Mammalian Orthoreovirus
LEVELS: Highly unlikely; Mild; Already in the herd; No validated assay; Minor; Negligible; Little; Available, or not needed
| Register id | porcine_reoviruses |
| Type | virus |
| Scientific name | Mammalian orthoreovirus |
| NCBI taxid | 351073 |
| Evidence | 2 document(s) |
| Assigned | 2026-09-06, against criteria version 093366e352a2 |
Overview
Mammalian orthoreovirus is the virus that gave rise to the word 'orphan' in virology — REO stands for respiratory and enteric orphan, because when it was found it appeared to cause nothing. Its clinical significance has since been established in some species, but the reference chapter is explicit that its role in disease in pigs still needs evaluation. Reoviruses are widely distributed in the environment, common in river water, sewage and standing water, and are transmitted by both the respiratory and the faecal-oral route. Types 1 to 3 have been found in the respiratory tract, gut and nervous system of healthy pigs as well as diseased ones, and healthy pigs are commonly infected with type 2 — which is exactly why finding one explains nothing. Since its status as a primary pathogen is debatable, a diagnostic investigation that turns up reovirus should go on to assess what else is contributing to the clinical picture. Reoviruses cause a variety of human illnesses and zoonotic transmission has been postulated, mainly between bats and people, but the evidence for it remains limited and no pig-to-human transmission is described.
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. ch38 38.3.1 gives the entry its character in one line: "The term 'REO' IS AN ACRONYM FOR 'RESPIRATORY AND ENTERIC ORPHAN' BECAUSE THE VIRUS WAS THOUGHT TO LACK CLINICAL SIGNIFICANCE. As reovirus research continued, its clinical significance was established, ALTHOUGH THE ROLE OF REOVIRUSES IN CAUSING DISEASE IN PIGS NEEDS FURTHER EVALUATION." ch38 38.3.3, the whole Public Health section: "REOVIRUSES CAUSE A WIDE VARIETY OF HUMAN DISEASES AND ZOONOTIC TRANSMISSION HAS BEEN POSTULATED, ESPECIALLY BETWEEN BATS AND HUMANS. HOWEVER, EVIDENCE OF ZOONOTIC TRANSMISSION REMAINS LIMITED." No pig-to-human transmission of any kind is described. 38.3.4 on where these viruses are found: "commonly found in river water, untreated sewage water, and stagnant water, reflecting FECAL CONTAMINATION FROM INFECTED INDIVIDUALS", and "NEARLY ALL MAMMALIAN SPECIES, as well as some reptilian and avian species, ARE SUSCEPTIBLE to reoviruses" (38.3.2). shic_orthoreovirus: "Zoonotic transmission of orthoreoviruses is possible, although sustained human-to-human transmission has not yet been observed... There have been no reports of porcine MRV infecting humans."
How this level was decided
Level 1. This is the pig-attribution question again and it resolves the same way it did on Actinobacillus suis and Staphylococcus hyicus, which you ruled to level 1 today: human disease with the organism is real, and a link attributing PIGS as the source is absent. The chapter names the species pair it thinks matters -- bats and humans -- and says even that evidence remains limited. Reoviruses infect nearly every mammal and are ubiquitous in surface water and sewage, so a human infection has no shortage of non-pig sources. Level 1 confirmed. Orthoreoviruses in general can reach people; porcine MRV never has.
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
INFERRED — reasoned, not stated. The evidence implies this level rather than stating it. The reasoning is below.
Corrected by Eric from L1 No human illness. Reason: MOVED L1 -> L2. "c2l2, we will hedge to slightly higher risk L2 based on broad view of the human evidence" -- answering the pass's flag, which had held level 1 on the ground that ch38 says reoviruses "cause a wide variety of human diseases" of the FAMILY rather than of the porcine members, and that no illness is attributed to a porcine reovirus. HE TAKES THE BROADER VIEW DELIBERATELY AND SAYS SO, which is worth recording as a reading of C2: where the agent is a group and the family plainly sickens people, hedge upward rather than score the silence. Human reovirus infection is usually mild -- level 2.
Basis. ch38 38.3.1 gives the entry its character in one line: "The term 'REO' IS AN ACRONYM FOR 'RESPIRATORY AND ENTERIC ORPHAN' BECAUSE THE VIRUS WAS THOUGHT TO LACK CLINICAL SIGNIFICANCE. As reovirus research continued, its clinical significance was established, ALTHOUGH THE ROLE OF REOVIRUSES IN CAUSING DISEASE IN PIGS NEEDS FURTHER EVALUATION." ch38 38.3.3: human reovirus disease exists, but "evidence of zoonotic transmission remains limited" and the postulated route is between bats and humans. No human illness acquired from a pig is described anywhere. shic_orthoreovirus: "Illness is most commonly mild and self-limiting; however, more severe respiratory or neurologic forms of the disease have occurred."
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. FLAG. ch38 says reoviruses "cause a wide variety of human diseases", but that is the family rather than the porcine members, and no illness is attributed to a porcine reovirus. Level 1 held on the agent as registered.
PREVIOUS ANSWER, written under the pig-attribution reading: Level 1, following C1. Under C2 as rescoped, level 1 says pigs are not a source of human illness with this agent. IT DOES NOT SAY REOVIRUSES ARE HARMLESS TO PEOPLE -- the chapter says they cause a wide variety of human diseases -- and the printed label must be the full one if this entry is ever excerpted. Level 2 confirmed, and this is Eric's own hedge on this entry -- "we will hedge to slightly higher risk L2 based on broad view of the human evidence". The factsheet puts the same picture in one sentence: mostly mild and self-limiting, occasionally severe.
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. ch38 38.3.1 gives the entry its character in one line: "The term 'REO' IS AN ACRONYM FOR 'RESPIRATORY AND ENTERIC ORPHAN' BECAUSE THE VIRUS WAS THOUGHT TO LACK CLINICAL SIGNIFICANCE. As reovirus research continued, its clinical significance was established, ALTHOUGH THE ROLE OF REOVIRUSES IN CAUSING DISEASE IN PIGS NEEDS FURTHER EVALUATION." ch38 38.3.4: "MAMMALIAN REOVIRUSES ARE WIDELY DISTRIBUTED and commonly found in river water, untreated sewage water, and stagnant water... Reoviruses are transmitted via RESPIRATORY AND FECAL-ORAL ROUTES. REOVIRUS TYPES 1-3 HAVE BEEN FOUND IN SWINE POPULATIONS IN INDIA, ITALY, THE UNITED STATES, JAPAN, SOUTH KOREA, AND CHINA." 38.3.6: "HEALTHY SWINE ARE COMMONLY INFECTED WITH TYPE 2 REOVIRUS." 38.3.2 on persistence: "Reoviruses are STABLE AT ACIDIC CONDITIONS (pH 3) AND RESISTANT TO MANY DISINFECTANTS, including ether and chloroform... resistant to extremes in environmental conditions, SURVIVING IN WASTEWATER FOR EXTENDED PERIODS." 38.3.9 records the practical consequence: "the discovery of chloroform- and trypsin-resistant, thermostable MRV isolates in United States swine indicated that EXISTING BIOSECURITY PRACTICES MAY BE INSUFFICIENT TO CONTROL REOVIRUS SPREAD IN SWINE." shic_orthoreovirus: "Orthoreoviruses are ubiquitous, infecting a large variety of animal species over a wide geographic range." "A novel pathogenic strain of serotype 3 (MRV3) was reported in U.S. swine herds in 2015." "the recently discovered U.S. strains are stable at 56C and remain viable after exposure to 80C or 90C for up to one hour."
How this level was decided
Level 1. Healthy US pigs are commonly infected, the virus is present in swine populations on three continents including this one, and it is ubiquitous in water and sewage besides. There is no introduction event to prevent. NOTE THE SENTENCE THAT LOOKS LIKE A LEVEL 4 ARGUMENT AND IS NOT: existing biosecurity may be insufficient to control reovirus SPREAD in swine. That is about containing an agent already inside the herd, not about excluding one from outside it, and C3 asks only the second question -- the same distinction that kept Streptococcus zooepidemicus off level 4 on the depopulation-failure statistic. Level 1 confirmed. In US herds since 2015, and the US strains are extraordinarily heat stable -- viable after an hour at 90C, which puts them beyond ordinary thermal treatment.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c4l4, likely to require unvalidated tests and research labs"
Basis. ch38 38.3.1 gives the entry its character in one line: "The term 'REO' IS AN ACRONYM FOR 'RESPIRATORY AND ENTERIC ORPHAN' BECAUSE THE VIRUS WAS THOUGHT TO LACK CLINICAL SIGNIFICANCE. As reovirus research continued, its clinical significance was established, ALTHOUGH THE ROLE OF REOVIRUSES IN CAUSING DISEASE IN PIGS NEEDS FURTHER EVALUATION." ch38 38.3.8, the whole diagnostic section: "SINCE THE ROLE OF REOVIRUSES AS A PRIMARY PATHOGEN IS DEBATABLE, the diagnostic investigation should assess the contributions of other respiratory and intestinal pathogens to the clinical picture. REOVIRUS DETECTION RELIES ON RT-PCR OR VIRAL ISOLATION. COMMERCIAL ANTIBODY ASSAYS ARE NOT AVAILABLE FOR REOVIRUSES, but VIRAL TYPING CAN BE DONE VIA VIRUS NEUTRALIZATION AND HEMAGGLUTINATION INHIBITION TESTS USING REFERENCE ANTISERA to the reovirus types. VIRUS TYPING CAN ALSO BE DONE BY SEQUENCING THE S1 GENE. More recently, WHOLE GENOME SEQUENCING of the 10 dsRNA segments has been used to subtype the virus." Isolation is a research-laboratory procedure: "MOUSE FIBROBLAST L929 CELLS ARE MOST COMMONLY USED to isolate reoviruses" (38.3.2). On the clinical side there is nothing to prompt a test: "reovirus infections have been found in the respiratory, gastrointestinal, and nervous systems of HEALTHY AND DISEASED PIGS." shic_orthoreovirus: "Reverse transcription polymerase chain reaction (RT-PCR) with primers specific to the S1 and L1 gene is used for diagnosis of MRV3 in swine." "Direct and indirect IFA specific to MRV3 are also available. The National Veterinary Services Laboratory (NVSL) in Ames, Iowa currently offers virus isolation and IFA for diagnosis of reovirus in swine." "Serum neutralization or hemagglutination inhibition (HI) are necessary for serotype identification."
How this level was decided
INFERRED, AND FLAGGED, because it is the same L3-or-L4 question you settled on other_actinobacillus_species today and I want it settled the same way rather than differently. LEVEL 4 IS SCORED: no commercial assay of any kind exists, there is no antibody test, typing needs reference antisera or S1 sequencing or whole-genome sequencing, isolation runs in mouse fibroblasts, and no clinical picture would prompt anyone to ask. That is the level 4 label -- no pathogen-specific test of known reliability, identification depending on sequencing. THE ARGUMENT FOR LEVEL 3 is your own ruling on the other actinobacilli: "conceivable that many labs could isolate and type to genus level but MAY LIKELY REQUIRE ASSISTANCE FROM A REFERENCE LABORATORY TO CONFIRM THE SPECIES." If US diagnostic laboratories run a reovirus RT-PCR in practice and send out only for typing, this is level 3. If reovirus is simply not on any panel here, level 4 stands. One line either way. *** LEVEL 4 HELD BUT THIS IS THE STRONGEST FLAG IN THE WHOLE ROUND. *** The factsheet names a laboratory that CURRENTLY OFFERS diagnosis of reovirus in swine -- NVSL in Ames -- with virus isolation and IFA. Level 4 means no reliable test exists anywhere, and a named national laboratory offering the test is close to a direct contradiction of that. On the 2026-08-29 axis, an assay you can send a sample to is validated and available, and the question becomes only WHERE it can be run: NVSL is a reference laboratory, which is level 3 exactly. I have not moved it because a level change on a signed-off entry is Eric's call, but of the several C4 flags in this round this is the one I would expect him to move.
C5 Production cost
Financial impact on the infected farm's cost of production
Minor: Small and generally short-lived losses with little effect on overall cost of production
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c5l2, experimental evidence shows can cause disease but apparently is a minor player in field settings"
Basis. ch38 38.3.1 gives the entry its character in one line: "The term 'REO' IS AN ACRONYM FOR 'RESPIRATORY AND ENTERIC ORPHAN' BECAUSE THE VIRUS WAS THOUGHT TO LACK CLINICAL SIGNIFICANCE. As reovirus research continued, its clinical significance was established, ALTHOUGH THE ROLE OF REOVIRUSES IN CAUSING DISEASE IN PIGS NEEDS FURTHER EVALUATION." ch38 38.3.6, and the evidence points both ways within one paragraph: "Reovirus infections (types 1-3) have been found in the respiratory, gastrointestinal, and nervous systems of HEALTHY AND DISEASED PIGS. HEALTHY SWINE ARE COMMONLY INFECTED WITH TYPE 2 REOVIRUS. EXPERIMENTAL INFECTION INDUCED TRANSIENT FEBRILE REACTIONS, BUT NOT OTHER CLINICAL DISEASE." 38.3.7: "GROSS LESIONS ARE RARELY ASSOCIATED WITH REOVIRUS INFECTIONS IN SWINE." Against that: "MRV3 WAS SHOWN TO BE ASSOCIATED WITH ENTERITIS, PNEUMONIA, AND ENCEPHALITIS IN PIGS" (38.3.4); "Type 3 reovirus produced gross lesions of CATARRHAL ENTERITIS AND INTUSSUSCEPTION in piglets, with clinical signs of diarrhea, lethargy, and weight loss"; and 38.3.5, "A RECENT STUDY IDENTIFIED A NOVEL REASSORTANT REOVIRUS IN PIGLETS THAT WAS ASSOCIATED WITH FEVER, DIARRHEA, NEUROLOGICAL SIGNS ("DOWN AND PEDDLING"), NASAL DISCHARGE, AND ~40% MORTALITY (Wang et al. 2021a)." Reovirus has also been recovered from aborted fetuses, and experimental inoculation of seronegative sows yielded mummified and stillborn pigs. shic_orthoreovirus: "Enteric disease is observed in swine infected with some strains of MRV... Along with enteritis, pneumonia and reproductive failure have also been reported in infected pigs. Encephalitis may occur, but appears to be uncommon." "Young pigs are more susceptible to MRV infection than immunocompetent adults."
How this level was decided
INFERRED. Level 2 -- small, sporadic losses. WHAT KEEPS IT OFF LEVEL 1: the disease reports are real and specific, not rumours -- catarrhal enteritis with intussusception, encephalitis confirmed histologically in a piglet, and a reassortant associated with 40% mortality. WHAT KEEPS IT OFF LEVEL 3 IS YOUR MENANGLE RULE: "a level is a claim about the general case, and one outbreak cannot support the general case however bad that outbreak was." The 40% mortality is one study of one novel reassortant. The general case is stated plainly elsewhere in the same chapter -- healthy pigs are commonly infected, experimental infection produces transient fever and nothing else, and gross lesions are rare. NOTE FOR ANY BRIEF: the 40% figure belongs with its qualifier or not at all. Level 2 confirmed.
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, currently present in healthy pigs, no reason to think diagnosis would trigger market imnpact"
Basis. ch38 38.3.1 gives the entry its character in one line: "The term 'REO' IS AN ACRONYM FOR 'RESPIRATORY AND ENTERIC ORPHAN' BECAUSE THE VIRUS WAS THOUGHT TO LACK CLINICAL SIGNIFICANCE. As reovirus research continued, its clinical significance was established, ALTHOUGH THE ROLE OF REOVIRUSES IN CAUSING DISEASE IN PIGS NEEDS FURTHER EVALUATION." Reovirus types 1-3 are present in US swine populations (ch38 38.3.4) and healthy pigs are commonly infected. No trade measure, movement restriction or consumer response to a porcine reovirus diagnosis is recorded anywhere in the chapter.
How this level was decided
INFERRED. Level 1 under the standing note SCORING/market_impact: an agent already resident and reported in US pigs with no market move ever observed. Nothing about a virus that healthy pigs commonly carry would reach a market. Unchanged.
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. "c7l1, insufficient clinical signficance to warrant treatment"
Basis. ch38 38.3.1 gives the entry its character in one line: "The term 'REO' IS AN ACRONYM FOR 'RESPIRATORY AND ENTERIC ORPHAN' BECAUSE THE VIRUS WAS THOUGHT TO LACK CLINICAL SIGNIFICANCE. As reovirus research continued, its clinical significance was established, ALTHOUGH THE ROLE OF REOVIRUSES IN CAUSING DISEASE IN PIGS NEEDS FURTHER EVALUATION." ch38 38.3.9: "given the lack of consistent clinical data and few clinical signs in pigs, VERY LITTLE INFORMATION IS AVAILABLE regarding reovirus immunity in swine and LITTLE EMPHASIS IS PLACED ON PREVENTION AND CONTROL of reovirus infections. LITHIUM CHLORIDE (LiCl) HAS SHOWN POTENTIAL AS AN ANTIVIRAL AGENT AGAINST MRV3 IN VITRO, necessitating further studies to assess its in vivo potential." On outcome without treatment, 38.3.6: "Experimental infection induced TRANSIENT FEBRILE REACTIONS, BUT NOT OTHER CLINICAL DISEASE", and 38.3.4 records that immunity persists about 11 weeks.
How this level was decided
INFERRED. Level 1 on the second clause of the label -- animals recover acceptably without treatment. The ground is the experimental finding, not the absence of a product: inoculated pigs get a transient fever and nothing else, which is an examined outcome rather than a silence. NOT SCORED LEVEL 1 BECAUSE NOBODY TREATS VIRAL DISEASE, which your 2026-09-02 rule forbids. NOTE THE ONE THING THAT WOULD MOVE IT: if the Wang 2021 reassortant with 40% mortality and neurological signs turns out to represent the general case rather than an outlier, a treatment would plainly be worth having, and this cell would follow C5 upward. Unchanged.
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 clinical imperative to develop vaccine"
Basis. ch38 38.3.1 gives the entry its character in one line: "The term 'REO' IS AN ACRONYM FOR 'RESPIRATORY AND ENTERIC ORPHAN' BECAUSE THE VIRUS WAS THOUGHT TO LACK CLINICAL SIGNIFICANCE. As reovirus research continued, its clinical significance was established, ALTHOUGH THE ROLE OF REOVIRUSES IN CAUSING DISEASE IN PIGS NEEDS FURTHER EVALUATION." No vaccine against porcine reovirus is described anywhere in the chapter, and none is in use. ch38 38.3.9: "LITTLE EMPHASIS IS PLACED ON PREVENTION AND CONTROL of reovirus infections... Greater understanding of MRV pathogenesis and epidemiology in swine is critical for implementing appropriate prevention and control measures, SUCH AS VACCINATION STRATEGIES, biosecurity practices, and surveillance." Pigs do develop immunity from natural infection, "which can protect to some degree against subsequent exposures", lasting about 11 weeks. shic_orthoreovirus: "There is no vaccine for MRV in swine."
How this level was decided
INFERRED. Level 1 on the "not needed" clause, by the FIRST route in the standing note SCORING/vaccine_availability -- low clinical significance. The claim is made affirmatively rather than by default, which is what your PCV4 rule requires: healthy pigs commonly carry this virus, experimental infection produces transient fever alone, gross lesions are rare, and the chapter itself places little emphasis on prevention and control. THE SENTENCE THAT ARGUES FOR LEVEL 3 IS QUOTED ABOVE AND IS WORTH WEIGHING: the chapter says greater understanding is critical for implementing measures SUCH AS VACCINATION STRATEGIES. That is a call for research into whether a vaccine is warranted, not a finding that one is warranted and missing -- which is what level 3 asserts. IF C5 MOVES UP, THIS CELL SHOULD BE REVISITED WITH IT. Level 1 held.
Levels and evidence are generated from data/assignments/porcine_reoviruses.yml; the overview is authored in data/overviews/porcine_reoviruses.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.