Influenza B Virus
LEVELS: Highly unlikely; Mild; Extraordinary biosecurity required; Reference or research laboratory required; Negligible; Negligible; Little; Available, or not needed
| Register id | influenza_b_virus |
| Type | virus |
| Scientific name | Influenza B virus |
| NCBI taxid | 11520 |
| Evidence | 2 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Influenza B is primarily a human respiratory virus. It can infect seals and pigs, but only influenza A is of routine clinical significance in swine, and influenza B has never caused a recognised natural outbreak in pigs. The one experimental study available shows both what it can and cannot do: pigs inoculated directly into the windpipe developed fever and lung lesions and shed virus from the nose, so the virus is capable of replicating and causing damage in a pig — but it did not pass to pigs housed with them. Without pig-to-pig transmission it cannot establish in a herd, and the source of any porcine infection would be a person. There is a practical diagnostic consequence worth noting: the routine swine influenza tests are built for influenza A, and a generic influenza A screening assay does not detect influenza B, so a case would be missed unless it were specifically looked for. There is no treatment and no vaccine for pigs — every swine influenza vaccine on the market targets influenza A.
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. ch32 32.1: "INFLUENZA B VIRUS MAINLY CAUSES RESPIRATORY DISEASE IN HUMANS, although it can also infect seals and pigs." The direction is stated, and lee_2019 tests what a pig does with it: pigs inoculated intratracheally with IBV developed fever and lung lesions and shed virus nasally, but "IBV AND IDV DISPLAYED DIFFERING LEVELS OF REPLICATION IN THE RESPIRATORY TRACT OF INFECTED PIGS, NO" transmission to contact animals occurred. The paper's conclusion is that both viruses "can replicate, and are pathogenic in pigs" -- pathogenic, but not transmissible pig to pig.
How this level was decided
Level 1, and the transmission failure is what carries it rather than a silence. Influenza B is a human virus that occasionally gets into a pig; the only experiment that put it there and watched found that an inoculated pig did not pass it to a penmate. A pig that cannot infect another pig is not a source for a person either. THE DIRECTION OF TRAFFIC IS THE OPPOSITE OF WHAT C1 MEASURES: influenza B moves from people to pigs, which is a reason to think about staff health rather than a zoonotic risk, and C1 as rescoped asks only about transmission FROM pigs.
C2 Zoonotic impact
Severity of human illness caused by the agent
Mild: Human illness is usually mild and self-limiting, and serious disease occurs only rarely
Basis. ch32 32.1: influenza B "mainly causes respiratory disease in humans". No case of human illness acquired from a pig is described anywhere in the chapter or in lee_2019.
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. Human seasonal influenza: usually self-limiting, serious disease uncommon. Scored level 2 to match Eric's influenza A ruling -- "most transmission events to humans are mild, there is always a risk of a major uptick in human severity but this remains unlikely".
PREVIOUS ANSWER, written under the pig-attribution reading: Level 1. C2 grades the severity of illness ACQUIRED FROM PIGS, and no such illness exists. THE LABEL MUST NOT BE READ AS "THIS VIRUS IS HARMLESS TO PEOPLE" -- influenza B is a substantial cause of human seasonal influenza and kills people every year. What level 1 records is that pigs are not where anyone gets it. C1L1 implies C2L1 and validate_criteria.py checks it.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Extraordinary biosecurity required: Exclusion needs measures beyond the routine, such as air filtration, thermally treated feed and bedding, or repeated costly testing to find carriers, and may still fail
Corrected by Eric from L3 Routine biosecurity keeps it out. Reason: "v3l4, humans are the source for pigs, and we must have people in farms to take care of the pigs. So, all we can do is ask people not to come to work if they are sick, but in my view this falls in the realm of L4 because compliance is hard" HE WROTE "v3l4" AGAINST THIS CELL. The criterion prefix is a typo -- the cell is C3 and no criterion is numbered v3 -- and the level is unambiguous, so it is read as c3l4. THE RULE HE STATES GOVERNS ANY AGENT WHOSE ROUTE INTO A HERD IS THE STAFF: "humans are the source for pigs, and WE MUST HAVE PEOPLE IN FARMS TO TAKE CARE OF THE PIGS. So, all we can do is ask people not to come to work if they are sick, but in my view this falls in the realm of L4 BECAUSE COMPLIANCE IS HARD." That is the level 4 label -- exclusion needs measures beyond the routine and may still fail -- and it puts influenza B alongside influenza A at level 4 for the same reason, which the chapter supports: human-to-swine transmission is "central in globally seeding swine populations with new viral diversity".
Basis. ch32 32.1: influenza B "mainly causes respiratory disease in humans, although it can also infect seals and pigs", so the source of a porcine infection is a person. lee_2019 shows what happens next: pigs given IBV intratracheally shed virus nasally but did NOT transmit it to contact pigs, so the infection does not establish or spread in a herd. For the analogous route in influenza A, ch32 32.1 records that "human-to-swine transmission of influenza virus is far more frequent than swine-to-human and is central in globally seeding swine populations".
How this level was decided
Level 3, and FLAGGED because the level rests on an inference about the route rather than on any prevalence data. The reasoning: the source is an infected stockperson, housing does nothing about people, and what keeps a human respiratory virus out of a barn is staff health policy, shower-in and exclusion of sick workers -- measures that belong to a routine biosecurity programme. That is level 3. IT IS NOT LEVEL 1, because there is no evidence the virus is present in most herds; it does not even transmit between pigs experimentally, so it cannot establish. It is not level 4, because nothing extraordinary is required. NOTE HOW LITTLE IS KNOWN: the chapter gives this virus one sentence and no seroprevalence figure for pigs anywhere.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c4l3, I believe this is correct. While Flu A is readily diagnosed in pigs locally, I don't think Flu B reagents are typically in local or regional vet labs - they would need to refer to confirm the diagnosis (i.e. histopathology may fit, but Flu A test is neg, hence refer the case to another lab)" HIS DESCRIPTION OF THE FAILURE MODE IS THE USEFUL PART, because it is how the gap would actually present: "histopathology may fit, but Flu A test is neg, hence refer the case to another lab". The herd's routine pan-influenza A screen returns a clean negative on an influenza B infected pig, because they are different genera.
Basis. ch32 32.8 describes the swine influenza diagnostic system, and it is built for influenza A: "RT-PCR assays targeting IAV are classified into two broad types. The first allow GENERIC DETECTION OF ANY IAV and are applicable to all swine IAV", the second are HA-subtype specific. A pan-influenza A assay does not detect influenza B, and no influenza B assay validated for porcine samples is described anywhere in the chapter. 32.2 gives the reason the two cannot be conflated: influenza A and B are different genera, and "REASSORTMENT BETWEEN MEMBERS OF DIFFERENT GENERA HAS NOT BEEN REPORTED". lee_2019 detected IBV in pigs using research assays.
How this level was decided
Level 3, and FLAGGED AS A LEVEL 3 OR 4 JUDGMENT. Level 3 is scored because validated influenza B assays plainly exist -- they are routine in human diagnostic medicine -- so this is not the level 4 situation of no reliable test anywhere. But none of them is validated on porcine specimens, no veterinary diagnostic laboratory offers an influenza B panel for pigs, and the routine swine influenza screen a laboratory does run would return a clean negative on an IBV-infected pig because the pan-influenza A primers do not see it. That is an assay held at reference or research level. THE PRACTICAL POINT FOR A BRIEF: influenza B in a pig is invisible to the test the herd is already using.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
Basis. ch32 32.1, the sentence that governs every cell of both these entries: "AMONG THE FOUR TYPES OF INFLUENZA VIRUSES, ONLY IAV IS OF ROUTINE CLINICAL SIGNIFICANCE IN SWINE. Therefore, the remainder of this chapter is focused on IAV." lee_2019 is the only experimental evidence and it cuts both ways: pigs inoculated INTRATRACHEALLY developed fever and lung lesions and IBV "had a higher replication efficiency in the lower respiratory tract", but the virus did not transmit to contact pigs. No natural outbreak, no field case and no production effect is reported anywhere.
How this level was decided
Level 1, and it rests on a positive statement rather than on absence. Diseases of Swine says that of the four influenza types only A is of routine clinical significance in swine, which is an assessment by authors who then wrote thirteen pages about A and one sentence about B. The experimental pathogenicity does not change it: fever and lung lesions after intratracheal inoculation is what most respiratory viruses do when delivered straight to the lung, and the finding that matters for a herd is that it did not spread to the pig in the next pen. No measurable effect on cost of production.
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 reason to think Flu B will cause any effect on markets, the agent is unlikely to transmit between pigs or farms"
Basis. No trade measure, movement control or consumer response involving influenza B in pigs is recorded anywhere in chapter 32, and no natural outbreak in pigs has been reported.
How this level was decided
INFERRED. Nothing about a human respiratory virus that occasionally infects a pig without spreading reaches pork supply or demand. NOTE THE ONE REASON THIS IS SCORED RATHER THAN LEFT NULL: influenza B in pigs has been looked for and found, in China and experimentally, so this is not the never-observed case the standing note reserves for a null cell -- it is an agent that has been seen and has caused nothing.
C7 Treatment potential
Potential for treatment to improve outcomes
Little: Effective pathogen-directed treatment is available and works, or animals recover acceptably without it
Basis. ch32 32.1, the sentence that governs every cell of both these entries: "AMONG THE FOUR TYPES OF INFLUENZA VIRUSES, ONLY IAV IS OF ROUTINE CLINICAL SIGNIFICANCE IN SWINE. Therefore, the remainder of this chapter is focused on IAV." No treatment for influenza B in pigs is described anywhere. lee_2019 records that the virus does not transmit between pigs.
How this level was decided
Level 1 on the second clause -- animals recover acceptably without treatment -- because on the evidence held they do not develop natural disease at all. Eric's rule of 2026-09-02 bars the lazy route to this answer, that nobody treats viral disease, and it is not the route taken: C7 asks whether outcomes would meaningfully improve if a treatment existed, and there is no field outcome to improve. This cell moves with C5 if influenza B is ever shown to cause disease in pigs.
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. No vaccine for influenza B in pigs is mentioned anywhere in chapter 32. ch32 32.1, the sentence that governs every cell of both these entries: "AMONG THE FOUR TYPES OF INFLUENZA VIRUSES, ONLY IAV IS OF ROUTINE CLINICAL SIGNIFICANCE IN SWINE. Therefore, the remainder of this chapter is focused on IAV." 32.10 describes the swine influenza vaccine market in detail and every product in it is an influenza A vaccine.
How this level was decided
Level 1 on the "not needed" clause of the 2026-08-28 rewording. There is no porcine influenza B vaccine, so the question is whether the disease would justify one on the agent's own weight, and a virus with no reported natural disease in pigs and no pig-to-pig transmission does not. NOTE THAT HUMAN INFLUENZA B VACCINE EXISTS AND IS GIVEN TO MILLIONS -- the quadrivalent human seasonal vaccine covers two B lineages -- so this is not a technical gap. It is an absence of demand in the species this MCDA prioritises.
Levels and evidence are generated from data/assignments/influenza_b_virus.yml; the overview is authored in data/overviews/influenza_b_virus.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.