Eastern Equine Encephalitis Virus
LEVELS: Highly unlikely; Severe; Extraordinary biosecurity required; Reference or research laboratory required; Minor; Negligible; Substantial; Available but inconsistent
| Register id | eastern_equine_encephalitis_virus |
| Type | virus |
| Scientific name | Eastern equine encephalitis virus |
| NCBI taxid | 11021 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Eastern equine encephalitis virus is an alphavirus maintained by mosquitoes and birds in eastern Canada, the United States east of the Mississippi, the Caribbean and Central and South America. It is a serious cause of encephalitis in people and horses. In pigs, infection is not uncommon — seropositive rates vary but exposure clearly occurs — yet clinical episodes are rare. When they happen they hit the very young: no natural outbreak was reported before 1972, and in a 1991 outbreak in pigs under two weeks old mortality reached 80%, with survivors left stunted. Infection in pigs over two months of age is inapparent, and there has been no apparent increase in episodes in pigs since. Diagnosis is not straightforward because gross lesions are unremarkable and pigs dying in the acute phase may have no visible changes in the brain at all, so it rests on virus isolation from nervous tissue or swabs followed by PCR. There is no treatment. Prevention is mosquito control and, where an outbreak is severe or the pigs valuable, vaccination — sow vaccination passes protective antibody to the piglets.
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. ch41 41.6.1: "EEEV is zoonotic and causes encephalitis in humans." 41.6.2.1: "Ornithophilic mosquitoes (e.g. Culiseta melanura) maintain endemic cycles. Epidemic cycles are mediated by arthropod vectors that feed on both birds and mammals." 41.6.3.1: "Nursing pigs are most severely affected, and infection in pigs aged 2 months or older is inapparent."
How this level was decided
Level 1 on Eric's standing note. EEEV reaches people readily, but from a bird-mosquito cycle -- the pig is a parallel dead-end mammal, infected by the same mosquito that infects the person, and not a source for it. Eric's standing note at chikungunya_virus/zoonotic_potential names this virus specifically: "Same dead-end reasoning applies to Ross River, Sagiyama, dengue, Zika, eastern equine encephalitis and Murray Valley." The framework scores pig-to-human only, and a pig that does not seed a mosquito is not a source however much virus is in the neighbourhood.
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. 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 "l4". REVERSES THE 2026-09-01 MOVE TO LEVEL 1, restoring the level this cell held before C2 was rescoped. EEE is one of the most severe arboviral diseases of people, with high case fatality and lasting neurological sequelae. C1 records separately, at level 1, that pigs are not a source.
Basis. ch41 41.6.1: "EEEV is zoonotic and causes encephalitis in humans." The chapter does not describe the human clinical course.
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. Returns to the level it held before the 2026-09-01 rescoping moved it to 1. The previous assessment states the basis: "Eastern equine encephalitis is one of the most severe arboviral diseases of people, with high case fatality and lasting neurological sequelae in survivors."
PREVIOUS ANSWER, written under the pig-attribution reading: MOVED L4 -> L1 on 2026-09-01. Eastern equine encephalitis is one of the most severe arboviral diseases of people, with high case fatality and lasting neurological sequelae in survivors, and none of that reaches a person from a pig. The maintenance cycle is passerine birds and Culiseta mosquitoes, and the pig is a dead-end host, which is why C1 is level 1. Moved by the 2026-09-01 C2 rescoping, which Eric accepted: "i accept the c2_review changes". C2 now grades the severity of illness acquired FROM PIGS, so an entry whose pig is a dead-end host has no pig-attributable illness to grade. THE LEVEL MUST NOT BE READ AS A FINDING THAT THE AGENT IS MILD -- it records that pigs are not a source. EEE is the sharpest illustration of why the rescoping was needed: at level 4 it carried the full additive weight of severe human disease into a pig-disease ranking.
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. ch41 41.6.2.1: "EEEV is endemic in eastern Canada, the United States east of the Mississippi River, the Caribbean islands, and Central and South America. Ornithophilic mosquitoes (e.g. Culiseta melanura) maintain endemic cycles."
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: 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. A US reservoir outside the pig industry, maintained in wild birds and delivered by mosquitoes, endemic across the entire eastern United States. No farm biosecurity closes it. Level 4, and unlike most exotic arboviruses this one needs no general-rule reasoning -- it is already here.
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. ch41 41.6.4: "Diagnosis of EEEV infection can be performed by viral isolation using CNS tissues and/or oropharyngeal and rectal swabs and tonsils in Vero cells, followed by identification by IF or RT-PCR." 41.6.3.1: "Gross lesions are unremarkable... Pigs that die in the acute phase may not have CNS lesions."
How this level was decided
Sudden death or neurological signs in nursing pigs has a long differential and the gross lesions are unremarkable, so nothing points here. Confirmation is virus isolation from CNS tissue in Vero cells with IF or RT-PCR, which is reference and research work for an arbovirus rather than a service a regional laboratory offers for swine. Level 3.
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
Basis. ch41 41.6.2.1: "EEEV infection in pigs is not uncommon, as shown by variable seropositive rates, but clinical episodes are rare. No natural EEEV outbreaks were reported until 1972... In 1991, it was reported in pigs under 2 weeks of age, with 80% mortality. There has been no apparent increase in EEEV episodes in pigs." 41.6.3.1: "Nursing pigs are most severely affected, and infection in pigs aged 2 months or older is inapparent. Growth is usually retarded in survivors."
How this level was decided
Level 2. When it breaks in a farrowing house it is severe -- 80% mortality in the one substantial report, with retarded growth in survivors -- but the chapter is emphatic that clinical episodes are rare and have not become more frequent, against seropositivity that is not uncommon. Small and episodic losses rather than a standing cost, which is level 2. The standing note SCORING/production_cost_impact points the same way: two reported outbreaks in fifty years is not modern US production experience.
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, EEE is well recognized in US and is a minor player if at all in pigs, diagnosis would have no effect on markets"
Basis. Section 41.6 records no trade measure, movement control or consumer response arising from EEEV in pigs. The virus is endemic across the eastern United States.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact, EEEV is already present and already diagnosed in US pigs, and no diagnosis has produced a market response in fifty years. Level 1.
C7 Treatment potential
Potential for treatment to improve outcomes
Substantial: No effective treatment (antimicrobial, antiviral, antiparasitic, herbal or other) is available, but outcomes would meaningfully improve if one existed
Basis. ch41 41.6.5: "There is no treatment for pigs clinically affected by EEEV." 41.6.2.1 records 80% mortality in pigs under two weeks of age in the 1991 outbreak.
How this level was decided
Level 3, and one of the few in this batch. The chapter states the absence directly, and the outcome a treatment would change is real -- 80% mortality in affected litters, with growth retardation in survivors. No effective treatment available, and outcomes would meaningfully improve if one existed. Both halves.
C8 Vaccine availability
Availability of effective vaccines or bacterins
Available but inconsistent: Commercial or autogenous vaccines exist in the US but protection may be inconsistent
Basis. ch41 41.6.5: "Prevention is achieved by vaccination of animals at risk and/or control of the vector population. Vaccines are available for humans and horses. Vaccination may be economically justifiable in severe outbreaks or for the protection of valuable pigs. Vaccination of sows provides protective maternal antibodies to piglets."
How this level was decided
Level 2. A vaccine exists, works, and has been used in pigs -- sow vaccination protecting piglets through maternal antibody is described directly. What holds it below level 1 is that the licensed products are equine and human, so use in swine is extralabel, and the chapter frames it as justifiable only in severe outbreaks or for valuable animals rather than as standard practice. Vaccines that exist but whose use is restricted and inconsistent is level 2 word for word.
Levels and evidence are generated from data/assignments/eastern_equine_encephalitis_virus.yml; the overview is authored in data/overviews/eastern_equine_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.