Weakly hemolytic Brachyspira spp. (brachyspiral colitis)
LEVELS: Highly unlikely; No human illness; Already in the herd; Laboratory-dependent; Minor; Negligible; Some; Available, or not needed
| Register id | brachyspira_weakly_hemolytic |
| Type | bacteria |
| Scientific name | — |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
This entry covers the weakly beta-haemolytic half of the Brachyspira genus in pigs — chiefly B. murdochii and B. intermedia — as distinct from the strongly haemolytic species that cause swine dysentery. Colonisation with them is common and largely harmless. B. murdochii is the species most often found alongside lesions of mild colitis, and experimental infection confirmed it is mildly pathogenic; some strains of B. intermedia are suspected of similar potential. Where they cause anything it is mild colitis and chronic loose faeces rather than the bloody diarrhoea of dysentery. Their real significance, as the reference chapter puts it, may be in complicating the diagnosis of the two diseases that do matter — swine dysentery and intestinal spirochetosis — both of which require rigorous and expensive control measures that no one wants to start on the strength of a misidentified commensal. Speciation therefore matters, and is done by MALDI-TOF or by PCR and sequencing rather than by biochemical testing, which cannot be relied upon. No human disease from either species is described.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c1l1, no evidence to support this agent is a meaningful zoonotic agent"
Basis. Chapter 45 gives two public health sections and neither covers these species. 45.2.3 disposes of the agents of swine dysentery -- "None of the agents of SD are known to infect humans" -- and 45.3.3 is entirely about B. pilosicoli, which is a different register entry. B. murdochii and B. intermedia are mentioned nowhere in either.
How this level was decided
INFERRED FROM A STRUCTURED SILENCE, and this chapter makes it unusually legible: it writes a public health section for each disease group and names the human question explicitly in both. Where a species has a human story the chapter tells it, at length in the case of B. pilosicoli. These two get no mention in either section. Level 1. NOTE ON WHAT THIS ENTRY IS: the register unit is the weakly haemolytic Brachyspira spp. OTHER than B. pilosicoli -- its members are Brachyspira murdochii and Brachyspira intermedia. B. pilosicoli is the separate entry brachyspira_pilosicoli, and B. innocens is a non-pathogenic commensal the chapter excludes from the pathogenic group.
C2 Zoonotic impact
Severity of human illness caused by the agent
No human illness: Agent does not cause illness in people
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c2l1, no evidence this agent is associated with human disease"
Basis. No human disease from B. murdochii or B. intermedia is described anywhere in chapter 45.
How this level was decided
INFERRED, on the same structured silence. C2 scores level 1 wherever pigs are not a source of human illness with the agent.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Already in the herd: Present in most herds, or carried by pigs themselves, so there is no introduction event to prevent
Basis. ch45 45.3.1: "Infections or colonization with one or more of the nonpathogenic or mildly pathogenic Brachyspira spp. are quite common in pigs." 45.1.2: seven Brachyspira spp. colonise swine, and "some strains of Brachyspira murdochii and Brachyspira intermedia occasionally are associated with colitis in swine", the remainder of the genus in pigs being the commensal B. innocens. 45.3.1: "Their greatest significance may be in complicating diagnosis of SD or PIS/PCS."
How this level was decided
Level 1. These are organisms pigs carry: the chapter groups them with the non-pathogenic commensals, says colonisation is quite common, and describes them chiefly as background flora that confuses the diagnosis of the two diseases that matter. There is no introduction event to prevent because they are already there. This is the same case as streptococcus_suis and the other biome organisms, which is what level 1 was rewritten for.
C4 Detection difficulty
Difficulty of recognizing and confirming infection
Laboratory-dependent: The presentation does not point to this disease specifically, but local or regional laboratories can confirm it with a validated assay once it is suspected
Basis. ch45 45.3.1: "B. murdochii is the most frequently reported species associated with lesions of mild colitis, and experimental infection of pigs with a B. murdochii strain confirmed that it was mildly pathogenic." 45.1.2: "Biochemical testing is now rarely undertaken in diagnostic laboratories, and no tests can be completely relied upon for identification." 45.2.8: speciation is by MALDI-TOF, which "is becoming standard practice in veterinary diagnostic laboratories", or by PCR and sequencing of 16S rRNA, 23S rRNA, nox or cpn60 targets.
How this level was decided
Level 2. Nothing points here clinically -- mild colitis is the least specific presentation in a chapter whose differential list runs to eight other diseases, and the chapter's own framing is that these organisms turn up while looking for something else. But once a spirochaete is cultured, speciating it is routine regional laboratory work by MALDI-TOF or PCR. NOTE the caution the chapter records about the older route: phenotypic and biochemical identification is unreliable and is no longer done, which is why the molecular and mass-spectrometric methods matter.
C5 Production cost
Financial impact on the infected farm's cost of production
Minor: Small and generally short-lived losses with little effect on overall cost of production
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c5l2, agree with assessment"
Basis. ch45 45.3.1: "Occasionally other weakly hemolytic Brachyspira spp. have been implicated in causing colitis and chronic diarrhea in pigs. Some strains of B. intermedia have been suspected to have pathogenic potential in pigs. B. murdochii is the most frequently reported species associated with lesions of mild colitis, and experimental infection of pigs with a B. murdochii strain confirmed that it was mildly pathogenic." "Their greatest significance may be in complicating diagnosis of SD or PIS/PCS, both of which are more economically important diseases that require rigorous control measures."
How this level was decided
Level 2. There is a real if small cost on the record: mild colitis and chronic diarrhoea, occasionally, and B. murdochii confirmed as mildly pathogenic in experimental infection rather than merely associated. Against that the chapter is explicit that the two diseases which matter economically are the other two, and that the main significance of these species is diagnostic confusion. Small and generally short-lived losses. FLAGGED as a level 1 or 2 judgment: an organism whose chief effect is to mislead a diagnostician has arguably no measurable effect on cost of production at all, and level 1 is defensible.
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, routinely identified in US pigs with no market effect"
Basis. Section 45.3 records no trade measure, movement control or consumer response for the weakly haemolytic Brachyspira spp., which it describes as commonly colonising pigs.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact, organisms that commonly colonise US pigs and are routinely cultured from them without consequence have no route to pork supply or demand. Level 1.
C7 Treatment potential
Potential for treatment to improve outcomes
Some: Treatment exists but is inconsistent, requires extralabel use, or works only in some circumstances
INFERRED — reasoned, not stated. The evidence implies this level rather than stating it. The reasoning is below.
Corrected by Eric from L1 Little. Reason: "c7l2, I'm inclined to upgrade this to L2 because there are no labelled treatments to my knowledge though probably a fair bit of extralabel treatment happening in the field. The clinical effect, if any, is poorly understood so in presence of diarrhea, and only identification of this organism, people tend to treat"
Basis. Chapter 45 describes no treatment aimed at B. murdochii or B. intermedia. Its two treatment sections address the agents of swine dysentery (45.2.10.1) and B. pilosicoli (45.3.10.1). 45.3.1: these species are grouped with "the nonpathogenic or mildly pathogenic Brachyspira spp." whose "greatest significance may be in complicating diagnosis".
How this level was decided
INFERRED. Under the standing note SCORING/treatment_potential, level 1 covers the case where there is no imperative to treat, and that is this one: the chapter never proposes treating these organisms because they are largely what a healthy pig carries, and the one confirmed effect is mild. The drugs used for the genus would work if anyone chose to use them, so this is not a therapeutic gap. FLAGGED as arguable at level 2 on the ground that any such use would be extralabel.
C8 Vaccine availability
Availability of effective vaccines or bacterins
Available, or not needed: Effective vaccines are widely available in the US or secured for national outbreak response, or the disease does not clinically or economically justify vaccinating
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c8l1, need further evidence on clinical signficance before vaccine development is warranted"
Basis. No vaccine for B. murdochii or B. intermedia is described in chapter 45, which discusses vaccination only for B. hyodysenteriae and B. pilosicoli.
How this level was decided
INFERRED. No vaccine exists, and under the reworded criterion the question is whether one is warranted. Mildly pathogenic organisms that commonly colonise healthy pigs do not clinically or economically justify vaccinating. Level 1 on the second clause.
Levels and evidence are generated from data/assignments/brachyspira_weakly_hemolytic.yml; the overview is authored in data/overviews/brachyspira_weakly_hemolytic.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.