Swine Brucellosis
LEVELS: Sometimes occurs; Severe; Routine biosecurity keeps it out; Reference or research laboratory required; Moderate; Temporary disruption; Some; Needed but not available
| Register id | brucella_suis |
| Type | bacteria |
| Scientific name | Brucella suis |
| NCBI taxid | 29461 |
| Evidence | 3 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Brucella suis is the bacterium that causes swine brucellosis. In pigs it is above all a reproductive disease — abortion, stillbirths and infertility in females, inflamed testes in males, and sometimes lameness when the organism settles in joints or bone. Entering a naive herd it spreads fast, infecting up to 70–80% of animals early in an outbreak, but once a herd is chronically infected the signs become mild or disappear altogether and the infection is easily missed. It is unusual among the brucellae in spreading venereally as well as by ingestion of infected tissue, which makes an infected boar or a batch of semen a very efficient way to seed a farm. Infected animals carry it for years: some sows shed for up to three years, and half of experimentally infected boars were still culture-positive at 42 months. Feral swine are a substantial reservoir in the United States. It is also a serious zoonosis — human brucellosis is the commonest bacterial zoonosis worldwide, and there is no commercial swine vaccine, so freedom depends on testing incoming stock and fencing wildlife out.
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. ch46 46.4: "HUMAN BRUCELLOSIS DUE TO B. SUIS MOST FREQUENTLY OCCURS IN FARM WORKERS, VETERINARIANS, AND ABATTOIR EMPLOYEES, but it can also be contracted through HUNTING OR ASSOCIATION WITH FERAL SWINE... TRANSMISSION OF B. SUIS FROM SWINE TO HUMANS PRIMARILY OCCURS BY DIRECT CONTACT WITH INFECTED ANIMALS OR CONTACT WITH ABORTED TISSUES AND FLUIDS. ROUTES OF ENTRY INCLUDE AEROSOL, ORAL, OR THROUGH BREAKS IN EPIDERMAL INTEGRITY. PROCESSING OF INFECTED SWINE THROUGH AN ABATTOIR IS ASSOCIATED WITH A HIGH RISK FOR INFECTION OF HUMAN WORKERS, MOST LIKELY THROUGH AEROSOL EXPOSURE. Some data suggests that zoonotic infection with B. suis CAN RESULT FROM HANDLING OR CONSUMPTION OF INFECTED RAW OR UNDERCOOKED PORK." 46.1: "Brucellosis is THE WORLD'S MOST COMMON BACTERIAL ZOONOSIS", and in the US before the 1959 eradication programme human cases peaked at 6321 in 1949, mostly B. abortus and B. suis.
How this level was decided
Level 3. The pig attribution is as direct as it gets -- swine are the reservoir host, the transmission is named as swine to human, and the exposed groups are farm workers, veterinarians, abattoir employees and feral swine hunters. What holds it at 3 rather than 4 is that every route named is a control point that can hold: abattoir process controls and personal protection, cooking pork, and above all keeping commercial herds free, which the US eradication programme has largely achieved. LEVEL 4 WOULD REQUIRE transmission through intact controls, and the chapter instead describes a high risk WHEN INFECTED SWINE ARE PROCESSED -- which is the failure of the upstream control. NOTE THE AEROSOL ROUTE, because it is the strongest argument the other way: abattoir infection is attributed to aerosol, and respiratory protection is the level 4 exemplar.
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. ch46 46.4: "BRUCELLA MELITENSIS AND B. SUIS ARE GENERALLY MORE VIRULENT HUMAN PATHOGENS THAN OTHER BRUCELLA SPP... Human brucellosis is also known as UNDULANT FEVER due to the single consistent symptom of fever that waxes and wanes. It is associated with A WIDE SPECTRUM OF NON-PATHOGNOMONIC CLINICAL MANIFESTATIONS depending on the organs or tissues of localization, with OSTEOARTICULAR, NERVOUS, PULMONARY, AND MUCOCUTANEOUS TISSUES MOST COMMONLY AFFECTED. COMPARED TO INFECTION WITH OTHER BRUCELLA SPP., HUMAN INFECTION WITH B. SUIS IS FREQUENTLY ASSOCIATED WITH HEPATIC AND SPLENIC ABSCESSES AND CHRONIC SUPPURATIVE LESIONS IN ORGANS." 46.1 records over half a million new human cases annually worldwide.
How this level was decided
Level 4, and it is the worked example at this level alongside Streptococcus suis, Taenia solium and Nipah virus. The label asks for a substantial risk of life-threatening illness, death or permanent disability even if infections are uncommon, and B. suis is named as one of the two most virulent Brucella species in people, producing hepatic and splenic abscesses, chronic suppurative organ lesions, and neurological and osteoarticular disease. Chronic brucellosis is a disabling illness measured in months to years.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Routine biosecurity keeps it out: Quarantine of incoming stock, transport and fomite control, cleaning and disinfection, and the usual monitoring reliably exclude it
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c3l3, transmission routes are well known and straightforward to control with routine biosecurity"
Basis. ch46 46.10: "CURRENTLY, THE ONLY EFFECTIVE STRATEGY TO MAINTAIN B. SUIS-FREE STATUS OF A SWINE HERD OR SYSTEM IS EXCELLENT BIOSECURITY. PRINCIPAL COMPONENTS MUST INCLUDE TESTING TO ENSURE B. SUIS-FREE STATUS OF ANY SOURCE OF PURCHASED SEMEN OR SEEDSTOCK PRIOR TO HERD ENTRY AND INSTALLATION OF ADEQUATE FENCING WHEN HUSBANDRY PRACTICES ALLOW POTENTIAL CONTACT WITH WILDLIFE RESERVOIRS. Given the FEW AND NONSPECIFIC CLINICAL SIGNS IN CHRONICALLY B. SUIS-INFECTED PIG HERDS and the degree of transport and intermingling of pigs in most countries, INTRODUCTION OF B. SUIS INFECTED PIGS IN SEEDSTOCK FARMS OR BOAR STUDS COULD RAPIDLY DISSEMINATE DISEASE." 46.3: "Direct or close contact is required for disease transmission... environmental persistence of B. suis is considered OF LOW EPIDEMIOLOGICAL IMPORTANCE... Unlike most other brucellae, B. SUIS CAN BE SPREAD BY VENEREAL TRANSMISSION, which appears to be important for transmission within and between herds", with sporadic long-duration shedding in boar semen.
How this level was decided
INFERRED. Level 3 -- routine biosecurity keeps it out. The chapter names the programme and it is the level 3 list: testing incoming semen and seedstock, quarantine, and fencing against the wildlife reservoir. The environment is not the route and does not need engineering out; direct contact and semen are. THE ARGUMENT FOR LEVEL 4 IS THE CARRIER-DETECTION CLAUSE, and it is real: 46.8.2 says a panel of serologic tests identified only 52% of naturally infected feral swine, and 17% of culture-positive swine were negative on all serologic tests, so "repeated costly testing to find carriers" is literally what this takes. IT DOES NOT CARRY because serology is reliable AT HERD LEVEL, which is the level at which exclusion is actually managed -- the chapter says so twice -- and because the US commercial herd has in fact been held free of B. suis by exactly this programme since the eradication campaign. NOTE THE FERAL RESERVOIR AND WHY IT DOES NOT LIFT THE LEVEL: under your 2026-09-03 rule the reservoir is not the question, the route from it is, and a fence answers it.
C4 Detection difficulty
Difficulty of recognizing and confirming infection
Reference or research laboratory required: A validated assay exists, but only at a national reference or research laboratory rather than at local or regional level
Basis. ch46 46.8.2, on why the individual animal cannot be tested: "THE DIAGNOSTIC VALUE OF SEROLOGICAL TESTS FOR BRUCELLOSIS IN SWINE IS ONLY USEFUL AT THE HERD LEVEL RATHER THAN FOR INDIVIDUAL SWINE. NONE OF THE CONVENTIONAL SEROLOGICAL TESTS used for the diagnosis of brucellosis in domestic ruminants HAVE ADEQUATE SENSITIVITY AND SPECIFICITY FOR THE DIAGNOSIS OF B. SUIS INFECTIONS IN INDIVIDUAL PIGS... a panel of serologic tests ONLY IDENTIFIED 52% OF NATURALLY INFECTED FERAL SWINE as seropositive, and 17% OF CULTURE-POSITIVE SWINE WERE NEGATIVE ON ALL SEROLOGIC TESTS. YERSINIA ENTEROCOLITICA O:9 INFECTION IN PIGS IS WIDESPREAD in many countries and is believed to be THE MAJOR SOURCE OF FALSE POSITIVE serological reactions." On culture, 46.8.1: it "is the definitive method", but "slow growth of Brucella in vitro, a tendency for reduced isolation from chronically infected swine, HIGH COSTS, AND BIOSAFETY CONCERNS make diagnosis by bacterial culture UNFEASIBLE IN SOME SITUATIONS", cultures need at least 8 days, and "differentiation of biovars 1, 2, and 3 may require additional testing and SHOULD THEREFORE BE CONDUCTED BY A BRUCELLOSIS REFERENCE LABORATORY."
How this level was decided
Level 3. A validated approach exists -- culture is definitive and herd-level serology works -- but the chapter sends the confirmatory work to a reference laboratory in terms, and the reasons are biosafety and biovar differentiation rather than convenience. That is the level 3 label, and it puts this beside classical swine fever, foot-and-mouth disease and African swine fever for the same structural reason. NOTE THE FALSE POSITIVE PROBLEM, because it is unusual and belongs in any brief: Yersinia enterocolitica O:9 is widespread in pigs and cross-reacts, so a positive brucellosis serology in a pig is more often Yersinia than Brucella.
C5 Production cost
Financial impact on the infected farm's cost of production
Moderate: Losses measurably increase cost of production but remain manageable within normal farm operations, whether acute, chronic, or associated with endemic disease
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c5l3, could easily make this a l2 based on the very low prevalence but if it does find a susceptible pig, the consequence to that individual is signficance."
Basis. ch46 46.1: in swine "Brucella can produce PREGNANCY LOSS, STILLBIRTHS, AND INFERTILITY IN FEMALES, AND EPIDIDYMITIS AND ORCHITIS IN MALES... LAMENESS following localization in joints or bone being most common" among the uncommon signs. 46.3 on the herd course: "WHEN FIRST ENTERING A NAIVE HERD, B. SUIS CAN QUICKLY SPREAD FROM A SINGLE INFECTED ANIMAL RESULTING IN INFECTION RATES OF UP TO 70-80% IN THE EARLY STAGES OF OUTBREAKS. IN ACUTELY INFECTED HERDS, PREGNANCY LOSS, INCREASED PERINATAL MORTALITY, AND INFERTILITY ARE COMMON. IN CHRONICALLY INFECTED HERDS, ONLY MILD TO MODERATE CLINICAL SIGNS ARE OBSERVED, AND INFECTION OFTEN GOES UNDETECTED." Persistence is long: a small percentage of females shed for up to 36 months, and 50% of experimentally infected boars remained culture positive at 42 months.
How this level was decided
INFERRED. Level 3 -- losses that measurably increase cost of production but remain manageable. On an infected breeding herd the loss is reproductive and large: 70-80% infection early, with pregnancy loss, perinatal mortality and infertility common, in animals that cannot be cured and must be removed. NOT LEVEL 4, because the chronic state is mild and often undetected and the herd continues to produce; the response is test and removal rather than the collapse of the unit. NOTE WHAT THIS CELL DOES NOT SCORE: the regulatory consequence of a B. suis detection in a US commercial herd is a trade and public health event rather than a production one, and under the swine vesicular disease rule that belongs in C6.
C6 Market impact
Duration of material disruption to pork and pig markets
Temporary disruption: Material negative effect on supply or demand lasting less than a month when disease occurs on one or more farms
INFERRED — reasoned, not stated. The evidence implies this level rather than stating it. The reasoning is below.
Corrected by Eric from L3 Significant disruption. Reason: "c6l2, diagnosis would have only a temporary effect, not signficant. We have dealt with singleton true and false reactors for many years and the market behaves responsibly."
Basis. ch46 46.1: "HUMAN BRUCELLOSIS IS THE PRIMARY BASIS FOR CONTROL OR ERADICATION PROGRAMS TARGETING BRUCELLA SPP. IN DOMESTIC LIVESTOCK HOSTS. Control or eradication programs are justified by data, which indicates that ADDRESSING THE DISEASE IN LIVESTOCK RESERVOIRS IS THE MOST EFFICIENT AND ECONOMICAL APPROACH FOR REDUCING HUMAN BRUCELLOSIS." The US programme began in 1959, after human cases peaked at 6321 in 1949. 46.10: "regulatory efforts to control or eradicate B. suis in domestic swine SHOULD BE DIRECTED TOWARD HERDS, RATHER THAN INDIVIDUAL ANIMALS." No market or trade consequence of a B. suis detection is quantified anywhere in the chapter.
How this level was decided
INFERRED, AND FLAGGED, because there is no US precedent to reason from and the standing note SCORING/market_impact is strict about that. WHAT THE NOTE ALLOWS: a null where nothing has ever been observed, and a scored level where a foreign or historical precedent exists. THE PRECEDENT HERE IS DOMESTIC AND HISTORICAL rather than absent -- the United States ran a national eradication programme against this organism for decades, which is the strongest possible evidence that a detection triggers a regulatory response, and the chapter says the programme exists because of human disease. LEVEL 3 IS SCORED: a B. suis detection in a US commercial herd would be a reportable zoonotic event with herd quarantine, tracing and test-and-removal, and regaining a clean status is measured in months rather than weeks -- the one-to-six month band. THE ARGUMENT FOR LEVEL 2 is your Menangle calibration, where a zoonotic novelty in a single herd got a temporary effect lasting under a month. THE ARGUMENT FOR A NULL is that the chapter never describes a market consequence at all. Your call, and it is the cell in this entry I am least sure of.
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. ch46 46.10: "As test and removal strategies are not economically feasible under all circumstances, some published studies have evaluated the ability of ANTIBIOTIC TREATMENT to control brucellosis in reservoir hosts, including swine. THESE STUDIES FOUND THAT ALTHOUGH DISEASE PREVALENCE WAS REDUCED, COST OF TREATMENT AND PERSISTENCE OF INFECTION IN SOME TREATED ANIMALS MADE LONG-TERM ANTIBIOTIC THERAPY NONVIABLE AS A REGULATORY STRATEGY. A small study of naturally infected swine (n = 8/treatment) found that ORAL TREATMENT WITH OXYTETRACYCLINE (20 mg/kg/daily for 21 days) ELIMINATED B. SUIS BIOVAR 2 INFECTION FROM ONLY 50% OF INFECTED SWINE."
How this level was decided
Level 2. Treatment exists and demonstrably does something -- prevalence fell, and oxytetracycline cleared half the treated pigs -- but it works only in some circumstances, which is the level 2 clause: half the animals stayed infected, the course runs three weeks, and the chapter judges it nonviable as a strategy on cost and persistence. NOT LEVEL 1, because a pig that remains infected after treatment is not an acceptable recovery in a disease whose whole importance is that it infects people. NOT LEVEL 3, because a treatment does exist and half of it worked; level 3 is for an absent treatment.
C8 Vaccine availability
Availability of effective vaccines or bacterins
Needed but not available: No effective vaccine is available in the US or has been developed, and the disease would justify vaccinating if one existed
Basis. ch46 46.10: "THERE ARE CURRENTLY NO COMMERCIALLY AVAILABLE VACCINES FOR PROTECTING DOMESTIC OR FERAL SWINE AGAINST B. SUIS INFECTION. Although initial reports were promising, currently available data suggests that THE ORAL B. SUIS STRAIN 2 VACCINE developed in China AND THE B. ABORTUS STRAIN RB51 VACCINE DO NOT ADEQUATELY PROTECT SWINE against B. suis infection. Recently, A ROUGH B. SUIS STRAIN that can be used as a DIVA vaccine WAS DEMONSTRATED TO HAVE EFFICACY after oral or parenteral delivery to domestic or feral swine. Other recombinant B. suis vaccines have been proposed, but immunogenicity and efficacy data in swine are lacking. AT THE CURRENT TIME, COMMERCIAL DEVELOPMENT OF A B. SUIS VACCINE IN THE UNITED STATES DOES NOT APPEAR TO BE IMMINENT."
How this level was decided
Level 3, and it is well evidenced in both halves. No effective vaccine is available in the US -- none is commercial, two existing products were tested and found not to protect swine, and the chapter says US commercial development is not imminent. And the disease warrants one on its own weight: swine are the reservoir for the world's most common bacterial zoonosis, the human illness is at C2 level 4, and the alternative is a national test-and-removal programme. THIS IS A DEMONSTRATED GAP RATHER THAN AN UNREPORTED ONE -- somebody tried strain 2 and RB51 in pigs and they failed -- which is the same quality of evidence that put Streptococcus zooepidemicus at this level.
Levels and evidence are generated from data/assignments/brucella_suis.yml; the overview is authored in data/overviews/brucella_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.