Porcine Hemagglutinating Encephalomyelitis Virus
LEVELS: Highly unlikely; No human illness; Already in the herd; Laboratory-dependent; Minor; Negligible; Substantial; Available, or not needed
| Register id | porcine_hemagglutinating_encephalomyelitis_virus |
| Type | virus |
| Scientific name | Porcine hemagglutinating encephalomyelitis virus |
| NCBI taxid | 42005 |
| Evidence | 2 document(s) |
| Assigned | 2026-09-06, against criteria version 093366e352a2 |
Overview
Porcine haemagglutinating encephalomyelitis virus is a coronavirus first isolated from pigs with encephalomyelitis in Canada in 1962. In susceptible piglets under three or four weeks old it produces one of two syndromes: vomiting and wasting disease, in which piglets vomit persistently, become constipated and waste away; or acute encephalomyelitis, with tremors, paddling and paralysis. A respiratory variant causing influenza-like illness has been described more recently. The striking thing about this virus is the gap between how common it is and how rarely it causes trouble. Surveys in the United States find about 53% of individual pigs and 96% of herds seropositive, yet clinical disease is uncommon. The explanation is maternal immunity: almost all breeding females have been infected without ever being ill, and the antibody they pass in colostrum protects piglets through exactly the window in which they would be vulnerable. Outbreaks therefore happen in the situations that break that pattern — non-immune gilts farrowing susceptible litters, or the virus entering a naive high-health or SPF herd where it has not been circulating.
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. ch28 28.5.3, the whole public health section: "Pigs are the only species known to be susceptible to pHEV and pHEV has no public health significance." shic_phev: "Swine are the only species in which PHEV naturally causes clinical disease. PHEV is not known to be zoonotic and poses no public health threat to humans."
How this level was decided
Level 1, stated outright and in the strongest terms in the chapter -- pigs are the ONLY species known to be susceptible. An examined negative rather than a silence. Level 1 confirmed by direct statement.
C2 Zoonotic impact
Severity of human illness caused by the agent
No human illness: Agent does not cause illness in people
Basis. ch28 28.5.3: "Pigs are the only species known to be susceptible to pHEV and pHEV has no public health significance." shic_phev: "PHEV is not known to be zoonotic and poses no public health threat to humans."
How this level was decided
Level 1. No human infection exists to grade. Level 1 confirmed.
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. ch28 28.5.4: "Serological surveys (1960-1990) revealed that pHEV infection in swine occurs worldwide and is endemic in both breeding and growing swine." "In a recent study, 2756 serum samples from breeding females on 104 commercial farms with no history of pHEV-associated disease located in 19 US states were tested by a pHEV S1 ELISA. The overall pHEV seroprevalence was 53.35% and HERD SEROPREVALENCE WAS 96.15%." "PHEV is maintained in swine populations by infecting successive groups of pigs after replacement or weaning." "Persistent virus carriers are not known to exist." 28.5.10: "On most breeding farms, pHEV infection persists endemically by pig-to-pig transmission and through subclinical respiratory infections." shic_phev: "PHEV infection is found nearly worldwide. Serological evidence of infection has been found in pigs throughout Europe, the Americas, Asia, and Australia." "Pig-to-pig transmission results in persistence of PHEV in large herds, where few outbreaks are seen. Small herds are more likely to experience outbreaks of PHEV due to their inability to maintain enzootic infection."
How this level was decided
Level 1, and it is the best-measured level 1 in the register. A US survey of 104 commercial farms across 19 states found 96.15% of HERDS seropositive, in farms with no history of clinical disease. The virus renews itself within the herd by infecting each replacement or weaned group. There is no introduction event to prevent because 24 herds in 25 already have it. NOTE the corollary the chapter draws, which matters for C8: the few herds at risk are the ones that do NOT have it -- newly populated farms, well-isolated gilts, SPF and small herds. Level 1 confirmed, with a mechanism worth recording: PHEV persists enzootically in LARGE herds, and it is small herds that see outbreaks because they cannot sustain the cycle. For the modern US reference system that is squarely already-in-the-herd.
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
Basis. ch28 28.5.8: "PHEV diagnosis can be made by virus isolation, IHC, or RT-PCR. Tonsils, brain stem, and lungs dissected aseptically from young acutely diseased piglets can be used for testing. Pen-based oral fluids and feces can also be used for pHEV PCR testing. It is difficult to isolate the virus from pigs that have been sick for more than 2-3 days." "Antibody titer results must be evaluated carefully because subclinical infections with pHEV are common and pigs may develop antibody titers early at 6-7 DPI, which often coincides with early disease, making an interpretation of paired serology more difficult. A pHEV S1-based ELISA was developed for antibody-based serodiagnosis of pHEV to differentiate pHEV antibodies from antibodies to other porcine CoVs." "Differential diagnosis must be made between pHEV encephalomyelitis, Teschen-Talfan disease, and pseudorabies." shic_phev: "Virus may be isolated from nasal swabs and identified by virus neutralization, hemagglutination, immunofluorescence, or hemadsorption plaque assay." "Virus antigen identification in tissues may be performed by the fluorescent antibody test (FAT), immunofluorescence, or immunohistochemistry. Viral RNA may be identified by reverse transcriptase polymerase chain reaction (RT-PCR)."
How this level was decided
Level 2. The clinical picture is suggestive but not decisive: vomiting and wasting or a tremoring, backward-walking piglet points at three different viruses, and the chapter names Teschen-Talfan and Aujeszky's disease as the differentials to exclude. Confirmation is RT-PCR on tonsil, brainstem, lung or pen-based oral fluid, plus a pHEV-specific S1 ELISA built to separate it from the other porcine coronaviruses -- all regional laboratory work. NOTE two real limits that do not change the level: virus isolation fails after two or three days of illness, and paired serology is hard to interpret because antibody appears at 6-7 days, which is when the disease appears. Level 2 confirmed -- multiple established methods on ordinary sample types.
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, agree with assessment. Disease is exceedingly rare in the US anymore, for unexplained reasons"
Basis. ch28 28.5.1: "PHEV is widespread among swine, but the infection is generally subclinical, although some outbreaks in SPF and high health gilt herds may cause losses." 28.5.4: "Since pHEV is endemic in most swine populations, most sows are immune and protect their offspring by maternal antibodies. Thus, clinical outbreaks are rare and usually occur in litters from nonimmune gilts or in SPF herds." Where it does occur: "Morbidity and mortality in neonatal pigs is usually 100%", and in the Argentine outbreak "An estimated 12.6% (3683) of the suckling pigs in the affected farrowing units died or were euthanized" with 29% of weaned pigs from affected units showing wasting. 28.5.10: "litters born 2-3 weeks after the onset of disease are protected because nonimmune gestating sows have become infected and immune by farrowing." shic_phev: "Clinical manifestations of PHEV, including vomiting and wasting and/or encephalomyelitis, are generally seen only in piglets less than 4-weeks-of-age." "Pig-to-pig transmission results in persistence of PHEV in large herds, where few outbreaks are seen."
How this level was decided
Level 2. The individual outbreak is severe -- effectively total loss in neonatally infected litters, and 3683 dead piglets in the Argentine case -- but the criterion asks about cost of production, and two facts hold this down. Outbreaks are rare, because 96% of herds are already seropositive and immune sows protect their litters. And they are self-limiting on a defined timetable: litters born two to three weeks after onset are protected, because the gestating sows have immunised themselves. That is the short-lived rather than merely manageable test Eric set on Salmonella. FLAGGED as a level 2 or 3 judgment for the SPF and gilt-heavy herds where it does land. 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, no evidence to suggest it has any impact on the market"
Basis. Section 28.5 records no trade measure, movement control or consumer response. pHEV has been recognised since 1962, is endemic worldwide, and 96.15% of surveyed US herds are seropositive.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact, a virus present in 24 of every 25 US herds for over sixty years, with no market move on record, is the clearest possible level 1. Unchanged.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED AT LEVEL 3 AFTER HE REVERSED HIS OWN MARKUP, and recorded because the reversal matters more than the ruling. On the gaps sheet he wrote "c7l1, i have reconsidered and am downgrading it. The virus does circulate in the US but in general seems to rarely produce clinical signs at a prevalence or severity that would warrant treatment." That was put back to him because it collides with his own standing C7 note, which uses pHEV as THE worked example of the neonate rule -- lethal in neonates, trivial in adults, score the neonate -- alongside TGEV, PEDV, PDCoV and SADS-CoV. Eric, 2026-09-06: "i reread the pHEV evidence in the folder and we will leave at c7l3 to be consistent with other examples." SO THE NEONATE RULE STANDS UNCHANGED and pHEV remains its example. Anyone finding the L1 markup in the sheet should read this rather than assume it was missed.
Basis. ch28 28.5.10: "Once clinical signs are evident, the disease will run its course; spontaneous recoveries are rare." "Small interfering RNAs targeting various pHEV genes prevented pHEV replication in vitro, but this approach has not been tested in pigs." No treatment of any kind is described. 28.5.6: "Morbidity and mortality in neonatal pigs is usually 100%", and affected older pigs enter a wasting state that "persists for several weeks and may require euthanasia". shic_phev: "It usually causes vomiting and wasting disease and/or encephalitis in neonatal pigs." "Clinical manifestations... are generally seen only in piglets less than 4-weeks-of-age."
How this level was decided
INFERRED, and FLAGGED as the most arguable cell in this entry. The case for level 3: no pathogen-directed treatment exists, the chapter says the disease runs its course and that spontaneous recovery is rare, and the outcome to be improved is total mortality in affected neonatal litters plus weeks of wasting in the survivors. The case against, which is why this is flagged: outcomes ARE reliably improved without a drug, by managed exposure so that gilts are immune at farrowing and by parenteral immune serum to piglets born early in an outbreak, so the unmet need is narrow and only bites in herds that failed to immunise their gilts. Level 3 confirmed, and this entry is the one the standing note was written around. Eric on pHEV, 2026-09-02: the disease is 100% lethal in newborns and a transient nuisance in adults, and "it would be good to have a treatment available, particularly to deal with chronic infections that cause persistent disease in breeding herds, or to treat affected juveniles". The factsheet confirms the age split exactly.
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, agree with assessment. Not clear from recent evidence whether US herds are keeping it out, or whether some form of underlying immunity/exposure is just minimizing clinical appearance. Regardless, no imperative for vaccine development"
Basis. ch28 28.5.10: "In the absence of pHEV vaccines, promoting virus circulation in the farm so that gilts are immune at farrowing can prevent disease in piglets." "Gilts usually contract the virus before their first farrowing and then provide protection to their offspring via colostral antibodies." 28.5.9: maternal antibodies protect to a mean of 10.5 weeks of age, and "the duration of immunity is less critical in pHEV because of the resistance to disease that naturally develops with age".
How this level was decided
Level 1 on the second clause, and the chapter shows why rather than merely asserting it. No vaccine exists, and the control programme that replaces it is deliberate natural exposure -- let the virus circulate so gilts seroconvert before farrowing. That works because the virus is in 96% of herds anyway and because age resistance is rapid. A vaccine would be a product for the 4% of herds that have kept it out, and those herds have kept it out precisely by not letting it in. FLAGGED as arguable at level 3 for exactly that minority -- SPF and newly populated units, where the chapter says outbreaks concentrate. Unchanged.
Levels and evidence are generated from data/assignments/porcine_hemagglutinating_encephalomyelitis_virus.yml; the overview is authored in data/overviews/porcine_hemagglutinating_encephalomyelitis_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.