Influenza C Virus
LEVELS: Highly unlikely; Mild; Extraordinary biosecurity required; Reference or research laboratory required; Negligible; Negligible; Little; Available, or not needed
| Register id | influenza_c_virus |
| Type | virus |
| Scientific name | Influenza C virus |
| NCBI taxid | 11552 |
| Evidence | 3 document(s) |
| Assigned | 2026-09-06, against criteria version 093366e352a2 |
Overview
Influenza C virus infects both people and pigs, and the traffic appears to run from humans outward — it crossed the species barrier from people to infect dogs, pigs, camelids and horses. Pigs are more than a dead end, though: they were the first animal species reported to show seasonal influenza C activity, when fifteen strains were isolated from apparently healthy pigs at a Beijing slaughterhouse across a full year in 1981, and experimental work showed the virus remains infectious for 25 days and transmits by contact between pigs. Nearly half of pigs surveyed in Bavaria carried antibody. But no clinical disease, lesion or production effect has ever been attributed to influenza C in pigs — the Beijing isolates all came from healthy animals — and only influenza A is of routine clinical significance in swine. In people it causes a mild upper respiratory illness, mostly in children. As with influenza B, the routine swine influenza assays would not detect it. There is no treatment and no vaccine for pigs.
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 C VIRUS IS INFECTIOUS FOR HUMANS AND PIGS." sreenivasan_2021 gives the detail, and the direction it records is human to pig: "influenza C viruses CROSSED THE SPECIES BARRIER FROM HUMANS to infect dogs, pigs, camelids, and horses." But it also records why pigs are not simply a dead end. 3.1.4: "PIGS ARE THE FIRST ANIMAL SPECIES REPORTED TO HAVE SEASONAL ICV ACTIVITY IN CHINA. FIFTEEN ICV STRAINS WERE ISOLATED FROM APPARENTLY HEALTHY PIGS AT A SLAUGHTERHOUSE IN BEIJING from January-December 1981 and experimental infection of porcine and human ICV strains in experimental pigs also showed that ICV CAN REMAIN INFECTIOUS FOR 25 DAYS AND CAN BE TRANSMITTED BY CONTACT." "The close homology of the swine and human ICVs and their serological and genetic similarities SUGGEST THE POSSIBILITY OF PIGS AS A NATURAL HOST FOR ICV." A 1996 Bavarian survey found 49.9% of 240 pigs seropositive. No human case attributed to a pig is reported. shic_2021_flu_cd: "ICV is mainly a pathogen of humans. Transmission of ICV between humans and pigs is strongly suggested, but the direction of transmission is unclear." "ICV causes sporadic outbreaks of mild respiratory disease in humans, the primary reservoir and host." "most people seem to develop antibodies against the virus by age ten." "In the 1990s, human ICV strains from Japan were found to be antigenically related to swine isolates from China. However, whether swine-to-human or human-to-swine transmission occurred was unknown."
How this level was decided
MOVED L2 -> L1, re-read 2026-09-04 AGAINST THE REWORDED LABELS. No human case has ever been attributed to pig contact, and every documented species jump ran the other way -- sreenivasan_2021: "influenza C viruses CROSSED THE SPECIES BARRIER FROM HUMANS to infect dogs, pigs, camelids, and horses."
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: Level 2, and the plausibility here is stronger than for influenza B on the facing entry. Fifteen strains were isolated from healthy pigs in a single year at one abattoir, the virus persists in an infected pig for 25 days and passes to penmates by contact, and half the pigs in a German survey had met it -- so unlike influenza B, this virus sustains itself in pigs and the authors raise pigs as a possible natural host. A pig that carries and transmits a virus people also get is a plausible source. WHAT KEEPS IT AT LEVEL 2 rather than level 3 is that no human case has ever been attributed to pig contact, and every documented species jump ran the other way, from people outward. Level 1 held, with a note. Humans are the PRIMARY reservoir and nearly everyone is seropositive by age ten, so human ICV infection is universal and none of it is attributed to pigs -- the second L1 clause exactly. The factsheet's "direction of transmission is unclear" is the only pull toward L2, and it is a statement that nobody knows which way the 1990s Japan/China isolates travelled, not a report of a pig-attributed human case. On this agent the traffic is more likely human-to-pig.
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 C virus is infectious for humans and pigs." Human influenza C is a mild upper respiratory illness, chiefly of children, and neither chapter 32 nor sreenivasan_2021 describes hospitalisation, severe disease or death from it; the review treats ICV as the least clinically consequential of the influenza types in people, which is why it is grouped with influenza D in a comparative host-range paper rather than in the clinical literature. No human case acquired from a pig is on record. shic_2021_flu_cd: "ICV causes sporadic outbreaks of mild respiratory disease in humans, the primary reservoir and host." In humans, "neuraminidase inhibitors used to treat IAV and IBV are ineffective for ICV."
How this level was decided
Level 2, grading the illness that would follow if the route C1 calls plausible were taken. Human influenza C is mild and self-limiting -- a cold-like upper respiratory illness that most people meet in childhood -- with serious disease rare. That is the level 2 label. IT IS NOT LEVEL 1, because level 1 says pigs are not a source and the chapter records the virus moving between the two species; and it is not level 3, because nothing about influenza C in a person commonly requires medical treatment. It sits below its own C1, which is unusual and correct: the route is more plausible than the consequence is serious. Level 2 confirmed and now directly supported -- the factsheet's own word for human ICV disease is "mild".
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. RULED L4 AND RELEASED FROM THE KISS SET. Eric, 2026-09-06: "flu c C3l4". SUPERSEDES HIS OWN NULL OF 2026-09-03, and the history matters because the cell moved twice. The pass first scored L1 on a 1996 Bavarian serosurvey and 1981 Beijing abattoir isolates; he nulled it because none of that was American and a level would have been indistinguishable from a real one, writing only "KISS analysis" against the cell. The SHIC influenza C/D factsheet placed on 2026-09-06 then supplied what was missing, though not in the form expected: still no US prevalence figure, but a direct statement that in swine ICV "has been described only in China, Japan, and Great Britain". That is evidence AGAINST level 1 rather than for it -- the agent is not established in US herds, so there is an introduction to prevent. At level 4 the reservoir is people: nearly all are seropositive by age ten, the route is respiratory aerosol, human ICV isolates replicate in pigs, and nothing routine excludes a ubiquitous human virus carried in by staff. THIS CELL WAS INFLUENZA C'S ONLY MISSING ONE, so the entry now has a complete score vector and leaves the unassessable set.
Basis. sreenivasan_2021 3.1.4: "Pigs are the first animal species reported to have SEASONAL ICV ACTIVITY in China. Fifteen ICV strains were isolated from APPARENTLY HEALTHY PIGS at a slaughterhouse in Beijing from January-December 1981", the virus "CAN REMAIN INFECTIOUS FOR 25 DAYS AND CAN BE TRANSMITTED BY CONTACT" in experimental pigs, and "the close homology of the swine and human ICVs... suggest the possibility of PIGS AS A NATURAL HOST for ICV". A 1996 survey in Bavaria found ICV antibody in 49.9% OF 240 PIGS, alongside 50.6% of 150 dogs. shic_2021_flu_cd: "In swine, however, ICV has been described only in China, Japan, and Great Britain." "ICV was first isolated from healthy pigs in China in 1981." "Influenza viruses spread through direct or indirect contact with respiratory droplets or by inhalation of infectious aerosols. Experimentally, human and swine ICVs can be transmitted to naive pigs through direct contact with infected animals." "Human ICV isolates are replication competent in experimentally infected pigs." "most people seem to develop antibodies against the virus by age ten."
How this level was decided
MOVED L1 -> L4 on Eric's ruling, 2026-09-06: "flu c C3l4". This closes an arc rather than making a fresh call. The pass originally scored L1 (already in the herd) on a 1996 Bavarian serosurvey and 1981 Beijing abattoir isolates; he NULLED the cell on 2026-09-03 because none of that evidence was American and a level would have been indistinguishable from a real one; the SHIC factsheet then supplied the missing statement -- in swine ICV "has been described only in China, Japan, and Great Britain" -- which is evidence AGAINST level 1 for a US herd, because the agent is not established here and there is therefore an introduction to prevent. AT LEVEL 4 THE RESERVOIR IS PEOPLE: nearly everyone is seropositive by age ten, the route is respiratory aerosol and direct contact, and human ICV isolates are replication competent in pigs. Nothing on the routine biosecurity list excludes a ubiquitous human respiratory virus carried in by staff, because no herd screens its workers for influenza C -- which is "extraordinary biosecurity required, and may still fail". PREVIOUS REASONING, KEPT: INFERRED at level 1, AND FLAGGED, because the evidence is real but old and thin. What supports it: half the pigs in a German survey were seropositive, the virus circulates seasonally in pigs, it transmits by contact between them, and it was isolated fifteen times in one year from HEALTHY pigs at slaughter -- an agent found at will in clinically normal animals is one the herd already has. That is level 1, present in most herds with no introduction event to prevent. WHAT MAKES IT AN INFERENCE RATHER THAN A FINDING: the seroprevalence figure is from one country in 1996, there is no US prevalence data at all, and nobody has looked recently. If a modern US survey found ICV to be uncommon, this cell would move to level 3, since the route in would be an infected person. *** FLAGGED FOR ERIC -- THIS IS THE CELL HE DEFERRED, AND THE NEW EVIDENCE CHANGES THE PICTURE RATHER THAN FILLING THE OLD GAP. LEVEL NOT MOVED; HIS NULL STANDS UNTIL HE RULES. *** He nulled this cell on 2026-09-03 because the level 1 rested on a 1996 Bavarian survey and 1981 Beijing abattoir isolates with no US prevalence figure at all. The factsheet does not supply a US figure either -- but it supplies something more useful, a direct statement that in swine ICV "has been described only in China, Japan, and Great Britain". That is evidence AGAINST level 1 for a US herd: the agent is not established here, so there is an introduction to prevent. WHAT I WOULD SCORE, IF HE WANTS A LEVEL: 4. The reservoir is people, essentially all of whom are infected by age ten; the route is respiratory aerosol and direct contact; and human ICV replicates in pigs. Nothing on the routine list excludes a ubiquitous human respiratory virus carried in by staff, because no herd screens its workers for influenza C. That is "extraordinary biosecurity required, and may still fail". THE CASE FOR LEAVING IT NULL: C3 measures effort to exclude, and it is odd to assign a high exclusion cost to an agent that has never been shown to cause disease in a pig -- the level would be true but would carry weight into the ranking for no reason. That tension is his to settle.
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, reagents would not typically be available at local or regional lab"
Basis. ch32 32.8 describes the routine swine influenza diagnostic system and it is built for influenza A: the screening assays "allow generic detection of any IAV", and 32.2 records that influenza A and C are different genera between which "reassortment... has not been reported". No influenza C assay validated for porcine specimens is described anywhere in the chapter. sreenivasan_2021 detected ICV in pigs using research methods, and notes that ICV is genetically stable -- "ICV is much more stable with fewer variations compared with IAV/IBV" -- which makes an assay easier to build than for influenza A but does not mean one is deployed. shic_2021_flu_cd: "ICV is more difficult to culture than IDV." "standard laboratory tests for influenza do not detect emerging strains." "Non-specific cross-reactivity occurs in human HI testing, complicating the interpretation of laboratory data. More information is needed on both ICV and IDV to develop diagnostic tests and vaccines."
How this level was decided
Level 3, and FLAGGED AS A LEVEL 3 OR 4 JUDGMENT alongside the influenza B cell. Validated influenza C assays exist in human diagnostic and research virology, so this is not the level 4 case of no reliable test anywhere; but none is validated on pig samples and no veterinary laboratory offers one, which is an assay held at reference or research level. THE PRACTICAL POINT IS THE SAME AS FOR INFLUENZA B AND WORTH CARRYING TO A BRIEF: the pan-influenza A screen a herd already runs does not detect influenza C, so a herd could have seasonal ICV activity of the kind described in Beijing and see nothing on its routine monitoring. Level 3 confirmed, and Eric's own ruling on this cell -- "C4l3, reagents would not typically be available at local or regional lab" -- is directly borne out: the factsheet says the standard influenza panel does not detect these species at all.
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 this entry: "AMONG THE FOUR TYPES OF INFLUENZA VIRUSES, ONLY IAV IS OF ROUTINE CLINICAL SIGNIFICANCE IN SWINE." sreenivasan_2021 3.1.4 supplies the field observation that matters: the fifteen Beijing isolates came from APPARENTLY HEALTHY PIGS at slaughter, across a full year. No clinical disease, lesion or production effect attributable to influenza C in pigs is described in either document. shic_2021_flu_cd: "clinical illness has not been associated with natural infection in pigs." "Experimentally, some pigs intranasally inoculated with ICV developed mild clinical signs (slight dyspnea, increased nasal secretion), while others remained asymptomatic. No postmortem lesions have been associated with ICV in pigs." "Currently, influenza A virus (IAV) is the only species of routine clinical significance in swine."
How this level was decided
Level 1, and this is an examined negative rather than a silence, which is the distinction that separates a supported level 1 from an unassessed cell. Somebody sampled pigs at an abattoir every month for a year, recovered the virus fifteen times, and recorded the animals as clinically normal; and the textbook, having considered all four influenza types, states that only influenza A matters clinically in swine. No measurable effect on cost of production. Level 1 confirmed and now well supported: no natural clinical illness, no lesions, and an explicit statement that influenza A is the only influenza of routine clinical significance in pigs.
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, unlikely to present as an outbreak or multifarm infection, market is unlikely to respond"
Basis. A virus of humans that also circulates in pigs without causing recognised disease. No trade measure, movement control or consumer response involving influenza C in pigs is recorded anywhere in chapter 32 or in sreenivasan_2021. shic_2021_flu_cd: "ICV and IDV are not OIE-listed diseases. There are no restrictions for importation of animals from countries or zones affected by ICV or IDV."
How this level was decided
INFERRED. The virus has been isolated from pigs since 1981 and found by serology in half a German pig population, with nothing following -- so this is an agent that has been observed rather than one never seen, which is the test the standing note SCORING/market_impact applies before a cell can be scored at all. Nothing about a subclinical human-origin respiratory virus reaches pork supply or demand. Level 1 confirmed, and the cell moves from inference to a direct statement. Consistent with Eric's confirmation, "c6l1, unlikely to present as an outbreak or multifarm infection, market is unlikely to respond".
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 this entry: "AMONG THE FOUR TYPES OF INFLUENZA VIRUSES, ONLY IAV IS OF ROUTINE CLINICAL SIGNIFICANCE IN SWINE." No treatment for influenza C in pigs is described anywhere. The fifteen Beijing isolates came from apparently healthy pigs. shic_2021_flu_cd: "There is no treatment for influenza virus infection in swine. Antimicrobials may be used to treat secondary infections."
How this level was decided
Level 1 on the second clause -- animals recover acceptably without treatment, because on the evidence held they do not become ill. Eric's rule of 2026-09-02 bars the answer "nobody treats viral disease" and that is not the answer given here: C7 asks whether outcomes would meaningfully improve if a treatment existed, and no adverse outcome has been recorded in a pig infected with influenza C. This cell moves with C5 if one ever is. Level 1 held. There is no product, but under the standing note that is not what decides the level: with no natural clinical illness in pigs there is nothing for a treatment to improve, which is the "absent or subclinical" clause.
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 C in pigs is mentioned anywhere in chapter 32; 32.10 describes the swine influenza vaccine market and every product in it targets influenza A. ch32 32.1, the sentence that governs this entry: "AMONG THE FOUR TYPES OF INFLUENZA VIRUSES, ONLY IAV IS OF ROUTINE CLINICAL SIGNIFICANCE IN SWINE." shic_2021_flu_cd: "There are no swine vaccines for ICV or IDV." "Vaccination is an important component of IAV-S control, but there are no swine vaccines for other influenza species." "Cross-protection does not occur between influenza species."
How this level was decided
Level 1 on the "not needed" clause of the 2026-08-28 rewording. There is no porcine influenza C vaccine and none is in development, so the question is whether the disease would justify one, and a virus recovered from clinically normal pigs and stated by the textbook to be of no routine clinical significance does not. Consistent with C5 at level 1 and C7 at level 1 on the same evidence. Level 1 held, and this is a supported use of it rather than the unsafe default the standing note forbids: the factsheet states affirmatively that natural infection causes no clinical illness in pigs, so "not needed" is a claim about a characterised situation. The contrast is instructive -- vaccination matters for influenza A in the same species, so the absence of an ICV vaccine is a judgment about ICV rather than about the difficulty of making influenza vaccines.
Levels and evidence are generated from data/assignments/influenza_c_virus.yml; the overview is authored in data/overviews/influenza_c_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.