Porcine norovirus
LEVELS: Highly unlikely; Mild; Already in the herd; No validated assay; Negligible; Negligible; Little; Available, or not needed
| Register id | porcine_norovirus |
| Type | virus |
| Scientific name | Norovirus |
| NCBI taxid | 142786 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Porcine noroviruses are relatives of the human noroviruses that cause winter vomiting disease, and they are extremely common in pigs — 97% of US pigs carry antibody to them. Almost nothing else about them is settled. Every field detection so far has been from pigs with no clinical signs, subclinically infected pigs are probably the natural reservoir, and little is known about any association with natural disease. One strain has been shown to replicate in germ-free pigs with mild diarrhoea. The virus has never been grown in the laboratory, so nothing is known about its basic properties, and no diagnostic test exists outside research settings — the sensitivity of the molecular assays used has never been evaluated, which means published prevalence figures are probably underestimates. Because they are likely to persist in the environment like rotaviruses, eliminating them from a herd is probably impossible. The public health question is genuinely open: porcine and human noroviruses are closely related genetically and antigenically, which raises the possibility of pigs acting as a reservoir, but there is no evidence that porcine noroviruses threaten human health directly.
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. ch26 26.3.1, covering both enteric genera: "Public health concerns over potential cross-species transmission and animal reservoirs for caliciviruses have been raised... However, the question is still open. BASED ON LIMITED DATA, THERE IS NO CLEAR EVIDENCE THAT KNOWN PORCINE CALICIVIRUSES POSE A THREAT TO HUMAN HEALTH." 26.3.2.2, the entry's own Public Health section: "The CLOSE GENETIC AND ANTIGENIC RELATIONSHIPS between human and porcine noroviruses raise public health concerns regarding the potential for cross-species transmission and THE POSSIBILITY FOR PIGS TO SERVE AS RESERVOIRS FOR HUMAN NOROVIRUSES. However, THERE IS NO EVIDENCE FOR THE DIRECT THREAT of porcine noroviruses to human health." The biology behind the concern, from 26.3.2.1 and 26.3.2.4: porcine noroviruses "are grouped with the GII HUMAN NOROVIRUSES but form three separate genotypes (11, 18, and 19) distinct from human noroviruses"; virus-like particles of two porcine strains "CROSS-REACT WITH ANTIBODIES AGAINST HUMAN GII NOROVIRUSES"; "human recombinant norovirus VLP binds specifically to histo-blood group antigens (HBGAs) in epithelial cells of PIG gastrointestinal tissue"; and in an experiment using GII.4 HUMAN norovirus, "48 (74%) of 65 gnotobiotic piglets developed mild diarrhea", with evidence of replication in intestinal enterocytes in 18 of 31 cases and seroconversion in 13 of 22, "indicating human norovirus replication in pigs".
How this level was decided
MOVED L2 -> L1, re-read 2026-09-04 AGAINST THE REWORDED LABELS. ch26 26.3.2.2: "THERE IS NO EVIDENCE FOR THE DIRECT THREAT of porcine noroviruses to human health." What the experiments demonstrate is HUMAN-TO-PIG infection. No human case has ever been attributed to a porcine norovirus.
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: Level 2, and the evidence for it is unusually specific for a level 2 cell. The label asks whether human infection from pigs is plausible but unreported, and every element of plausibility is present: the porcine viruses sit inside the human genogroup, their capsids cross-react with anti-human antibodies, human norovirus binds the receptor on pig gut, and human norovirus demonstrably replicates in pigs and makes them scour. What is absent is any case of the reverse. NOTE THE DIRECTION THE EXPERIMENTS ACTUALLY SHOW, because it matters for how this level is read: what has been demonstrated is HUMAN-TO-PIG infection, not pig-to-human. That still supports level 2 under C1 as rescoped, because a pig that can be infected by and shed a human strain is a route back to people -- which is the reservoir concern the chapter names -- but it is not a report of a porcine strain infecting anyone.
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. ch26 26.3.1: caliciviruses have a "recognized role in SPORADIC AND EPIDEMIC ACUTE GASTROENTERITIS IN HUMANS", which is the illness in question. 26.3.2.2: "there is no evidence for the direct threat of porcine noroviruses to human health." No human case attributable to a pig is described.
How this level was decided
Level 2, grading the illness that would follow if the route C1 calls plausible were ever taken. Norovirus in people is acute gastroenteritis: unpleasant, sometimes epidemic, and in the ordinary case self-limiting over a few days, with serious disease confined to the very young, the very old and the immunocompromised. That is the level 2 label. It is not level 1, because level 1 says pigs are not a source and the chapter is explicit that the question is open rather than closed; and it is not level 3, because norovirus gastroenteritis does not commonly require hospitalisation.
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
Basis. ch26 26.3.2.4: "THE SEROPREVALENCE OF GII NOROVIRUS IN SWINE WAS 97% IN THE UNITED STATES and 36% in Japan." 26.3.2.6: "Assuming that porcine norovirus epidemiology and immunology are similar to porcine rotaviruses, it is likely that these viruses PERSIST IN THE ENVIRONMENT and IT MAY BE IMPOSSIBLE TO ELIMINATE THE INFECTION FROM PIG HERDS AND PREVENT NATURAL INFECTION OF PIGLETS." 26.3.2.3: detection by RT-PCR is worldwide -- Japan, the Netherlands, the United States, Hungary, Italy, Belgium, Canada, China, South Korea, Slovenia, Brazil -- and "the geographical distribution indicates the WORLDWIDE OCCURRENCE of porcine noroviruses among pigs on farms". 26.3.2.1: "in general, noroviruses are characterized by STABILITY IN THE ENVIRONMENT and relative resistance to inactivation."
How this level was decided
Level 1, and the number that settles it is 97% seroprevalence in US swine. That is the level 1 label without qualification -- present in most herds, so there is no introduction event to prevent -- and the chapter reinforces it from the other side by saying elimination from a herd may be impossible and natural infection of piglets cannot be prevented. NOTE THE GAP BETWEEN SEROLOGY AND RT-PCR, which explains why the same virus looks rare in one place and universal in another: RNA was found in only 0.4 to 6% of pigs sampled, because those surveys caught a short shedding window in mostly adult animals using primers designed for human strains, while 97% of pigs carry antibody to having met it.
C4 Detection difficulty
Difficulty of recognizing and confirming infection
No validated assay: No pathogen-specific test of known reliability exists anywhere, and identification depends on sequencing or on primers that have not been critically evaluated
Basis. ch26 26.3.2.5, the whole Diagnosis subsection: "NO DIAGNOSTIC TESTS FOR PORCINE NOROVIRUSES HAVE BEEN DEVELOPED FOR USE OUTSIDE THE RESEARCH LABORATORY. Porcine noroviruses have been detected using electron microscopy, RT-PCR, and real-time RT-PCR. HOWEVER, THE SENSITIVITY OF MOLECULAR TESTS FOR LABORATORY DIAGNOSIS OF PORCINE NOROVIRUS INFECTIONS HAS NOT BEEN EVALUATED." 26.3.2.1: "Porcine noroviruses have not been cultured in vitro, and NOTHING IS KNOWN about their physiochemical and other biological properties." 26.3.2.3 notes that the prevalence figures are underestimates in part because "PCR PRIMERS DESIGNED FOR HUMAN NOROVIRUSES WERE USED".
How this level was decided
Level 4, and this entry is the framework's own worked example at that level. It is also the cleanest statement of what the C4 rework was for: primer sets exist and have been published, but the chapter says in terms that no test has been developed for use outside a research laboratory and that the sensitivity of the molecular tests has never been evaluated. Under the 2026-08-29 rewording, VALIDATION IS THE TEST -- an assay of unknown accuracy does not yield a trustworthy answer at all, which is why this sits at level 4 rather than at level 3. The detail that some surveys used primers designed for human noroviruses is the concrete form of the problem.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
Basis. ch26 26.3.2.4: "ALL PORCINE NOROVIRUSES WERE DETECTED FROM PIGS WITH NO CLINICAL SIGNS. Subclinically infected pigs may be natural reservoirs for noroviruses. The virus QW101/2003/US (genotype 18) replicated in gnotobiotic pigs with fecal shedding coincident with MILD diarrhea." 26.3.2.3: "LITTLE IS KNOWN about the association of porcine noroviruses with natural disease." 26.3.1: "Porcine caliciviruses have not been widely studied and MUCH IS UNKNOWN concerning their role in naturally occurring swine disease."
How this level was decided
Level 1, and the distinction between this cell and an unassessed one is worth stating because the chapter says both kinds of thing. The unassessable version would be "little is known about the association with natural disease", which is an admission of ignorance. But the chapter also makes a positive statement: EVERY field detection of a porcine norovirus has been in a pig with no clinical signs, and the only diarrhoea on record came from experimental inoculation of gnotobiotic piglets, which are germ-free animals with no competing flora and are not a field model. On the evidence held, no measurable effect on cost of production. This would move if a natural outbreak were ever attributed to it, as happened for sapovirus in 2022.
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 evidence to support a diagnosis will effect markets"
Basis. A subclinical enteric virus of worldwide distribution carried by nearly every US pig, with no trade measure, movement control or consumer response recorded anywhere in chapter 26.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact, an agent already endemic in US pigs with no market move on record is level 1. Nothing that is only ever found in pigs without clinical signs reaches pork supply or demand.
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. ch26 26.3.2.6: "TREATMENT WITH ORAL REHYDRATION FLUIDS IS LIKELY TO BE SUCCESSFUL." No antiviral is described, and no clinical disease requiring one is described: 26.3.2.4 records that all field detections were from pigs with no clinical signs.
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 in the first place. C7 asks what a treatment that does not exist would be worth, and Eric's rule of 2026-09-02 says the answer must not be "nothing, because nobody treats viral disease": the question is whether outcomes would meaningfully improve. Here there is no outcome to improve. NOTE THE DIFFERENCE FROM THE ENTERIC CORONAVIRUSES, which he put at level 3 -- TGEV, PEDV, PDCoV, SADS-CoV and pHEV are lethal in neonates, and this virus is not associated with disease at all. If porcine norovirus is later shown to cause natural disease, this cell moves with C5.
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 porcine norovirus is mentioned anywhere in chapter 26. 26.3.2.6 gives the control approach in full: elimination from herds is likely impossible, "it is also likely that sows pass on maternal antibody in colostrum and milk and thus limit infection and damage in the gut of nursing piglets", and oral rehydration is likely to be successful. 26.3.2.4: "Immune responses against porcine noroviruses, protective immunity, and/or the role of maternal antibodies HAVE NOT BEEN ASSESSED."
How this level was decided
Level 1 on the "not needed" clause of the 2026-08-28 rewording, not on availability -- no vaccine exists. The test is whether the disease would clinically or economically justify vaccinating, judged on the agent's own weight, and an infection found only in pigs without clinical signs does not. This is the reading Eric applied to Taenia solium on 2026-09-02: no need for the vaccine regardless of whether it would work. It is not level 3, which is reserved for a genuine gap where the disease would warrant a product.
Levels and evidence are generated from data/assignments/porcine_norovirus.yml; the overview is authored in data/overviews/porcine_norovirus.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.