Listeria monocytogenes
LEVELS: Sometimes occurs; Severe; Already in the herd; Laboratory-dependent; Negligible; Negligible; Some; Available, or not needed
| Register id | listeria_monocytogenes |
| Type | bacteria |
| Scientific name | Listeria monocytogenes |
| NCBI taxid | 1639 |
| Evidence | 2 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Listeria monocytogenes is carried by most food animals and is common in pigs without causing them any trouble: roughly 10% of slaughter pigs carry it in the intestine and tonsils, and the rate rises where liquid feed or silage is used. Clinical listeriosis in pigs is uncommon — the disease belongs mainly to cattle, sheep and goats — and the few swine reports are septicaemia in nursing piglets and one outbreak in fatteners traced to poor-quality silage. Nothing about the clinical picture is specific, and pseudorabies in particular can look very similar in neonates, so diagnosis depends on culture. The organism's real significance is in food: it survives and multiplies at refrigeration temperature, up to 30% of raw minced pork may be contaminated through cross-contamination during processing, and some strains persist in slaughterhouses because they resist disinfectants. Human listeriosis is uncommon but serious, particularly in pregnancy and in the immunocompromised. Pasteurisation and thorough cooking destroy it.
C1 Zoonotic potential
Likelihood of transmission from pigs to humans
Sometimes occurs: Known risk of transmission to humans (foodborne or occupational), but requires failure of one or more control points for transmission to humans
Basis. "the organism is an uncommon cause of serious foodborne disease in humans." "While the organism is present in the environment, food, particularly meat products, are an important source." "Contamination increases from farm to raw meat because of cross-contamination and multiplication of the organism. Up to 30% of raw minced pork meat may be contaminated." "Some strains have been shown to persist in swine slaughterhouses, and persistence was associated with resistance to disinfectants." "L. monocytogenes is destroyed by pasteurization, but food recontamination can occur after heating." (ch58 58.8.1, 58.8.2)
How this level was decided
Pork is named as an important source, carriage moves from farm to carcass to retail meat, and up to 30% of raw minced pork carries the organism. The control points are explicit -- slaughter hygiene, plant disinfection, pasteurization and post-heating handling -- and transmission follows when they fail, which is level 3. NOTE for Eric: level 4 is arguable here in a way it is not for Salmonella, because two of those control points leak by design. The organism "grows at temperatures as low as -1 C", so refrigeration does not hold it, and slaughterhouse strains persist because they resist the disinfectants used against them. If you read those as transmission occurring despite intact control points, this is level 4.
C2 Zoonotic impact
Severity of human illness caused by the agent
Severe: Infection carries a substantial risk of life-threatening illness, death, or permanent disability, even if human infections are uncommon
Basis. "Listeria monocytogenes is a cause of septicemia, abortion, and meningitis in humans. Pregnant women, newborn infants, the elderly, and the immunosuppressed are most at risk." (ch58 58.8.1)
How this level was decided
Septicaemia, meningitis and abortion are the characteristic presentations, not rare extremes -- listeriosis is defined by them. Each carries a substantial risk of death or permanent disability, and the named risk groups are the ones in which it is most often fatal. Level 4.
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. "Listeria monocytogenes occurs worldwide and is carried by most food animal species, and clinical disease occasionally develops." "Carriage of L. monocytogenes is common in pigs in the intestine and tonsils. Studies in various countries suggest that about 10% of slaughter pigs are carriers." "The carriage rate seems to increase when liquid feed or silage is fed." "The organism can be detected in manure for at least 55 days." "While the organism is present in the environment" (ch58 58.8.1, 58.8.2)
How this level was decided
MOVED L5 -> L1 on 2026-09-03, when C3 was reduced to four levels and the whole level 5 block was re-read against the exclusion test rather than the reservoir test. Eric: "C3 is about exclusion, not reservoirs per se, though the two are related", and "who cares about reservoirs? Reservoirs really don't matter as long as you keep domestic pigs from becoming infected through biosecurity implementation." THE PREVIOUS LEVEL WAS NEVER JUDGED: this entry carried a RENUMBERED L4 -> L5 stamp from 2026-09-01, meaning it was moved past the newly created housing level rather than assessed against it. carriage "is common in pigs in the intestine and tonsils", with "about 10% of slaughter pigs" carriers and the rate rising on liquid feed. Carried by the pigs themselves, which is the level 1 label.
EARLIER REASONING, kept because it is the evidence read: RENUMBERED L4 -> L5 on 2026-09-01, when C3 went from four levels to five. The argument below is unchanged and so is the judgment. What changed is the scale it sits on. C3 now runs on the EFFORT REQUIRED TO EXCLUDE: L1 already in the herd, L2 housing alone keeps it out, L3 routine biosecurity keeps it out, L4 extraordinary biosecurity required, L5 exclusion may not be achievable. The reservoir sits outside the pig industry on two counts: the environment itself, where the organism persists and from which silage and liquid feed carry it in, and most other food animal species. Neither is closed by herd or transport biosecurity, and feed is a route C3 names specifically. Level 4.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "l2, straightforward to diagnose though disease is uncommon in pigs"
Basis. "Subclinical infection is common; however, clinical listeriosis is uncommon." "Clinical signs and lesions are not specific to listeriosis in any of its various forms. Other causes of each clinical syndrome must be included in a differential diagnosis. Of particular concern is pseudorabies virus that can cause rapid death of neonates and produce multifocal hepatic and splenic necrosis similar to listeriosis." "The organism is present in clinical specimens in high numbers and produces typical hemolytic colonies on blood agar." "For contaminated specimens... it is preferable to use cold enrichment at 4 C overnight in an enrichment medium... followed by subculture onto a chromogenic selective medium." (ch58 58.8.3)
How this level was decided
The chapter says plainly that no form of listeriosis is clinically specific and names pseudorabies as a look-alike, so level 1 is out. Confirmation is blood agar culture with cold enrichment and chromogenic media, plus PCR and MALDI-TOF -- all standard regional veterinary diagnostic laboratory work. Not recognised clinically but locally confirmable is level 2.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. "this should be c5l1, the literature is citing the exception rather than the rule. Clinical disease is essentially never seen in domestic pigs."
Basis. CFSPH 2019: "Clinical cases are seen most often in cattle, sheep and goats", with pigs named far down a list of other affected species. The only swine-specific reports are septicaemia in nursing piglets and one outbreak in fattening pigs traced to poor-quality silage that also contained mycotoxins. ch58 58.8.3: "Subclinical infection is common; however, clinical listeriosis is uncommon."
How this level was decided
MOVED L2 -> L1, agreeing with Eric's correction. He ruled: "the literature is citing the exception rather than the rule. Clinical disease is essentially never seen in domestic pigs." The factsheet supports him -- pigs are a minor host, and the one fattening-pig outbreak in the literature is a silage-quality case report rather than a production norm. Under the standing note SCORING/production_cost_impact this is scored on modern US production, where the disease does not occur at measurable cost.
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. "this should be c6l1, there is no historical evidence that would suggest the market would react to a diagnosis of this disease in pigs"
Basis. CFSPH 2019 treats Listeria as a food-processing and ready-to-eat product problem rather than a live-pig one, and records no movement control, export restriction or trade measure arising from a swine diagnosis. Eric, 2026-08-27: "there is no historical evidence that would suggest the market would react to a diagnosis of this disease in pigs."
How this level was decided
MOVED L2 -> L1 under the standing note SCORING/market_impact. Listeria is endemic, carried by about 10% of slaughter pigs, and has been throughout the period pork has traded normally. My L2 scored a product recall, but a recall is a processing event and the criterion measures disruption following disease occurring ON FARMS. No US swine listeriosis diagnosis has ever moved the market.
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. "The organism is sensitive to several antimicrobials, including penicillins and aminoglycosides, and affected pigs may recover if treated early in the course of the disease. However, in vitro multidrug resistance to antimicrobials, including fluoroquinolones and macrolides, has been observed in some isolates collected at slaughter (Rugna et al. 2021)." "Paralyzed pigs should be humanely euthanized." (ch58 58.8.4)
How this level was decided
Effective drugs exist and are named, so not level 3. But recovery is conditioned on treating early in the course, multidrug-resistant isolates are recorded, and the chapter still directs that paralysed pigs be euthanised rather than treated. Treatment that works only in some circumstances is level 2.
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 AT LEVEL 1. "c8l3 is correct, but there is currently no market incentive to develop or use a vaccine even if it were available" Eric, 2026-08-28: "all seven to L1". Re-ruled after the C8 rewording of the same day. His 2026-08-27 words are unchanged and are quoted above in full -- what has moved is the level they map to. He wrote each of them to say that no vaccine is NEEDED here, and level 3 was then the only place a missing vaccine could go, so the reasoning and the level contradicted each other. The reworded level 1 holds exactly what he was describing: no vaccine exists, and the disease does not justify one. Recorded as a confirmation of the reworded pass rather than an override, because the pass now agrees with him.
Basis. ch58 58.8.4 names antimicrobials, euthanasia and manure composting, and no vaccine. Eric, 2026-08-27: "there is currently no market incentive to develop or use a vaccine even if it were available".
How this level was decided
MOVED L3 -> L1 by the rewording. He also corrected C5 to level 1 on the ground that "clinical disease is essentially never seen in domestic pigs". A vaccine against a disease pigs essentially do not get is not an unmet need. NOTE: the human burden is real and severe, C2 level 4, but that is carried by the zoonotic criteria and the control point is the plant, not the pig.
Levels and evidence are generated from data/assignments/listeria_monocytogenes.yml; the overview is authored in data/overviews/listeria_monocytogenes.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.