Clostridial histotoxic infections
LEVELS: Highly unlikely; No human illness; Already in the herd; Routine; Minor; Negligible; Some; Available, or not needed
| Register id | clostridial_histotoxic |
| Type | bacteria |
| Scientific name | — |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
This entry covers the histotoxic clostridia as one group — Clostridium chauvoei, C. novyi and C. septicum — because although the diseases they cause differ in detail, they behave as a single risk to a pig herd. These are soil and gut organisms whose tough spores accumulate on premises that have carried livestock for many years. They do not cause disease until they find dead or oxygen-starved tissue to grow in, and then they do so explosively: gas gangrene is nearly always acute and often kills in under 24 hours. Most cases in pigs follow a wound, and contaminated injection sites are the classic trigger — outbreaks of gas gangrene in young piglets frequently follow iron injections given to prevent anaemia, with around half of affected pigs dying. Cases are sporadic rather than epidemic, they are not contagious from pig to pig, and they are not a public health concern. Because the spores are permanent residents of the environment, prevention is a matter of injection hygiene and wound management rather than of keeping the organism out.
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. ch47 47.3, opening the histotoxic section: "Histotoxic clostridial diseases of swine are not zoonotic; hence, there is no public health concern."
How this level was decided
Level 1, stated outright. This is an examined negative rather than a silence -- chapter 47 gives Public Health subsections where a public health question exists, and here it disposes of the whole group in one sentence.
C2 Zoonotic impact
Severity of human illness caused by the agent
No human illness: Agent does not cause illness in people
Basis. ch47 47.3: "Histotoxic clostridial diseases of swine are not zoonotic; hence, there is no public health concern."
How this level was decided
Level 1. C2 scores level 1 whenever the agent does not cause disease in people, and the chapter says these do not.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c3l1, agree with assessment. REality is we don't fully understand why this organism causes diseases sometimes, but not others because it seems like the agent is widespread."
Basis. ch47 47.3.1.1: C. septicum "is found in soil and feces... and is a frequent postmortem invader". "The incidence of gas gangrene is particularly high on certain premises that have had large populations of livestock for many years, suggesting that there is a buildup of spore numbers in the environment of these farms." 47.3: "a small portion caused by C. chauvoei (blackleg) or C. perfringens type A appear spontaneously and are thought to be endogenous, resulting from oral exposure, where organisms may lie dormant in various tissues until a microenvironment favorable for their growth is generated."
How this level was decided
Level 1. The chapter puts these organisms in two places a farm cannot separate them from: the faeces of its own pigs, and the tissues of its own pigs, lying dormant until an injection site or a castration wound gives them the anaerobic microenvironment they need. That is carried by the pigs themselves, so there is no introduction event to prevent. FLAGGED as a level 1 or 4 judgment: the environmental spore buildup on old livestock premises is real and disinfectant-resistant, which is the level 4 shape. Scored level 1 because the chapter says the spontaneous cases are ENDOGENOUS, and because what actually triggers disease is a wound rather than an exposure.
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. ch47 47.3.1.4: "A presumptive diagnosis is based on typical history of rapid progression and gross lesions." "Bacteria can be observed in direct smears of affected subcutis or muscle, and fluorescent-labeled antibody staining is a rapid and accurate method to positively identify the species of clostridia present." "Confirmation can also be done by immunohistochemistry for each of the Clostridium spp.... In addition, rapid identification of several clostridia involved in gas gangrene may also be attempted by PCR on bacterial isolates or DNA extracted from affected tissues." 47.3.1.3: crepitant swelling with "blotchy reddish-purple discoloration", often fatal in under 24 hours.
How this level was decided
Both halves of level 1. A crepitant, discoloured swelling at an injection or castration site with rapid death is recognised in normal practice -- the chapter treats the history and gross lesions as sufficient for a presumptive diagnosis -- and confirmation is direct smear, fluorescent antibody, immunohistochemistry or PCR, all standard veterinary diagnostic laboratory work. NOTE two cautions the chapter records, neither of which is a barrier to testing: clostridia from the gut overgrow the primary pathogen if the sample is faecally contaminated, and swarming C. septicum can look predominant when it is not.
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
Basis. ch47 47.3: "Histotoxic clostridial infections are sporadic in swine and are usually exogenous, being associated with wound infections, especially contaminated injection sites." 47.3.1.2: "High intraherd incidence of C. perfringens type A-associated gas gangrene in young piglets is often a complication that follows injection of iron-containing preparations for prevention of nutritional anemia... The case fatality rate approaches 50%." 47.3.1.3: "Gas gangrene is nearly always acute and is often fatal in less than 24 hours."
How this level was decided
Level 2 under the standing note SCORING/production_cost_impact. Sporadic is the chapter's own word for the baseline, which alone would be level 1 -- but iron injection is universal in US piglets, and the chapter records that gas gangrene following it can reach high intraherd incidence with a case fatality rate approaching 50%. That is a real and recurring exposure in modern US production rather than an artefact of older systems. It is level 2 rather than 3 on the mechanism Eric supplied for Salmonella: the losses are short-lived, because they follow a correctable break in injection hygiene rather than persisting in the herd.
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, old disease with no evidence to suggest diagnosis would impact markets"
Basis. Section 47.3 records no trade measure, movement control or consumer response, and states that these diseases are not zoonotic.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact these are agents long present in US herds and diagnosed without market consequence, and the chapter removes the one route that might have produced one by saying there is no public health concern. 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
Basis. ch47 47.3.1.5: "Prevention is preferred to treatment, given the fulminant clinical course." "Treatment with antimicrobials may be successful if applied early. Experimental prophylactic use of tetracyclines, penicillin, or chloramphenicol prevents disease in mice."
How this level was decided
Level 2, and again the label fits word for word: treatment exists but works only in some circumstances. Penicillin and tetracyclines are effective against these organisms and are labelled for swine, but the chapter says the clinical course is fulminant -- often fatal inside 24 hours -- so the treatment window is narrow and prevention is preferred. This is not level 1, because the animals do not recover acceptably without treatment, and not level 3, because an effective drug exists and works when it is in time.
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
Basis. ch47 47.3.1.5: "Immunization against toxin antigens provides lifelong immunity, and the use of vaccines may be considered on premises where the disease recurs." 47.3: histotoxic infections "are sporadic in swine".
How this level was decided
Level 1 on the first clause. Clostridial bacterin-toxoids covering these organisms are available in the US, the chapter says immunisation gives lifelong immunity, and it names the situation where they are worth using -- premises where the disease recurs. That is an available and effective vaccine deployed selectively because the disease is sporadic, which is level 1 read either way.
Levels and evidence are generated from data/assignments/clostridial_histotoxic.yml; the overview is authored in data/overviews/clostridial_histotoxic.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.