water: "Lagoon water containing effluent can be a source of infection." While phenolics and sodium hypochlorite are effective disinfectants and "drying of dysenteric feces rapidly kills B. hyodysenteriae," the multiple transmission routes and prolonged environmental survival create biosecurity challenges.
DIFFICULTY OF DETECTING AND CONFIRMING INFECTION
Level: Easy: Distinct clinical signs and/or existing test(s) available at local/regional laboratory(s)
Multiple validated diagnostic methods exist: (1) Clinical/pathological: Typical mucohemorrhagic diarrhea and colitis allow presumptive diagnosis; (2) Culture: Grows on selective media (CVS, BJ medium) with characteristic strong beta-hemolysis and "ring phenomenon" after 3-5 days; (3) Microscopy: Spirochetes visible under phase contrast or dark-field; (4) PCR: Multiple targets available (nox, 23S rRNA, tlyA genes); real-time PCR allows quantitation; (5) MALDI-TOF MS: "rapid methodology for Brachyspira speciation" now "standard practice in veterinary diagnostic laboratories"; (6) FISH: For tissue localization; (7) Serology: ELISA available for herd-level detection; (8) Whole genome sequencing: Increasingly used for strain characterization. The main diagnostic challenges are: detecting asymptomatic carriers (who "may only periodically shed the organism at detectable levels"), distinguishing from other strongly hemolytic Brachyspira species (B. hampsonii, B. suanatina), and interpreting weakly hemolytic atypical strains.
FINANCIAL IMPACT ON FARM'S COST OF PRODUCTION
Level: Substantial: Unsustainable acute or chronic losses related to severe clinical signs in a high prevalence of animals
SD causes substantial economic losses through multiple mechanisms: (1) Mortality: "In outbreaks of SD, morbidity in weaner pigs may approach 90% and mortality may reach 30% if effective treatment is delayed"; "Experimentally, mortality in untreated pigs may exceed 50%"; (2) Growth performance: "considerable financial loss due to...slow growth rates, poor feed conversion"; "Prolonged diarrhea leads to dehydration and the animals become weak and emaciated"; (3) Treatment costs: Antimicrobials, veterinary services; (4) Prevention costs: "Costs also arise from the need to implement preventative measures"; (5) Movement restrictions: "disruption to the supply and movement of pigs when the disease is identified in seedstock herds"; (6) Welfare concerns: "Uncontrolled disease can be a welfare issue"; (7) Endemic losses: "On endemically infected farms, clinical signs often recur cyclically at 3- to 4-week intervals." The chapter notes that "herd prevalence differed significantly in six European countries, ranging from 4.2 to 45.8%," indicating widespread endemic impact.
EFFECT ON DOMESTIC OR EXPORT MARKETS
Level: Temporary disruption: Measureable negative effect on demand for less than a month when disease occurs on one or more farms
SD status significantly affects pig trade and breeding stock movement: (1) Seedstock restrictions: "disruption to the supply and movement of pigs when the disease is identified in seedstock herds"; (2) SPF/high-health programs: SD-free status is a key certification criterion; "Closed herds, or those in closed pyramids, should remain free of SD"; (3) Quarantine requirements: "Purchased animals should be tested, quarantined for at least 3 weeks, and treated"; "a reliable history of the source herd is essential"; (4) Slaughterhouse findings: Colonic lesions affect carcass processing. SD is not a WOAH-listed disease triggering international trade bans, but positive status creates significant commercial disadvantages for breeding stock sales and affects high-health herd certification programs.
PATHOGEN'S ABILITY TO DEVELOP AND SPREAD RESISTANCE
Level: High risk: Resistance to antibacterial or antiviral treatments is, or can be expected to be a common problem
Antimicrobial resistance is a major and growing problem: (1) Multidrug resistance: "Overlying these developments is the worldwide emergence of pathogenic Brachyspira species and strains with reduced susceptibility to commonly used antimicrobials"; "Multidrug-resistant strains of B. hyodysenteriae are increasingly being found"; (2) Limited effective options: "only a few effective antimicrobials remain available for treatment"; (3) Pleuromutilin resistance: "Decreased susceptibility to tiamulin among B. hyodysenteriae isolates has been widely reported"; "pleuromutilin resistance has been linked to the presence of the tva(A) gene"; (4) Macrolide/lincosamide resistance: "Resistance to both tylosin and lincomycin frequently occurs"; "tylosin resistance developing within 2 weeks in vitro"; (5) Transnational spread: "evidence for transnational spread of resistant strains"; (6) Selection pressure: "Tiamulin usage selects for B. hyodysenteriae clones with decreased tiamulin susceptibility"; (7) Gene transfer: Resistance genes increasingly identified through whole genome sequencing; gene transfer agents (GTAs) may contribute to resistance spread.
AMR DEVELOPMENT DRIVEN BY DISEASE MANAGEMENT
Level: High risk: Antibacterial or antiviral treatments commonly used on affected groups, or for prophylaxis
SD management drives significant antimicrobial use: (1) Outbreak treatment: Water or feed medication for entire groups; "water medication for 5–7 days is effective"; "in-feed medication for 7–10 days"; (2) Prolonged prophylaxis: "Treatment of acute SD may be followed by in-feed medication at reduced levels for 2–4 weeks to prevent reinfection"; (3) Endemic herd maintenance: "In chronically affected herds, particularly if they are being medicated, the disease may not be clinically evident"—implying ongoing medication to suppress clinical signs; (4) Eradication programs: Require "short periods of intensive medication of all pigs"; (5) Historical suppression: "Routine carbadox medication may have been contributing to suppression of the disease, as the incidence increased after carbadox was withdrawn"; (6) GTA induction: "Both carbadox and metronidazole may induce expression of the B. hyodysenteriae GTA, which, in turn, may increase transfer of resistance genes."
AVAILABILITY OF EFFECTIVE TREATMENT OPTIONS
Level: Available but with uncertain efficacy: Limited treatments available in US or are only effective in some situations
Treatment options exist but are increasingly limited by resistance: (1) Primary drugs: "Pleuromutilins (tiamulin and valnemulin) and lincomycin are the drugs most commonly used to treat SD"; (2) Efficacy when susceptible: "water medication with tiamulin resolved clinical disease within 24 hours and eliminated viable spirochete shedding within 72 hours"; (3) Alternative options: Tylvalosin, tylosin (where susceptible); (4) MIC testing essential: "Before treatment, it is recommended that agar or broth dilution methods should be used to determine the minimum inhibitory concentration (MIC)"; (5) Limitations: Widespread resistance to tylosin and lincomycin; increasing pleuromutilin resistance; many previously used drugs (carbadox, dimetridazole, metronidazole) withdrawn or restricted; (6) Standardized testing: "A new, standardized broth dilution protocol for B. hyodysenteriae and B. pilosicoli has been proposed and validated through an international ring trial."
AVAILABILITY OF EFFECTIVE VACCINES OR BACTERINS
Level: No availability: Effective vaccines not currently available in the US (or have not been developed)
Commercial vaccines provide limited protection: (1) Commercial bacterins: "Bacterin vaccines for B. hyodysenteriae are available commercially in some countries and may provide a degree of protection"; (2) Serotype-specific limitation: "Bacterin vaccines tend to be LOS serogroup-specific, which then requires the use of autogenous or multivalent preparations"; (3) Production challenges: "relatively difficult and costly to produce on a large scale because of the fastidious growth requirements"; (4) Potential harm: "One publication reported that immunization with a B. hyodysenteriae bacterin actually exacerbated dysentery"; (5) Incomplete protection: Natural immunity after recovery only protects 57-93% of pigs; "approximately 10% may only become protected after two bouts of disease"; (6) Research vaccines: Recombinant proteins and live attenuated strains under investigation but not commercially available; (7) Multi-species challenge: Vaccines would need to protect against B. hyodysenteriae, B. hampsonii, and potentially B. suanatina.
FEASIBILITY OF ERADICATING THE DISEASE FROM THE US
Level: Possible: Eradication possible but likely to require major changes into logistic systems, regulatory environment, infrastructure, and producer behaviors
Herd-level eradication is achievable but challenging; national eradication would require massive coordinated effort: (1) Documented herd eradication: "The cost of eliminating SD may be recouped in 6–12 months by improved production"; multiple methods described (total depopulation, partial depopulation, medication programs); (2) Success factors: "Only four of the farms successfully eradicated the disease, and these practiced total or partial depopulation combined with implementing strict biosecurity measures"; (3) Challenges: "Programs based on antimicrobials without depopulation or partial depopulation without strictly adhering to all suggested biosecurity measures were not successful"; (4) Scale issues: "eradication becomes more difficult as herd size increases"; (5) Wildlife reservoirs: Rodents, waterfowl, and feral pigs harbor the organism; "it is virtually impossible to prevent mechanical transmission...via birds, waterfowl, insects, and other potential wildlife vectors in outdoor facilities"; (6) Reintroduction risk: Even successful eradication requires ongoing biosecurity vigilance. The chapter notes: "Whether or not B. hyodysenteriae can be totally eliminated by these methods remains unclear" in some situations.