Actinobacillus suis
LEVELS: Highly unlikely; No human illness; Already in the herd; Laboratory-dependent; Minor; Negligible; Some; Available but inconsistent
| Register id | actinobacillus_suis |
| Type | bacteria |
| Scientific name | Actinobacillus suis |
| NCBI taxid | 716 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Actinobacillus suis is a gram-negative bacterium that lives harmlessly in the upper respiratory tract of most pigs and only occasionally causes disease. A Canadian survey found 94% of herds were infected without a single clinical case. It is best understood as an opportunist: when it does cause disease it takes one of three forms — sudden death from septicaemia in suckling and newly weaned piglets, a haemorrhagic pneumonia in growers and finishers that looks very like APP, or septicaemia in adults with red diamond-shaped skin lesions easily mistaken for erysipelas. It produces the same family of Apx toxins as APP but at lower levels, which probably explains why it is the milder organism of the two. Counter-intuitively, clinical disease is most often seen in high-health herds and in newly populated units, where pigs have had little previous exposure and so little immunity. Because every one of its presentations mimics a more common disease, it is usually identified only when a laboratory cultures it.
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 high quality evidence to suggest this is a zoonotic agent"
Basis. ch43 gives A. suis NO Public Health section, in a chapter that gives APP one (43.1.3) in order to say there is no risk. The only human sentence in the chapter is in 43.3, on the other Actinobacillus species: "A CASE OF HUMAN INFECTION BY A. SUIS OR A. EQUULI HAS BEEN REPORTED RECENTLY IN ITALY (Montagnani et al. 2015)." Nothing states where that person acquired it, whether they had pig contact, or which of the two organisms it was. 43.2 adds that "occasionally, A. suis can be isolated from species other than swine (Ishihara et al. 2022)."
How this level was decided
Level 1, and FLAGGED because it is the pig-attribution question you have ruled on twice and this is a third instance of it. THE FACTS: a single human infection with A. suis OR A. equuli has been reported, once, in Italy, with no source given. THE RULE APPLIED: your Clostridium perfringens type C ruling of 2026-09-01 -- "there are conditions in humans and neonates that seem to have pathology (and some bacteriological evidence) that CPC is involved but I AM NOT REALLY AWARE OF ANY LINK THAT ATTRIBUTES PIGS AS THE SOURCE" -- scored level 1 on exactly this shape of evidence, and C1 was rescoped to transmission FROM PIGS on the chikungunya ruling. Human disease with the organism is on record; pig attribution is not. THE ARGUMENT FOR LEVEL 2 is that A. suis colonises 94% of herds, so anyone in a barn is exposed daily, which makes pig-source infection plausible-but-rarely-reported -- the level 2 label almost word for word. You may prefer that reading. Level 1 is scored because nothing in the folder ties the Italian case to a pig, and level 2 would assert a route nobody has demonstrated.
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, not a zoonotic agent"
Basis. ch43 describes no human illness with A. suis anywhere. The single Italian case (43.3) is reported as a fact of isolation with no clinical description, no outcome and no attribution.
How this level was decided
RE-SCORED 2026-09-04. C2 now grades the severity of the illness the agent causes in a person, independent of how the person was exposed -- the 2026-09-01 pig-attribution reading is reversed, and C1 alone carries the source question. FLAG, and this is the case the previous assessment predicted: "NOTE WHAT HAPPENS IF C1 MOVES: this cell cannot simply move to level 2, because the folder says nothing at all" about the severity of human A. suis infection. Human cases exist in the literature. The folder cannot grade them, so level 1 is held rather than a level invented.
PREVIOUS ANSWER, written under the pig-attribution reading: Level 1, FOLLOWING C1 AND FLAGGED WITH IT. Under C2 as rescoped, level 1 says pigs are not a source of human illness with this agent, which is the position C1 takes. NOTE WHAT HAPPENS IF C1 MOVES: this cell cannot simply move to level 2, because the folder says nothing at all about how severe the Italian infection was -- it would become a null rather than a mild. That is worth knowing before ruling C1.
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. ch43 43.2: "The gram-negative bacterium A. suis is A UBIQUITOUS OPPORTUNISTIC PATHOGEN THAT COLONIZES THE UPPER RESPIRATORY TRACT OF PIGS (MacInnes and Desrosiers 1999). In a Canadian survey, MacInnes et al. (2008) showed that AS MANY AS 94% OF THE TESTED HERDS WERE PROBABLY INFECTED BY A. SUIS, although no clinical cases were observed." On where disease appears: "disease can also be observed in fattening pigs and even adult animals, ESPECIALLY IN HIGH HEALTH STATUS HERDS (Yaeger 1996). Prevalence of disease may be greater in NEWLY POPULATED HERDS BEFORE ANIMALS DEVELOP IMMUNITY."
How this level was decided
Level 1. Ninety-four percent of herds carry it and the pigs themselves are the reservoir, so there is no introduction event to prevent -- that is the level 1 label. NOTE THE INVERSION THE CHAPTER RECORDS, because it is counter-intuitive and worth carrying into a brief: THE HERDS THAT GET A. SUIS DISEASE ARE THE HIGH-HEALTH ONES. Excluding it is what leaves a naive population for it to hurt when it arrives. That does not lift the level -- C3 asks about the effort to keep it out and for the industry as it stands the answer is that it is already in -- but it is why an SPF herd manager cares about an organism that is otherwise universal.
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. ch43 43.2 on recognition, and the chapter is explicit that nothing points at this organism. The septicaemic form: "Very often, PIGS ARE SIMPLY FOUND DEAD... DIFFERENTIAL DIAGNOSIS SHOULD INCLUDE OTHER AGENTS OF SEPTICEMIA, SUCH AS STREPTOCOCCUS SUIS AND KLEBSIELLA PNEUMONIAE." The respiratory form: "DIFFERENTIAL DIAGNOSIS SHOULD INCLUDE MAINLY PORCINE PLEUROPNEUMONIA CAUSED BY APP." The adult form: "red, rhomboid skin lesions RESEMBLING ERYSIPELAS... THIS FORM, PARTICULARLY WHEN SKIN LESIONS ARE PRESENT, MAY BE CONFUSED WITH ERYSIPELAS." On confirmation: "Preliminary diagnosis is based on typical clinical signs and gross lesions. DIAGNOSIS IS CONFIRMED BY CULTURE AND IDENTIFICATION OF A. SUIS FROM TISSUES with typical microscopic lesions. A strain-specific ELISA test has been shown to be useful to follow up maternal and active antibodies in an infected herd (Lapointe et al. 2001)." The laboratory trap is named twice: "BIOTYPE II (NAD-INDEPENDENT) ISOLATES MIGHT BE MISIDENTIFIED AS A. SUIS" (43.1.9.1), and conversely "A. suis isolates may present cross-reactions with rabbit antisera against serotypes 3, 6, or 8 of APP, and WITHOUT A CORRECT IDENTIFICATION, THESE ISOLATES CAN BE ERRONEOUSLY REPORTED AS APP."
How this level was decided
Level 2. The presentation does not point to A. suis specifically -- the chapter lists three clinical forms and gives each a different disease it gets mistaken for, and sudden death is often the only sign -- so this is not level 1. But once it is suspected, an ordinary diagnostic laboratory confirms it by culture and identification from lesioned tissue, which is the level 2 label. NOTE THE CAVEAT THAT MATTERS FOR THE REGISTER RATHER THAN FOR THE LEVEL: A. suis and APP are misidentified as each other in both directions, so historical prevalence figures for either organism carry that error.
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, some suggestion from the field and laboratories that this agent is re-emerging but the evidence as it stands today is that there is only minor clinical disease associated with infection"
Basis. ch43 43.2 gives no economic figure of any kind. What it gives is the clinical picture and the prevalence: 94% of Canadian herds infected with "NO CLINICAL CASES OBSERVED"; disease "originally reported as causing septicemia and death in suckling and recently weaned pigs only", now also in fatteners and adults "ESPECIALLY IN HIGH HEALTH STATUS HERDS"; "prevalence of disease may be greater in NEWLY POPULATED HERDS BEFORE ANIMALS DEVELOP IMMUNITY"; and "A. suis produces LOWER LEVELS OF APX TOXINS THAN DOES APP, WHICH MAY EXPLAIN WHY A. SUIS IS GENERALLY LESS VIRULENT." The severe end is real where it occurs -- sudden death, septicaemia with haemorrhage in multiple organs, abortion in sows.
How this level was decided
INFERRED, AND FLAGGED, because the chapter carries no economic data at all and this is a cell your field experience settles better than the text does. Level 2 is scored on the shape of the disease as described: ubiquitous, usually silent, and when it does appear it is sporadic individual losses rather than a herd-level event, which is the level 2 label -- small and generally short-lived losses with little effect on overall cost of production. It is scored level with Actinobaculum suis and the colibacilloses. THE ARGUMENT FOR LEVEL 3 is that the losses concentrate in high-health and newly populated herds, which are the expensive ones, and sudden death in suckers plus sow abortion in a start-up unit is not a trivial event for that farm. If A. suis costs more in US production than this reads, say so and it moves.
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 market effect"
Basis. A. suis is ubiquitous in US and Canadian herds -- 94% infected in the MacInnes survey (43.2) -- and no trade measure, movement restriction or market response to an A. suis diagnosis is recorded anywhere in chapter 43.
How this level was decided
INFERRED. Level 1 under the standing note SCORING/market_impact: this is an agent already endemic and routinely diagnosed in US pigs, and no market move has ever followed, so the absence is an observation rather than an assumption.
C7 Treatment potential
Potential for treatment to improve outcomes
Some: Treatment exists but is inconsistent, requires extralabel use, or works only in some circumstances
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c7l2, effective treatment is available but I believe use is extralabel"
Basis. ch43 43.2: "ANTIBIOTIC TREATMENT SHOULD BE APPLIED EARLY IN THE APPEARANCE OF SIGNS; CEFTIOFUR, GENTAMICIN, AND TRIMETHOPRIM/SULFADIAZINE SEEM TO BE THE ANTIBIOTICS OF CHOICE, followed by ampicillin, sulfadimethoxine, and tiamulin (MacInnes and Desrosiers 1999). THERE ARE ALMOST NO DATA IN THE LITERATURE SUGGESTING RESISTANCE of numerous A. suis strains to different antimicrobials and NO CLEAR BREAKPOINTS ARE AVAILABLE (Hayer et al. 2020)." The obstacle to treating is in the clinical description rather than the pharmacology: in the septicaemic form "VERY OFTEN, PIGS ARE SIMPLY FOUND DEAD", and in the respiratory form "SUDDEN DEATH MAY ALSO BE THE SOLE SIGN". THE FOLDER CONTAINS NO US LABEL INFORMATION FOR ANY PRODUCT.
How this level was decided
FLAGGED FOR YOUR RULING, and this is the case your 2026-09-02 note said would arrive: "this matters most for the multi-species entries still to come in the Actinobacillus, Mycoplasma and Salmonella chapters... WHERE THE LABEL WORDING IS BROAD, ABSENT FROM THE FOLDER, OR AMBIGUOUS ABOUT THE SPECIES, DO NOT GUESS -- flag the cell and ask." THE QUESTION IS NARROW: does any US swine label name Actinobacillus suis, or do the respiratory labels name only Actinobacillus pleuropneumoniae? My reading is that ceftiofur and the other swine respiratory labels name APP and not A. suis, which would make treatment of A. suis extralabel and the level 2. LEVEL 2 IS SCORED PROVISIONALLY on that reading, and it is reinforced independently of the label by the second clause of the level 2 label -- treatment "works only in some circumstances" -- since the chapter says drugs must be given early in the appearance of signs and that the commonest sign is a dead pig. If a label does name A. suis, the label test would argue level 1 and I would still resist it on the sudden-death ground; tell me which way you read it.
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. ch43 43.2, the whole of what the chapter says on vaccination: "VACCINATION HAS BEEN IMPLEMENTED IN SOME HERDS USING AUTOGENOUS VACCINES WITH VARIABLE RESULTS. In one of these herds, gilts and sows were vaccinated, although these animals ALREADY PRESENTED HIGH TITERS OF ANTIBODIES BEFORE VACCINATION (Lapointe et al. 2001)."
How this level was decided
Level 2, and the chapter supplies the level label almost verbatim. Autogenous vaccines exist and are used in the US -- autogenous production is exactly what a ubiquitous farm-specific opportunist gets -- and results are stated to be variable. That is "available but inconsistent". It is not level 3, because a product exists and is used; it is not the "not needed" half of level 1, because herds are in fact paying to make and give it. The Lapointe observation is worth noting for anyone reading the brief: the vaccinated animals were already seropositive, so what that programme added is unclear.
Levels and evidence are generated from data/assignments/actinobacillus_suis.yml; the overview is authored in data/overviews/actinobacillus_suis.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.