Staphylococcus aureus

LEVELS: Likely to occur; Moderate; Already in the herd; Laboratory-dependent; Minor; Negligible; Some; Available, or not needed


Register id	`staphylococcus_aureus`
Type	bacteria
Scientific name	Staphylococcus aureus
NCBI taxid	1280
Evidence	2 document(s)
Assigned	2026-09-04, against criteria version `093366e352a2`

Overview

Staphylococcus aureus is common on the skin of healthy pigs and throughout pig buildings, and it is only an occasional pathogen. When it does cause disease it does so in individuals rather than groups: abscesses, septicaemia, mastitis, vaginitis and metritis, bone and joint infection, and endocarditis. Abscesses in skin, bone, joints and organs found at slaughter are the usual reason it comes to notice. Its real importance is a public health one. Pigs are a reservoir for livestock-associated MRSA, chiefly the sequence types ST398, ST9 and ST5, and nasal or skin colonisation without any illness is common. People who work with pigs carry LA-MRSA in the nose considerably more often than the general population, although these livestock-adapted strains appear less virulent in humans than the hospital-associated ones. It is a One Health surveillance issue rather than a herd health problem.

C1 Zoonotic potential

Likelihood of transmission from pigs to humans

Likely to occur: Transmission from pigs can occur even when control points are intact, or is prevented only by extraordinary control measures such as respiratory protection or vector control

CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.

Confirmed by Eric. CONFIRMED. "c1l4, there are important strain differences between human and pig isolates and the epidemiology is not completely clear but I think it is safe to say farm workers are probably getting colonized"

Basis. SCOPE OF THE ENTRY, which decides how the MRSA material is read: this is the SPECIES, not the resistance phenotype. Eric, 2026-08-18, renaming it off "Abscesses/MRSA": "IGNORE ANY REFERENCE TO ABCSESS OR MRSA IN THE NAMING -- THIS IS JUST PART OF THE NARRATIVE FOR S. AUREUS". So MRSA is evidence about this organism, not a separate agent. ch56 56.3.2, a Public Health section of its own: "SWINE HAVE BEEN IDENTIFIED AS A RESERVOIR FOR METHICILLIN-RESISTANT S. AUREUS (MRSA)... nasal and skin colonization without clinical infection in pigs is COMMON... PEOPLE WHO SPEND CONSIDERABLE TIME IN CONTACT WITH INFECTED PIGS ARE AT AN INCREASED RISK FOR TESTING POSITIVE FOR LA-MRSA FROM NASAL SWABS RELATIVE TO THE GENERAL POPULATION (Smith et al. 2009; Voss et al. 2005). IN ONE STUDY, ISOLATES FROM PIGS AND EMPLOYEES FROM THE SAME FARM WERE INDISTINGUISHABLE, SUGGESTING TRANSMISSION BETWEEN PIGS AND HUMANS ON FARM (Khanna et al. 2008)." cfsph_2016_mrsa puts numbers on it, including US ones: general population carriage "often estimated to be < 1% to 5%"; against that, "one U.S. STUDY DETECTED MRSA IN 64% OF SWINE WORKERS on a farm colonized with CC398, but no colonized workers (or pigs) on another farm. A larger study by this group found that APPROXIMATELY 21% OF SWINE WORKERS CARRIED MRSA... In North Carolina, <5% of people who worked in industrial and antibiotic-free swine or poultry operations carried MRSA. ANOTHER STUDY FOUND NO EVIDENCE FOR INCREASED MRSA CARRIAGE in people with livestock contact in Iowa." European farm-worker rates "range from < 5% to 44%, and are exceptionally as high as 72-100%". On what stops it: "A FEW STUDIES HAVE SUGGESTED THAT FACE MASKS REDUCE THE RISK OF COLONIZATION WHEN WORKING WITH LIVESTOCK, COMPARED TO GLOVES ALONE", and "PEOPLE WHO WORK WITH CC398-COLONIZED LIVESTOCK OFTEN BECOME RECOLONIZED FROM THIS SOURCE." The Netherlands treats it as a standing occupational category: "High risk patients, INCLUDING PEOPLE WHO WORK WITH PIGS or veal calves, ARE ISOLATED [on hospital admission] until the screening test demonstrates that they are MRSA-free."

How this level was decided

Level 4, and FLAGGED -- not because the evidence is thin but because only influenza A virus and Japanese encephalitis virus currently sit at this level, and you warned two batches ago that "THE LITERATURE OVERRATES THE POTENTIAL" on zoonoses where human data is scarce. Here the human data is not scarce. BOTH LEVEL 4 CLAUSES FIRE INDEPENDENTLY. Transmission occurs with ordinary controls intact -- coveralls, boots and gloves are worn in every barn and swine workers still carry the organism at many times the background rate, with pig and worker isolates indistinguishable on the same farm. And the label's named example of an extraordinary measure IS the measure: face masks reduce colonisation compared with gloves alone, which is respiratory protection in terms. THE HONEST WEAKNESS, STATED RATHER THAN BURIED: the US studies do not agree. One found 64% of workers on a colonised farm and none on another; a larger one found 21% overall but 87% of positives came from four farms; North Carolina found under 5%; an Iowa study found no increase at all. What that scatter means is that colonisation follows the herd's own MRSA status, not that transmission is rare where the herd carries it. IF YOU READ THE US PICTURE AS PATCHY ENOUGH TO BE LEVEL 3, the argument would be that a control point does exist -- keeping the herd MRSA-free -- and this then looks like the occupational-risk-with-a-control-point shape of level 3.

C2 Zoonotic impact

Severity of human illness caused by the agent

Moderate: Clinically important illness is common and may require medical treatment or hospitalization, but death or permanent disability is uncommon

CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.

Confirmed by Eric. CONFIRMED. "c2l3, not all S.aureus are drug resistant so need to avoid the temptation to put this as l4. l3 is suitable for a more general risk rating"

Basis. SCOPE OF THE ENTRY, which decides how the MRSA material is read: this is the SPECIES, not the resistance phenotype. Eric, 2026-08-18, renaming it off "Abscesses/MRSA": "IGNORE ANY REFERENCE TO ABCSESS OR MRSA IN THE NAMING -- THIS IS JUST PART OF THE NARRATIVE FOR S. AUREUS". So MRSA is evidence about this organism, not a separate agent. cfsph_2016_mrsa on what the pig lineage actually does in people: "CC398 HAS MAINLY BEEN FOUND IN SUPERFICIAL SKIN AND SOFT TISSUE INFECTIONS, BUT SOME CASE REPORTS DESCRIBE CONDITIONS SUCH AS AGGRESSIVE WOUND INFECTION, NECROTIZING FASCIITIS, DESTRUCTIVE BONE AND JOINT INFECTIONS, SINUSITIS, ENDOCARDITIS, NOSOCOMIAL BACTEREMIA, PNEUMONIA, AND SEVERE INVASIVE INFECTION WITH MULTIORGAN FAILURE." On virulence relative to human strains: "WHILE CC398 CAN CAUSE SEVERE ILLNESS, SOME STUDIES HAVE SUGGESTED THAT THIS LINEAGE MIGHT BE LESS VIRULENT than lineages adapted to humans. An alternative explanation is that most people colonized with CC398 are generally younger and in better health." ch56 56.3.2 says the same from the pig side: the livestock strains "APPEAR TO BE LESS VIRULENT AND INFREQUENTLY CAUSE DISEASE IN COLONIZED PEOPLE compared with other human MRSA strains... attributed to an adaption to pigs leading to a reduction in pathogenicity in humans, such as a REDUCED CAPACITY FOR ADHERENCE TO HUMAN SKIN and the NEAR ABSENCE OF HUMAN-SPECIFIC S. AUREUS VIRULENCE FACTORS." Both documents also record that most carriage is asymptomatic and often transient.

How this level was decided

Level 3, and FLAGGED as a level 3 or 4 judgment. WHAT MAKES IT 3: the typical outcome of a pig-acquired CC398 infection is a skin or soft tissue infection needing medical treatment, both documents state that the pig-adapted lineages are less virulent in people and rarely cause disease in those they colonise, and the mechanism is given -- these strains have lost the human-specific virulence factors. Clinically important illness requiring treatment, with death uncommon, is the level 3 label. WHAT ARGUES FOR 4: necrotizing fasciitis, endocarditis and severe invasive infection with multiorgan failure are all on the record for CC398, and the label asks only for a SUBSTANTIAL RISK of life-threatening illness EVEN IF HUMAN INFECTIONS ARE UNCOMMON -- and methicillin resistance is itself a mortality multiplier, since ch56 says MRSA strains are "of great concern given their POOR RESPONSE TO TREATMENT AND CONSEQUENT HIGHER MORTALITY RATES". I have taken 3 because those severe outcomes are case reports against a background of mainly superficial disease, and because the folder twice says this lineage is the less virulent one. It is a close call and it is yours.

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

How this level was decided

Level 1. The organism is on the skin of healthy pigs and throughout the buildings they live in, so there is no introduction event to prevent -- the level 1 label. NOTE THE DISTINCTION THE ENTRY SCOPE FORCES, because it would change the answer if read the other way: a herd CAN be MRSA-free and acquire MRSA with incoming pigs, and if this entry were the resistance phenotype the cell would be arguable at level 3. It is not the phenotype. On your 2026-08-18 ruling this entry is the species, and the species is already everywhere.

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

How this level was decided

Level 2, and the chapter supplies both halves of the label in successive sections. The presentation does not point at this organism -- the chapter says so in terms, because abscesses, septicaemia, endocarditis and arthritis have many causes -- so it is not level 1. Confirmation is ordinary aerobic culture with a distinctive colony morphology plus routine biochemistry or MALDI-TOF, all within any regional diagnostic laboratory. NOTE THAT LINEAGE TYPING IS A DIFFERENT QUESTION AND IS NOT WHAT THIS CRITERION ASKS: establishing that an isolate is CC398 rather than a human strain needs MLST or spa typing, which is a research-laboratory activity, but C4 asks about recognizing and confirming INFECTION, and that is culture.

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, not a common pathogen of pigs though the organism is readily found on farms and in pigs, can cause disease however under some circumstances"

How this level was decided

INFERRED. Level 2 -- small, sporadic, individual-animal losses with little effect on overall cost of production. The chapter states the shape of the loss three separate ways: not a common pathogen, no herd outbreaks, animal-to-animal spread rare, and no need to treat the group. That is the level 2 label, and it puts the entry level with the colibacilloses and Actinobaculum suis. IT IS NOT LEVEL 1 because the individual losses are real and countable -- a dead neonate, a stunted piglet, a condemned or trimmed carcass, a culled lame sow with vertebral osteomyelitis -- rather than absent. NOTE WHAT IS DELIBERATELY NOT COUNTED HERE: MRSA carriage costs the farm nothing in production terms, because colonised pigs are not sick. Its cost is borne in human medicine and is scored in C1 and C2, not here.

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, thus far no market impact from S.aureus occurrence in pigs. Despite the worry from public health, most MRSA's in people are attributed to self, other people, or the hospital environment"

How this level was decided

INFERRED. Level 1 under the standing note SCORING/market_impact: an organism long resident and repeatedly reported in US pigs, with no market move ever observed. NOTE THE ARGUMENT AGAINST, BECAUSE IT IS THE STRONGEST OF ANY C6 IN THE REGISTER AND STILL FAILS: "pig MRSA" is the one item in this chapter with genuine public salience -- it has had press coverage, and the Netherlands isolates swine workers on hospital admission. Nearly twenty years of that has produced no observed effect on US pork demand or trade, and the note is explicit that where an agent is already reported here, the absence of a historical move is the evidence 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, in rare cases where treatmetn of pigs is required, drugs are available (usualy with a label for the bacteria in other species like cattle, so remains extralabel even though is a vet label available)"

How this level was decided

Level 2, and FLAGGED, but on a narrower question than the usual label one. THE LEVEL ITSELF IS SECURE WITHOUT YOUR RULING: treatment exists and works, and it works only in some circumstances -- resistance to the common first-line drugs is frequent enough that the chapter recommends susceptibility testing before choosing, and severe abscessation requires surgical drainage as well. That is the level 2 clause satisfied on efficacy alone. THE QUESTION I CANNOT ANSWER FROM THE FOLDER is whether the chapter's three recommended drugs are lawfully usable for this indication in US swine. My understanding is that S. aureus is not named on the swine labels for ceftiofur or enrofloxacin, which would make both extralabel -- and extralabel fluoroquinolone use in food animals is federally prohibited, while extralabel cephalosporin use in swine is restricted. If that is right, the practical US answer is narrower than the chapter implies and it is worth recording; if I have the regulatory position wrong, say so and I will strike it. Either way the cell stays at 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. "c8l1, no clinical imperative for vaccine development for S.aureus infection in pigs"

How this level was decided

INFERRED, AND FLAGGED because it is the first application of the standing note SCORING/vaccine_availability written on 2026-09-03, and you should see the note working before it governs forty more entries. LEVEL 1 ON THE "NOT NEEDED" CLAUSE, by the note's FIRST ROUTE -- low prevalence and low clinical significance, the ground of your "all seven to L1" ruling of 2026-08-28. The claim is made affirmatively rather than by default, which is what your PCV4 rule requires: the chapter states that this is not a common pathogen, produces no herd outbreaks, spreads between animals rarely, and is treated one animal at a time. An agent whose disease is an individual-animal problem does not justify vaccinating a herd. THE ARGUMENT FOR LEVEL 2 is the bare fact that bacterins have been described, which is the "available" half of that level -- but "not widely available or extensively used" is closer to a product nobody wants than to a product with inconsistent protection, and the reason nobody wants it is the reason this is level 1.

Levels and evidence are generated from data/assignments/staphylococcus_aureus.yml; the overview is authored in data/overviews/staphylococcus_aureus.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.

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