Influenza D Virus
LEVELS: Rarely occurs; No human illness; Housing alone keeps it out; Reference or research laboratory required; Negligible; Negligible; Little; Available, or not needed
| Register id | influenza_d_virus |
| Type | virus |
| Scientific name | Influenza D virus |
| NCBI taxid | 1511084 |
| Evidence | 11 document(s) |
| Assigned | 2026-09-06, against criteria version 093366e352a2 |
Overview
Influenza D was first isolated in 2011 from a pig with influenza-like illness in Oklahoma, but cattle were quickly recognised as its primary reservoir — archived samples show it circulating in US cattle as early as 2004 — and the pig is an incidental spillover host. Experimental infections indicate it does not by itself cause significant illness in healthy animals: inoculated five-week-old pigs and feral pigs developed no clinical signs or meaningful gross lesions, though some ran a fever or showed mild microscopic changes in the airway, and preliminary evidence suggests the virus does not persist long-term under modern swine farming. Surveys find limited exposure in domestic pigs against high seroprevalence in wild boar in some regions. In people, exposure is common and appears to cause nothing — one study found the non- occupational control group more seropositive than cattle workers, and documented human infections are subclinical, possibly because the virus provokes a notably muted immune response. Detection requires looking for it deliberately with influenza D-specific assays, because the standard swine influenza tests do not find it. There is no swine vaccine and no programme to develop one.
C1 Zoonotic potential
Likelihood of transmission from pigs to humans
Rarely occurs: Human infection from pigs is plausible but has only rarely been reported, and specific control points are not commonly implemented to prevent transmission to humans
Basis. The human exposure data in this folder are about CATTLE, not pigs. Trombetta 2026 sampled cattle workers, and found the non-occupational control group MORE seropositive (65.0%) than the workers (42.9%), concluding "IDV exposure is not restricted to occupational settings involving direct contact with cattle". Sanders 2026 places the reservoir the same way -- IDV was "first identified in 2011 after being isolated from pigs with influenza-like illness in Oklahoma, with cattle later recognized as the primary reservoir" -- and reports ambient detections, "aerosols captured at an airport and a hospital". The one pig-linked human datum is a single case: "genetic material for IDV has been confirmed in a nasal swab from a pig farmer". On the pig side, Falsini 2025 found swine "demonstrated limited exposure, suggesting they are infrequent spillover hosts", though Lee 2019 showed pig-to-pig transmission does occur. shic_2021_flu_cd: "Whether IDV is zoonotic is not definitely known. Human infections have not been reported, but people can develop antibodies to the virus, particularly those with cattle exposure." "Cattle are the primary host." "IDV replicates in ferret and guinea pig models, as well as human airway epithelial cells." "people with and without occupational exposure to cattle can become seropositive."
How this level was decided
RE-READ AND HELD AT L2 2026-09-04 AGAINST THE REWORDED LABELS. Exactly one pig-attributed human datum exists -- "genetic material for IDV has been confirmed in a nasal swab from a pig farmer" -- which is the definition of rarely reported. The rest of the human exposure data in the folder is about cattle.
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: MOVED FROM LEVEL 4, AND THIS IS A CORRECTION OF MY OWN ERROR, NOT NEW EVIDENCE. C1 was renamed "transmission FROM PIGS to humans" on 2026-08-26 on Eric's chikungunya ruling. This cell was scored L4 the same day, against the old label, on evidence that IDV reaches humans -- human airway replication, a ubiquitous lung receptor, ambient aerosols, high seroprevalence. All true, and none of it about pigs. It is precisely the error the ruling was made to fix, repeated on the next entry. Scored from pigs: pigs are an infrequent spillover host of a cattle virus, they do shed and transmit to each other, and exactly one human case has ever been linked to pig contact. Human infection from pigs is therefore plausible but rarely reported, and no control point is implemented for swine workers -- both clauses of level 2. Not level 3, which needs a KNOWN risk requiring a control point to fail; one nasal swab is not that, and there is no control point to fail. NOTE FOR ERIC: this changes a level on an entry he has signed off, so it will show as REVIEW AGAIN. He confirmed C6 and C7 on this entry, not C1, so no ruling of his is overridden. Level 2 held, and the reasoning is Eric's West Nile ruling applied to a second agent: "pigs don't really seem to be responsible for human cases but it certainly is plausible, so risk should be greater than l1." No human IDV infection has been reported at all, which alone would read as level 1 -- but people seroconvert, the virus replicates in human airway epithelial cells, and pigs are infected. The attributable exposure in the seroprevalence studies is CATTLE, not pigs, which is what keeps this at 2 rather than higher.
C2 Zoonotic impact
Severity of human illness caused by the agent
No human illness: Agent does not cause illness in people
Basis. Sanders 2026: IDV "has attracted limited attention because it is not yet known to cause clinical disease in humans", and "human infections documented so far appear subclinical". The paper offers a mechanism rather than an absence: "most IDV isolates tested elicited a muted innate immune response... The muted host response elicited by IDV may result in limited immunopathology, potentially explaining why IDV infections in humans are currently asymptomatic or subclinical, despite evidence of repeated zoonotic exposure." shic_2021_flu_cd: "Human infections have not been reported."
How this level was decided
Level held at 1, but this is no longer INFERRED. The previous assessment reasoned from silence -- no folder paper reported human illness -- and flagged that absence of reported disease is not evidence of no disease. Sanders states the position directly and, more importantly, explains it: the muted interferon response limits immunopathology, which is why exposure this widespread has produced no recognised illness in fifteen years. C2 says to score level 1 "whenever the agent does not cause disease in people". The paper warns the profile could shift with adaptation, but general_rule forbids scoring hypothetical future variants. Level 1 confirmed. No human illness has ever been described for this agent.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Housing alone keeps it out: Raising pigs indoors and off soil breaks the cycle, so no ongoing biosecurity program is needed and only outdoor herds stay exposed
Basis. Falsini 2025: wild boars "exhibited high titers (>=40), with an overall seroprevalence (>=10) of 80%", against 31% and 29% in the other species surveyed; swine "demonstrated limited exposure, suggesting they are infrequent spillover hosts". Cattle are the maintenance host throughout -- Sanders 2026: IDV "was first identified in 2011 after being isolated from pigs with influenza-like illness in Oklahoma, with cattle later recognized as the primary reservoir", with "archival samples confirming circulation in US cattle as early as 2004". Kwasnik 2023 confirms IDV in feral pigs, wild boars and other wildlife. shic_2021_flu_cd: "IDV is primarily found in cattle. However, it has been detected in domestic and feral swine, as well as other species." "IDV has been identified in swine in France, Ireland, Italy, Luxembourg, China, and the United States." "IDV was initially detected in U.S. swine with influenza-like illness in 2011." "Most studies have found that seroprevalence of ICV and IDV in swine is relatively low." "IDV seems to be less transmissible in pigs compared to bovids."
How this level was decided
MOVED L5 -> L2 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. cattle are the maintenance host and pigs are "infrequent spillover hosts". Not housing pigs beside cattle closes it, which is the same shape as Eric's malignant catarrhal fever ruling on sheep.
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. Unchanged. Two reservoirs sit outside the pig industry and both are established in the US: cattle as the maintenance host, and feral swine and wild boar. Neither can be excluded by on-farm biosecurity. This is a US reservoir in the sense the level 4 label now requires, not an imported one. Sanders adds US swine circulation without changing the level -- IDV was recovered from national jackpot shows in Kentucky, Ohio and Georgia between 2017 and 2020. Level 2 held, and the new evidence is worth stating because it looks at first like an argument for level 1: IDV HAS been detected in US swine, since 2011. It is not, because level 1 means there is no introduction event to prevent, and here there plainly is one -- the reservoir is cattle, seroprevalence in pigs is low, and the virus is less transmissible pig-to-pig than in bovids, so each appearance in a herd is a fresh spillover. Housing pigs away from cattle is what closes that route, which is the level 2 claim.
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. "l3, may have indicative diagnosis at L2 lab but confirmation almost certainly would involve l3 lab" Transcribed by the assistant and confirmed by Eric on 2026-08-31 -- "i confirm the six". The harvester refused it because the comment names two levels, but the second is a noun phrase for a level-N-capable laboratory rather than a second assignment.
Basis. Sanders 2026 found IDV only by going back and looking for it specifically: "some of the IAV-negative samples collected between 2018 and 2020 from swine with clinical signs were rescreened using IDV-specific primers and probes... A total of 446 samples in 2018 and 497 samples in 2020 were screened for IDV." The 2017 detection was likewise "one rRT-PCR-positive, but IAV-isolation-negative sample collected from a subclinical infection". Detection in every folder study rests on IDV-specific rRT-PCR, hemagglutination inhibition or virus neutralisation run in a university or reference laboratory. Chapter 32 gives IDV a single sentence and describes no swine diagnostic for it at all. shic_2021_flu_cd: "Reverse transcriptase polymerase chain reaction (RT-PCR) assays based on the highly conserved PB1 region have been used to detect IDV in swine samples. There are a few multiplex RT-PCR assays that detect all influenza species. Immunohistochemistry has also been used to detect IDV antigen in swine tissues." But "There is no long-term surveillance for these species, and standard laboratory tests for influenza do not detect emerging strains."
How this level was decided
Unchanged at level 3, and the new paper is the clearest demonstration of why. Routine influenza diagnostics do not find IDV: these isolates surfaced only because IAV-negative samples were deliberately rescreened with IDV-specific primers. A validated assay does exist, so this is not level 4, but it is not something a local or regional laboratory runs -- confirmation depends on a reference or research laboratory. Level 3 confirmed. Assays exist and have been used on swine samples, so this is not level 4 -- but the routine influenza panel a diagnostic laboratory runs does not detect IDV, so finding it takes a laboratory that has chosen to look. That is the level 3 shape after the 2026-08-29 rework: the axis is where a validated assay can be run, not whether one is theoretically possible.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
Basis. CFSPH 2021: "experimental infections suggest that, by itself, influenza D does not cause major illness in healthy animals." "Experimentally infected 5-week-old pigs and feral pigs did not develop any clinical signs or significant gross lesions, though some animals had a fever and/or mild microscopic lesions in the respiratory tract." "preliminary evidence suggests that they do not persist long term under modern swine farming methods." shic_2021_flu_cd: "An experimental study found that intranasal inoculation of 10-week-old pigs with IDV failed to produce clinical disease. In feral swine inoculated intranasally with IDV, pigs developed viremia, but none became clinically ill. No postmortem lesions have been associated with IDV in pigs." "clinical illness occurs rarely, if at all."
How this level was decided
MOVED L2 -> L1 on the new factsheet and the standing note together. Experimental infection of pigs produced no clinical signs and no significant gross lesions, and the virus does not appear to persist under modern swine farming -- which is the standing note SCORING/production_cost_impact answered directly by the document rather than inferred. IDV was first found in pigs with respiratory signs, but those were co-infections; by itself it does not make pigs ill. No measurable effect on cost of production. Level 1 confirmed. Two experimental inoculations, one in domestic and one in feral swine, produced no clinical disease and no lesions.
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. "agree c6l1". IDV is not notifiable, carries no trade restriction, and causes little or no overt disease in pigs, so an occurrence on a farm has no route to disrupting supply or demand.
Basis. CFSPH 2021 records no trade measure, movement control or consumer response associated with influenza D in swine, and describes the virus as widespread in cattle with pigs an incidental host. 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
Level 1 unchanged, now supported. IDV is already present in US cattle and swine and has been since at least 2011 without any market effect, which is the standing note SCORING/market_impact returning level 1 on an agent already here and already diagnosed. Level 1 confirmed, now on a direct statement rather than inference. IDV has been in US swine since 2011 with no market consequence, which is the endemic-and-already-diagnosed case the standing note puts at level 1.
C7 Treatment potential
Potential for treatment to improve outcomes
Little: Effective pathogen-directed treatment is available and works, or animals recover acceptably without it
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "agree c7l1". Cattle are the only species in which IDV produces reproducible respiratory disease; pigs recover acceptably without treatment, so the opportunity is negligible.
Basis. CFSPH 2021: "by itself, influenza D does not cause major illness in healthy animals", and experimentally infected pigs "did not develop any clinical signs or significant gross lesions". shic_2021_flu_cd: "There is no treatment for influenza virus infection in swine."
How this level was decided
Level 1 unchanged but now SUPPORTED rather than inferred. The previous score reasoned from the absence of disease; the factsheet states it. The second clause of level 1 is met -- animals do acceptably without treatment because there is no illness to recover from. Level 1 held, on the absent-disease clause rather than on the absence of a product.
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 imperative given it appears to have very limited penetration into the pig industry"
Basis. The folder documents no swine IDV vaccine and no programme to develop one. Swine are an infrequent spillover host of a cattle virus, C2 is level 1 no human disease, and the pig disease is mild. shic_2021_flu_cd: "There are no swine vaccines for ICV or IDV." "Cross-protection does not occur between influenza species, including ICV and IDV."
How this level was decided
MOVED L3 -> L1 by the rewording. INFERRED. No vaccine exists, but IDV causes little disease in pigs and none in people, and cattle are the reservoir any vaccination programme would have to target. A swine IDV vaccine is not a justified unmet need. NOTE: this entry is separately stale on evidence and awaits a re-read. Level 1 held. No clinical or market imperative: experimental infection of pigs produces no disease, so there is nothing for a vaccine to prevent in this species.
Levels and evidence are generated from data/assignments/influenza_d_virus.yml; the overview is authored in data/overviews/influenza_d_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.