Brachyspira hyodysenteriae (swine dysentery)
LEVELS: Highly unlikely; No human illness; Routine biosecurity keeps it out; Routine; Moderate; Negligible; Little; Available but inconsistent
| Register id | brachyspira_hyodysenteriae |
| Type | bacteria |
| Scientific name | Brachyspira hyodysenteriae |
| NCBI taxid | 159 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Brachyspira hyodysenteriae is the classical cause of swine dysentery, a severe mucohaemorrhagic diarrhoea of weaned and growing pigs. In an outbreak up to 90% of weaners may be affected and 30% may die if treatment is delayed; experimentally, untreated mortality exceeds 50%. Most pigs eventually recover, but they grow slowly afterwards, so the cost is a combination of deaths, poor feed conversion, medication and — for seedstock herds — disrupted pig movement once the diagnosis is known. It nearly always arrives in a herd inside an apparently healthy carrier pig that was not quarantined or treated, though rodents, birds, contaminated feed, trucks and visitors who have handled infected pigs can all bring it in. Closed, geographically isolated herds that source replacements carefully stay free of it. Eradication is expensive but usually pays for itself within six to twelve months through better growth and lower drug use.
C1 Zoonotic potential
Likelihood of transmission from pigs to humans
Highly unlikely: Human infection with the agent has never been reported, or has been reported but is not attributed to pig or pork exposure
Basis. ch45 45.2.3, the whole of the public health section for swine dysentery and covering all three of its agents: "None of the agents of SD are known to infect humans."
How this level was decided
Level 1, stated outright. An examined negative rather than a silence -- the chapter gives swine dysentery its own public health section and disposes of the question in one sentence covering B. hyodysenteriae, B. hampsonii and B. suanatina together.
C2 Zoonotic impact
Severity of human illness caused by the agent
No human illness: Agent does not cause illness in people
Basis. ch45 45.2.3, the whole of the public health section for swine dysentery and covering all three of its agents: "None of the agents of SD are known to infect humans."
How this level was decided
Level 1. C2 grades illness acquired from pigs and the chapter says none of the agents of swine dysentery infects people.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Routine biosecurity keeps it out: Quarantine of incoming stock, transport and fomite control, cleaning and disinfection, and the usual monitoring reliably exclude it
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED at level 3 on the C3 review sheet. "l3, as above for B.hyo"
Basis. ch45 45.2.4: "Outbreaks of SD typically occur following introduction of asymptomatic carrier pigs that are not quarantined, treated prophylactically, or both. Outbreaks also occur in herds following transmission via rodents or birds from adjacent infected farms, introduction through contaminated feed or animal trucks, or by visitors who have had contact with infected pigs." "heightened biosecurity, such as the provision of boots and protective clothing for visitors, the presence of security fencing, onsite feed mixing, and obtaining replacement breeders from a consistent source, were protective." 45.2.10.2: "Closed herds, or those in closed pyramids, should remain free of SD if they are geographically isolated and use precautions to prevent contamination. Introduction of new stock is the greatest risk, so a reliable history of the source herd is essential. Purchased animals should be tested, quarantined for at least 3 weeks, and treated." A European study found herd prevalence of 4.2 to 45.8%, with lower detection associated with "well-performed biosecurity measures".
How this level was decided
Level 3, the framework's worked example at that level, and the chapter writes the level 3 programme out almost as a checklist: quarantine and test incoming stock, control visitors and their boots, fence the site, mix feed on site, source replacements from one herd, control rodents. Every one of those is routine biosecurity run continuously and capable of failing, which is exactly what the level says. The evidence that it works is the observed six-country spread in herd prevalence, from 4.2% to 45.8%, with the low end attributed to biosecurity. NOTE why this is not level 5 although rodents, dogs and birds are named as sources: the chapter treats them as mechanical carriers between adjacent infected FARMS rather than as a reservoir maintaining the agent independently of pigs. Contrast brachyspira_hampsonii in this batch at level 5, where migratory waterfowl are a true reservoir.
C4 Detection difficulty
Difficulty of recognizing and confirming infection
Routine: The disease would be suspected from clinical signs, history and epidemiology in normal practice, and confirmed by tests that local or regional laboratories run routinely
Basis. ch45 45.2.8: "In pigs with typical dysentery, a presumptive diagnosis of SD can be made by examining fecal or colonic mucosal smears for the presence of large numbers of typical serpentine spirochetes using nonspecific stains or dark-field microscopy. A definitive diagnosis requires demonstration that the spirochetes are strongly beta-hemolytic pathogenic Brachyspira spp." "Selective anaerobic culture, which has the capacity to detect all known agents of SD and provides a hemolytic phenotype, is an integral part of Brachyspira detection, diagnosis, and surveillance." MALDI-TOF speciation "is becoming standard practice in veterinary diagnostic laboratories". Samples from colonic mucosa and faeces contain 10^8-10^9 cells per gram.
How this level was decided
Both halves of level 1, and it is the framework's worked example there. Mucohaemorrhagic diarrhoea in grower-finisher pigs is recognised in normal practice, and the chapter allows a presumptive diagnosis from a smear read under dark field in the barn. Confirmation is selective anaerobic culture plus MALDI-TOF, both of which the chapter describes as standard veterinary diagnostic laboratory work. NOTE the interpretive cautions, which are limits on an available test rather than barriers to getting one: asymptomatic carriers shed intermittently, so pooling five rectal swabs is recommended, and false negatives follow drying or transport delay.
C5 Production cost
Financial impact on the infected farm's cost of production
Moderate: Losses measurably increase cost of production but remain manageable within normal farm operations, whether acute, chronic, or associated with endemic disease
Basis. ch45 45.2.1: "SD causes considerable financial loss due to mortality, slow growth rates, poor feed conversion, and costs of treatment. Costs also arise from the need to implement preventative measures in herds that do not have SD and from the disruption to the supply and movement of pigs when the disease is identified in seedstock herds." 45.2.6: "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%." "Most pigs recover over several weeks, but their growth rate remains depressed." 45.2.10.3: "The cost of eliminating SD may be recouped in 6-12 months by improved production and reduced drug usage."
How this level was decided
Level 3. The outbreak figures look like level 4 -- 90% morbidity and 30% mortality -- but both are qualified as what happens when effective treatment is DELAYED, and treatment resolves clinical disease within 24 hours where the isolate is susceptible. What the criterion is asking about is the standing cost, and the chapter answers it: continuous medication, depressed growth in recovered pigs, preventive spending in herds that do not even have it, and eradication programmes that pay for themselves in 6 to 12 months. Losses that measurably increase cost of production and are managed within normal farm operations. It is not level 4, because nothing here is unsustainable -- the disease has been endemic in the industry for a century.
C6 Market impact
Duration of material disruption to pork and pig markets
Negligible: Little or no material disruption to supply or demand when disease occurs on one or more farms
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c6l1, no evidence to suggest market is or would react to a diagnosis"
Basis. Section 45.2 records no trade measure, movement control or consumer response at market level. SD has been recognised since the 1920s, is endemic worldwide, and a recent European study found herd prevalence between 4.2 and 45.8%.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact this is an agent long endemic and routinely diagnosed in US herds with no historical market move on record. FLAGGED, because the chapter does name a supply effect and it is worth Eric seeing the words: costs arise "from the disruption to the supply and movement of pigs when the disease is identified in seedstock herds". That is real, but it is one herd's sales rather than pork supply or demand, and it sits alongside Mycoplasma hyopneumoniae and APP -- both level 1 worked examples -- which do the same thing. Level 1.
C7 Treatment potential
Potential for treatment to improve outcomes
Little: Effective pathogen-directed treatment is available and works, or animals recover acceptably without it
Corrected by Eric from L2 Some. Reason: MOVED L2 -> L1. His ruling of 2026-09-02, made on the suanatina C7 cell and explicitly directed at all three agents of swine dysentery: "c7l2, check your work on hampsonii and hyodysenteriae. The US tiamulin label says \"For control of porcine proliferative enteropathies (ileitis) associated with Lawsonia intracellularis and for control of swine dysentery associated with Brachyspira (formerly Serpulina or Treponema) hyodysenteriae susceptible to Tiamulin.\" In other words, for this criteria B.hyo should be C7L1, but hampsonii and suanatina would be extralabel C7L2 by my reading." THE TEST IS THE LABEL TEXT, NOT THE SUSCEPTIBILITY. The assessment had scored these two cells the other way round, putting B. hampsonii at level 1 because a 2024 US survey found it MORE susceptible to tiamulin than B. hyodysenteriae. That is the wrong question. The standing note SCORING/treatment_potential says level 1 is where a labelled product already works and is used, and level 2 where treatment requires extralabel use -- and the label names one species. B. hyodysenteriae is the species named on the US tiamulin label, so treating it is labelled use of a product the chapter shows works: water medication with tiamulin "resolved clinical disease within 24 hours and eliminated viable spirochete shedding within 72 hours". The assessment had held this at level 2 on the rising multidrug resistance the chapter documents, and that concern is real -- but the label carries the phrase "susceptible to Tiamulin", so susceptibility testing is part of the labelled use rather than a departure from it.
Basis. ch45 45.2.10.1: "Multidrug-resistant strains of B. hyodysenteriae are increasingly being found, and only a few effective antimicrobials remain available for treatment." "Before treatment, it is recommended that agar or broth dilution methods should be used to determine the minimum inhibitory concentration (MIC) of the antimicrobials." "In pigs experimentally infected with susceptible isolates of either B. hyodysenteriae or B. hampsonii, water medication with tiamulin resolved clinical disease within 24 hours and eliminated viable spirochete shedding within 72 hours." "Decreased susceptibility to tiamulin among B. hyodysenteriae isolates has been widely reported, and there is evidence for transnational spread of resistant strains." Resistance to tylosin and lincomycin "frequently occurs".
How this level was decided
Level 2, and the label fits word for word: treatment exists but is inconsistent. Where the isolate is susceptible, tiamulin is close to a cure -- clinical resolution in 24 hours, shedding stopped in 72 -- which is why this is not level 3. What stops it being level 1 is that the chapter opens the treatment section by saying only a few effective antimicrobials remain, recommends MIC testing BEFORE treating, and records transnational spread of resistant strains. A drug you must test for before you can rely on it is not a drug that reliably works.
C8 Vaccine availability
Availability of effective vaccines or bacterins
Available but inconsistent: Commercial or autogenous vaccines exist in the US but protection may be inconsistent
Basis. ch45 45.2.10.1: "Bacterin vaccines for B. hyodysenteriae are available commercially in some countries and may provide a degree of protection." "Bacterin vaccines tend to be LOS serogroup-specific, which then requires the use of autogenous or multivalent preparations. These are relatively difficult and costly to produce on a large scale because of the fastidious growth requirements of B. hyodysenteriae. One publication reported that immunization with a B. hyodysenteriae bacterin actually exacerbated dysentery." Recombinant Bhlp29.7 gave "a 50% reduction in incidence of disease".
How this level was decided
Level 2, and this is the label's definition rather than a judgment: commercial or autogenous vaccines exist and protection may be inconsistent. Every clause is on the page -- commercial bacterins in some countries giving "a degree of protection", serogroup specificity forcing autogenous or multivalent products, a published report of a bacterin making dysentery WORSE, and the best experimental candidate reaching 50% protection. Contrast brachyspira_hampsonii in this batch at level 3, where no vaccine exists at all.
Levels and evidence are generated from data/assignments/brachyspira_hyodysenteriae.yml; the overview is authored in data/overviews/brachyspira_hyodysenteriae.md. Do not hand-edit this page — corrections go in data/assignments/CORRECTIONS.yml.
Quoted material marked Basis is from Zimmerman JJ, Karriker LA, Ramirez A, Schwartz KJ, Stevenson GW, Zhang J, eds. Diseases of Swine, 12th edition. Hoboken: Wiley Blackwell, 2025 — except where another source is named in the quotation itself.