Murray Valley Encephalitis Virus
LEVELS: Highly unlikely; Severe; Extraordinary biosecurity required; Reference or research laboratory required; Negligible; NOT ASSIGNED; Little; Available, or not needed
| Register id | murray_valley_encephalitis_virus |
| Type | virus |
| Scientific name | Murray Valley encephalitis virus |
| NCBI taxid | 11079 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Murray Valley encephalitis virus is a mosquito-borne flavivirus and an important cause of neurological disease in people in Australia, as well as fatal encephalitis in horses. Its natural cycle runs between Culex annulirostris mosquitoes and water birds — herons, pelicans, ducks and their relatives. The pig's role is unclear and probably minor. Serosurveys have found high seroprevalence in feral pigs in eastern Australia, but experimentally infected pigs produce only moderate to low levels of virus in the blood, and studies of what the vector mosquitoes actually feed on do not support pigs as an important source. It has been suggested that where feral pig densities are very high even a small proportion of adequately viraemic animals could contribute to maintaining the virus, but this remains a proposal. In the pig itself nothing happens: no disease or clinical signs following infection have ever been reported, and a pregnant sow inoculated experimentally farrowed normally with no sign of infection in the fetuses. There are no antivirals and no commercial vaccines.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. REVIEWED ON C1_C2_RECLASSIFICATION.md, 2026-09-04, after the C1/C2 rewording that separated exposure from consequence. Eric marked up every moved cell and reversed his own earlier setting on twenty of the twenty-one: "In almost every case, I agreed with new opinion and reversed my old setting." HIS MARKUP HERE WAS "l1". Reverses his 2026-09-02 level 2. ch30 30.3 says outright that pig involvement in the transmission cycle is unlikely, the vector prefers marsupials, and no human case is attributed to a pig. Scored with west_nile_virus as the pass said it should be.
Basis. ch30 30.3.2: "The role of pigs in the ecology of MVEV remains unclear. Serosurveys demonstrated a high prevalence of seropositive feral pigs in eastern Australia. However, pigs were found to produce only moderate to low-grade viremia following experimental infection." "It has been proposed that, in areas where feral pig densities are high, if only a small proportion of animals respond to infection with viremia capable of infecting mosquito vectors, a significant contribution may be made to the maintenance or amplification of MVEV. However, a study of host-feeding preferences of C. annulirostris in northern Australia showed that the majority of blood meals were obtained from marsupials, suggesting that host-seeking vectors are diverted away from feral pigs." 30.3: "it is considered unlikely that pigs are involved in the transmission cycles of these viruses."
How this level was decided
MOVED L2 -> L1, re-read 2026-09-04 AGAINST THE REWORDED LABELS. ch30 30.3: "it is considered unlikely that pigs are involved in the transmission cycles of these viruses", the vector prefers marsupials, and no human case has been attributed to a pig. Scored with west_nile_virus, as the previous assessment said it should be.
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: INFERRED, and the chapter argues both ways in one paragraph. Some pigs do make viraemia sufficient to infect C. annulirostris, which makes a pig-to-mosquito-to-human route plausible -- but the vector prefers marsupials, the chapter says outright that pig involvement in the transmission cycle is unlikely, and no human case has been attributed to a pig. Plausible, never reported, no control point implemented: level 2. Scored the same as WNV in this batch for the same reasons, and FLAGGED with it -- if Eric reads "the pig must be the source" strictly, both are level 1 and their C2 follows.
C2 Zoonotic impact
Severity of human illness caused by the agent
Severe: Infection carries a substantial risk of life-threatening illness, death, or permanent disability, even if human infections are uncommon
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c2l4, the reason MVV is on the radar in Australia is because of the severity of human infections, in high enough proportion of cases that it should not be ignored"
Basis. ch30 30.3.2: "MVEV is an important cause of arboviral neurological disease in humans in Australia and can cause fatal encephalitis in horses." The chapter gives no case-fatality or sequelae figures for human MVE.
How this level was decided
INFERRED, and it rests on the shape of the disease rather than on numbers the folder supplies. Murray Valley encephalitis is a neuroinvasive disease: clinical cases are encephalitis, and encephalitis carries a substantial risk of death or permanent disability, which is the level 4 test. FLAGGED as the least evidenced C2 in this batch. The counter-argument is the same one that holds WNV at level 2 -- the great majority of MVEV infections are asymptomatic -- but unlike WNV there is no common mild presentation between silence and encephalitis. Scored level 4 and level 3 is defensible.
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. CONFIRMED at level 4 on the C3 review sheet. "l4, if was present in the us, may very well take l4 biosecurity to deal with the vector" His level 3 ruling of 2026-09-02 is superseded by this one, made against the four-level scale. EARLIER RULING, 2026-09-02, kept because a later decision does not erase the reasoning it replaced: MOVED L5 -> L3 on 2026-09-02, REVERSING HIS OWN SHEET ENTRY of the same round. On the gaps sheet he had written "c3l5, all elements are in the US to make it a challenge to control if an incursion occurred". He then went looking for the evidence and could not find it: "MVE - can't really find much literature but we can change C3 to match the hendra setting." Hendra sits at C3 level 3 on his 2026-08-26 ruling -- an agent arriving here brings itself, not its ecosystem, so a reservoir that exists only in the agent's native range does not follow it. The assessment had scored level 5 on the argument that US Culex already transmit two viruses of the same serogroup and that the water-bird hosts are abundant here, and had flagged the gap in that argument: NO STUDY ANYWHERE IN THE FOLDER TESTS A NORTH AMERICAN CULEX FOR MVEV COMPETENCE. He has ruled that the gap decides it. Culex annulirostris, which the chapter calls the principal vector and the source of most MVEV isolates, does not occur in the US. NOTE THE CONTRAST WITH DENGUE AND ZIKA in this same batch, both of which he confirmed at level 5: Aedes aegypti and Aedes albopictus are established in the US and have already produced local transmission of both viruses, so for those the vector is present and proven rather than assumed. RESCALED 2026-09-03, when C3 went from five levels to four and level 4 absorbed level 5. A ruling of his is never overturned by a scale change, so what is recorded here is the same judgment expressed on the new scale. Recorded on the old scale as level 3, from_level 5.
Basis. ch30 30.3.2: "The principal vector, C. annulirostris, accounts for the majority of MVEV isolates." "Based on serosurveys and experimental evidence, the main vertebrate hosts are thought to be water birds belonging to the orders Ciconiiformes, Pelecaniformes, and Anseriformes." 30.1: MVEV is one of the orthoflaviviruses whose range has expanded, and 30.2.4 records that JEV "can readily move into and establish in new areas if there are receptive vector mosquito species and susceptible vertebrate hosts".
How this level was decided
RENUMBERED L5 -> L4 on 2026-09-03, when C3 was reduced to four levels and level 4 absorbed level 5. The judgment is unchanged; Eric's ruling of level 3 on this cell stands and is unaffected.
EARLIER REASONING, kept because it is the evidence read: RENUMBERED L5 -> L4 on 2026-09-03, when C3 was reduced to four levels and level 4 absorbed level 5. The judgment is unchanged; Eric's ruling of level 3 on this cell stands and is unaffected.
EARLIER REASONING, kept because it is the evidence read: INFERRED, AND THIS IS THE VECTOR-COMPETENCE QUESTION ERIC RAISED, so it is set out rather than asserted. MVEV is Australian and its principal vector, Culex annulirostris, does not occur in the US -- which under the bat rule is an argument against level 5. But the bat rule turns on whether the reservoir COULD follow, and unlike a flying fox the ingredients here are already present: the vertebrate hosts are Ciconiiformes, Pelecaniformes and Anseriformes water birds, all abundant across the US, and the vector genus is Culex, of which US species already transmit two viruses of the same serogroup -- Saint Louis encephalitis and West Nile. Scored level 5 on that reading. FLAGGED for his ruling: the honest gap is that no study in the folder tests a North American Culex for MVEV competence, and if he judges that unproven this becomes level 3 on the Hendra pattern.
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
Basis. ch30 30.3.2: "No disease or clinical signs following MVEV infection of pigs have been reported." "The majority of MVEV infections are diagnosed using serological assays. HI, VN, and ELISA have been used for antibody detection in serological surveys of feral swine or following experimental infections of pigs. A range of molecular diagnostics assays have been reported for the detection of MVEV genome, including conventional and real-time RT-PCR and RT-LAMP. IHC for detecting viral antigen has also been described." 30.3.1: serological interpretation must account for cross-reaction with JEV, Usutu, SLEV and MVEV.
How this level was decided
Level 3. Validated assays exist -- RT-PCR, RT-LAMP, IHC, HI, VN and ELISA are all named -- so this is not level 4. But MVEV is exotic to the US, no US laboratory holds the reagents or the virus for a neutralisation test, and pigs show no clinical signs to prompt one. Confirmation would go to a national reference laboratory, which is level 3. Contrast west_nile_virus in this batch at level 2: the difference is entirely that WNV is endemic here and MVEV is not.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
Basis. ch30 30.3.2: "Although pigs can be infected with MVEV, no disease association has been reported." "No disease or clinical signs following MVEV infection of pigs have been reported. There is also no available evidence that infection leads to secondary amplification in reproductive organs or fetuses." A single pregnant sow inoculated experimentally farrowed normally 14 weeks later with "no evidence of in utero infection of fetuses".
How this level was decided
Level 1, on an examined negative. The chapter states twice that no disease association exists, and the specific worry for an orthoflavivirus in pigs -- transplacental infection, which is the whole of JEV's cost -- was looked for in an experimentally infected pregnant sow and not found. No measurable effect on cost of production.
C6 Market impact
Duration of material disruption to pork and pig markets
NOT ASSESSED — no evidence in folder. No level was assigned on this criterion. This is a statement about what the evidence folder holds, not a finding about the agent.
Corrected by Eric from L1 Negligible. Reason: CONFIRMED AS NOT ASSESSABLE. "i think we need to make this not assessed like other similar situations. I would anticipate there would be some reaction if a diagnosis were made but we simply don't know. KISS analysis." Note what he is saying, because it is not that the market impact is nil: he expects a reaction and cannot size it. The assessment had scored level 1 on the ground that MVEV is not WOAH-listed and causes no disease in pigs, and had flagged in the same cell that a novel arbovirus with human encephalitis cases would attract attention. He has ruled that the attention cannot be quantified.
Basis. Section 30.3.2 records no trade measure, movement control or consumer response for MVEV in pigs, and MVEV is not among the orthoflaviviruses ch30 30.1 identifies as WOAH-listed -- that list names JEV and WNV only.
How this level was decided
INFERRED, and it is the exotic case rather than the endemic one, so the standing note SCORING/market_impact does not apply directly and this is scored on what would happen. An incursion of an agent that is not WOAH-listed and causes no disease in pigs gives no route to pork supply or demand. NOTE FOR ERIC: the human disease is severe, and a novel arbovirus in the US with human encephalitis cases would attract attention -- but the attention would attach to mosquitoes rather than to pork, as it did for West Nile in 1999. 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
Basis. ch30 30.3.2: "There are no specific antivirals available to treat MVEV infections." "No disease or clinical signs following MVEV infection of pigs have been reported."
How this level was decided
Level 1 on the second clause. No antiviral exists, and none is needed: the chapter says twice that pigs develop no disease. Under the standing note SCORING/treatment_potential there is no imperative to treat, so there is no outcome a treatment could improve.
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. ch30 30.3.2: "there are no commercially available vaccines" for MVEV. "Although pigs can be infected with MVEV, no disease association has been reported."
How this level was decided
Level 1 on the second clause of the reworded criterion. No vaccine exists anywhere, for any species -- but the question the criterion asks is whether one is warranted, and for a virus that causes no disease in pigs it is not. Contrast japanese_encephalitis_virus in this batch at level 3, where vaccines exist elsewhere and the gap is genuine because the disease is real.
Levels and evidence are generated from data/assignments/murray_valley_encephalitis_virus.yml; the overview is authored in data/overviews/murray_valley_encephalitis_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.