West Nile Virus
LEVELS: Highly unlikely; Mild; Extraordinary biosecurity required; Laboratory-dependent; Negligible; Negligible; Little; Available, or not needed
| Register id | west_nile_virus |
| Type | virus |
| Scientific name | West Nile virus |
| NCBI taxid | 11082 |
| Evidence | 2 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
West Nile virus is a mosquito-borne flavivirus that arrived in the United States in 1999 and spread across the whole continental country within a few years; it has been recovered from around 60 mosquito species, more than 300 bird species and over 30 non-avian hosts in the US alone. Pigs are exposed — serological evidence of infection has been reported in domestic and feral swine in the United States and elsewhere, and a Serbian survey found 37% of wild boar seropositive — but they do not get sick. Experimentally infected pigs showed no clinical signs in three separate studies, and a sow infected in mid-gestation produced normal fetuses. That combination, common exposure and no disease, is why the practical suggestion for pigs is to use them as sentinels for tracking where the virus is active rather than as animals needing protection. In people the picture is different: about 20% of infections cause a self-limiting fever, under 1% progress to neurological disease, and among those cases 10-30% are fatal with lasting effects in over half of survivors. There is no specific treatment. Vaccines exist for horses but none for pigs, and none is needed.
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. veljovic_2025 proposes wild boar as a SENTINEL species, which marks transmission rather than driving it; people acquire WNV from mosquitoes and no human case is attributed to a pig. The pass had flagged exactly this outcome.
Basis. Veljovic 2025 surveyed 522 wild mammals in Serbia and found 37% of 100 wild boar seropositive for WNV, with seropositive juveniles proving transmission during the study year -- far above the 0% reported from Poland and the 4-10% of other European surveys. The paper's own framing is that these species "may serve as useful SENTINEL species for monitoring WNV prevalence and geographic spread".
How this level was decided
MOVED L2 -> L1, re-read 2026-09-04 AGAINST THE REWORDED LABELS. veljovic_2025 proposes wild boar as a SENTINEL species, which is a statement that they mark transmission rather than drive it. No human case has ever been attributed to a pig; people acquire WNV from mosquitoes. The previous assessment flagged exactly this: "if Eric reads 'the pig must be the source' strictly this is level 1".
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: Level 2 unchanged, and the new paper sharpens why rather than moving it. A sentinel is an animal that reliably catches a virus and does nothing with it -- proposing wild boar as one is a statement that they mark transmission rather than drive it. So 37% seroprevalence in an endemic region is not evidence that pigs are a source; it is evidence that they are exposed, which is C3. What holds this at level 2 rather than level 1 remains the chapter's finding that weaner pigs can briefly reach a titre sufficient to infect Culex pipiens. FLAGGED as before: if Eric reads "the pig must be the source" strictly this is level 1, and C2 follows it down.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c2l2, agree with assessment. Textbook examnple aligned with level description - usually mild, rarely severe. good way to deal with this and related situations where pigs don't really seem to be responsible for human cases but it certainly is plausible, so risk should be greater than l1"
Basis. ch30 30.3.1: "In humans, approximately 20% of WNV infections are symptomatic and typically manifest as a self-limited fever, with less than 1% developing into neurological disease. Case fatality rates range between 10 and 30% during outbreaks and neurological sequelae can persist in over half of survivors 1 year after infection."
How this level was decided
Level 2. Four fifths of infections produce nothing at all, and the typical symptomatic case is a self-limited fever that needs no treatment -- usually mild and self-limiting, with serious disease occurring only rarely, which is level 2 stated. FLAGGED as a level 2 or 3 judgment, because the tail is severe: under 1% become neuroinvasive, and among those the case fatality is 10-30% with lasting sequelae in over half the survivors. C2 says rare extreme outcomes should not set the score unless they are a recognised and meaningful part of the distribution, and neuroinvasive WNV arguably is -- it is the reason anyone tracks the virus.
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
Basis. ch30 30.3.1: WNV "has rapidly spread throughout the continental United States" since 1999 and has been isolated from "approximately 60 species of mosquitoes, >300 species of birds, and >30 species of non-avian hosts" in the US alone. Veljovic 2025 adds a second geography and a measured exposure rate: 37% of wild boar, 50.6% of red deer, 32.4% of golden jackals and 11.9% of nutria seropositive in Serbia, with juveniles positive in every one of those species, which the authors read as "active transmission during the sampling year".
How this level was decided
RENUMBERED L5 -> L4 on 2026-09-03, when C3 was reduced from five levels to four and level 4 absorbed level 5. THE JUDGMENT IS UNCHANGED and this one really is a renumber: the merged level 4 label carries what level 5 said, ending "and may still fail". IT WAS STILL CHECKED rather than assumed, because the last renumber on 2026-09-01 was mechanical and moved twenty-one entries past a level they should have been judged against. The check is the exclusion test -- does housing close the route? For this entry it does not.
EARLIER REASONING, kept because it is the evidence read: Level 5 unchanged and now measured rather than asserted. The Serbian survey is what unavoidable exposure looks like when someone counts it: better than a third of wild boar in the study area had met the virus, across five unrelated mammal species, in animals young enough that the exposure happened that season. No farm or industry action removes a bird-and-Culex cycle, and the paper shows the exposure reaching well beyond the birds.
C4 Detection difficulty
Difficulty of recognizing and confirming infection
Laboratory-dependent: The presentation does not point to this disease specifically, but local or regional laboratories can confirm it with a validated assay once it is suspected
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c4l2, this sits between l2 and l3 but my belief is that in the areas where WNV is known to circulate (and therefore where pigs are most likely to become infected), testing is available locally or regionally"
Basis. ch30 30.3.1: "No clinical signs were observed in experimentally infected pigs." "Laboratory diagnosis of WNV infections uses the same general approach and techniques described for JEV and other members of the JE serological complex." "Several molecular diagnostic assays have been reported for the detection of WNV RNA in clinical specimens or mosquitoes, including conventional or real-time RT-PCR assays, RT-LAMP, and RPA assays. A pan-WNV real-time RT-PCR assay has also been reported for detection of viruses belonging to all recognized lineages. An IHC method for detecting WNV antigen in brain and spinal cord tissues of pigs has also been reported." "Interpretation of serological tests should consider the presence of cross-reactions to closely related orthoflaviviruses, such as JEV (Australia, India), Usutu (Europe), SLEV (North America), and MVEV (Australia)."
How this level was decided
INFERRED. Nothing points to WNV in a pig, because infected pigs show no clinical signs at all -- so level 1 is out. Unlike JEV this is not an exotic agent: WNV is endemic in the US, validated real-time RT-PCR is run routinely by state and regional laboratories for equine and avian cases, and a pan-lineage assay and a porcine IHC method are both published. Level 2. NOTE: serology still needs a reference laboratory to resolve cross-reaction with SLEV, which co-circulates here, but the molecular route does not.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
Basis. ch30 30.3.1: "No clinical signs were observed in experimentally infected pigs" in three independent studies, and a sow infected mid-gestation produced normal fetuses. Veljovic 2025 found 37% of wild boar seropositive and reports no disease in any species surveyed, proposing them instead as sentinels.
How this level was decided
Level 1 unchanged, and the new paper converts the experimental negative into a field one. Three laboratory studies found no clinical signs; a field survey now finds that a third of wild boar in a high-transmission region have been infected, and the authors' conclusion is about surveillance value rather than about disease. Infection without illness at that scale is the strongest form this cell could take. No measurable effect on cost of production.
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, the virus has been present for a long time and pigs seem to have little role in the diseaes ecology, and the market hasn't and isn't likely to respond to a diagnosis"
Basis. ch30 30.1: WNV is WOAH-listed as a multiple-species disease. 30.3.1: it has been endemic across the continental United States since 1999, and serological evidence of infection in domestic and feral pigs has been reported in the United States among other regions.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact this is the clearest kind of case: WNV has been endemic in US pigs, birds and horses for a quarter of a century, seropositive pigs have been reported here, and no market move has ever followed. NOTE the one complication, recorded rather than scored: WNV is WOAH-listed, so a formal report is possible in principle -- but it is listed for a disease pigs do not get, and reporting an endemic agent changes nothing. 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 30.3.1: "There is no specific treatment available for WNV infection." "No clinical signs were observed in experimentally infected pigs."
How this level was decided
Level 1 on the second clause. No treatment exists in any species, but under the standing note SCORING/treatment_potential level 1 also covers the case where there is no imperative to treat, and that is this one exactly: pigs do not become ill. A pathogen-directed treatment would improve no outcome, because there is no outcome to 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 30.3.1: "Several veterinary vaccines are commercially available, including cell culture-derived inactivated whole virus vaccines (WN Innovator, Vetera WNV, and Prestige WNV) and a live canary poxvirus-vectored vaccine expressing the WNV prM and Env genes (Recombitek Equine WNV) which have been approved for use in horses." An inactivated vaccine is authorised for geese in Israel and one is conditionally licensed in the US for alligators. No swine vaccine is described. "No clinical signs were observed in experimentally infected pigs."
How this level was decided
Level 1 on the second clause of the reworded criterion -- the disease does not clinically or economically justify vaccinating pigs. The chapter shows the technology is not the obstacle: four licensed equine products, one for geese, one for alligators. Nobody has made one for pigs because pigs do not get sick. This is the same shape as botulism in ch47, where the chapter said prophylactic toxoid "is not practical in swine because of the infrequent occurrence of the disease".
Levels and evidence are generated from data/assignments/west_nile_virus.yml; the overview is authored in data/overviews/west_nile_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.