Porcine Lymphotropic Herpesviruses
LEVELS: Highly unlikely; No human illness; Already in the herd; Reference or research laboratory required; Negligible; Negligible; Little; Available, or not needed
| Register id | porcine_lymphotropic_herpesviruses |
| Type | virus |
| Scientific name | Macavirus |
| NCBI taxid | 548687 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
The porcine lymphotropic herpesviruses are three related gammaherpesviruses carried by pigs and wild boar around the world at high prevalence — roughly half to three-quarters of German domestic pigs depending on which of the three is measured, 95% of New Zealand pigs for one of them. They have never been associated with any clinical disease in pigs under field conditions. The one situation in which they do cause disease is deliberate and iatrogenic: immunosuppressed miniature swine given bone marrow transplants developed a lymphoproliferative disease with high mortality, with the virus present in high copy number in the affected animals and absent in the healthy ones — a close parallel to the post-transplant lymphoproliferative disease seen in immunosuppressed people. That is why they matter. They are relatives of the viruses that cause malignant catarrhal fever, harmless in their natural hosts and lethal in others, and their combination of high prevalence and demonstrated pathogenic potential under immunosuppression makes them a recognised concern for pig-to-human xenotransplantation. No one has been able to grow them in cell culture, so the genomes were assembled directly from pig samples, and detection is by PCR. There is no vaccine; the control objective, where there is one, is producing virus-free donor pigs by caesarean derivation and barrier rearing.
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. ch31 31.4.3, a Public Health section of its own, and it reports no human infection at all: "PLHVs have the potential to cause a lymphoproliferative disease of high mortality IN EXPERIMENTALLY IMMUNOSUPPRESSED PIGS that resembles the post-transplantation lymphoproliferative disease (PTLD) in humans. PLHVs are closely related to AlGHV-1 and OvGHV-2, which are innocuous in their natural hosts, but cause MCF in other animals... the pathogenic potential and worldwide prevalence of PLHVs in pigs has raised CONCERNS regarding the consequences of pig-to-human xenotransplants in immunosuppressed humans." 31.4.5 states the concern in full and it is explicitly theoretical: "THE THEORETICAL CONCERN IS THAT PLHVS MIGHT RECOMBINE WITH HUMAN HERPESVIRUSES in the human xenotransplant recipients, giving rise to recombinants with novel pathogenic properties." 31.5.4, on the group these viruses belong to: "in general, GAMMAHERPESVIRUSES HAVE A NARROW HOST RANGE."
How this level was decided
Level 1, and it rests on a section written to answer the question rather than on a silence. The chapter gives PLHVs a Public Health heading, reviews what is known, and what is known is that no human infection has ever been reported -- including in xenotransplant recipients, where people have been looking specifically. The concern it does raise is a hypothetical about recombination in an immunosuppressed recipient, which is a statement about what might one day happen inside a transplant patient rather than about transmission from pigs. The biology supports the level: gammaherpesviruses are narrow-host-range viruses, and the PLHV relatives that do cross species -- AlGHV-1 and OvGHV-2 -- cross into ruminants and pigs, not into people.
C2 Zoonotic impact
Severity of human illness caused by the agent
No human illness: Agent does not cause illness in people
Basis. No human illness with any PLHV is described anywhere in chapter 31; 31.4.3 raises only a theoretical recombination risk in immunosuppressed xenotransplant recipients.
How this level was decided
Level 1. Under C2 as rescoped, level 1 means pigs are not a source of human illness with this agent, and no human illness exists to grade. This is the straightforward case rather than one where the label needs reading carefully.
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. ch31 31.4.4: "there is growing evidence that they are present WORLDWIDE in pigs and their relatives in the family Suidae", detected in domestic pigs from Germany, Italy, France, Belgium, Denmark, Ireland, the United Kingdom, the United States, Australia and Vietnam, and in wild boar from Germany, the United States, Australia and Brazil. The prevalences: "48-62% (PLHV1), 16-41% (PLHV2), and 54-78% (PLHV3) were estimated in German domestic pigs"; Irish spleens 74%, 21% and 45%; "in New Zealand, 95% of domestic piglets and adult pigs were PCR-positive for PLHV2"; in the United Kingdom "80% and 67% of adult large white pigs and adult miniature swine". "These data indicated a HIGH PLHV PREVALENCE IN COMMERCIAL AND EXPERIMENTAL PIG HERDS." On transmission: "de novo PLHV infection by contact to the infected dam and subsequent seroconversion". 31.4.8: "EARLY WEANING OF PIGLETS FAILED TO EXCLUDE PLHV, but a markedly reduced prevalence of PLHVs was achieved using CESAREAN-DERIVED, BARRIER-REARED breeding conditions."
How this level was decided
Level 1. Between half and 95% of pigs carry these viruses depending on country and virus, they pass from dam to piglet by contact after birth, and the only intervention that reduces them is caesarean derivation with barrier rearing -- which is a research colony technique, not something a commercial herd can do. There is no introduction event to prevent. NOTE that even early weaning, which removes several other agents, failed 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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c4l3, not routinely tested for in local or regional labs but the methodology is well described and validated well enough in research settings"
Basis. ch31 31.4.2: "propagation of these viruses in cell culture was NOT SUCCESSFUL... a LYTIC CELL CULTURE SYSTEM IS NOT AVAILABLE", and the genomes had to be amplified directly from positive pig samples by genome walking. 31.4.6: "THREE INDEPENDENT REAL-TIME PCR ASSAYS have been developed to quantify PLHV1, PLHV2, or PLHV3 genome copy numbers in porcine samples", plus conventional single-round assays and a degenerate panherpes consensus PCR. "The very low level of PLHV intraspecies variation (<1% in coding regions) ENSURES THE CONSERVATION OF THE PRIMER BINDING SITES used in the PCR and real-time PCR assays." Two ELISAs using recombinant PLHV1 gB have been described, and one was checked against PCR across age groups. Two limits: the consensus PCR cannot resolve mixed infections, which are common, and "since organ samples are not readily available from live pigs, routine PLHV diagnosis relies on testing white blood cells. SUCH TESTING SHOULD BE INTERPRETED WITH CAUTION since PLHVs are more frequently detected in the spleen and lungs than in the blood."
How this level was decided
Level 3, and FLAGGED AS A LEVEL 3 OR 4 JUDGMENT. Under the 2026-08-29 rewording the axis is whether a validated assay exists and where it can be run, and these assays are better characterised than the level 4 group: three virus-specific real-time PCRs, a documented reason to trust the primer sites, and two ELISAs one of which was cross-checked against PCR. That is not "primers that have not been critically evaluated". But they exist to serve xenotransplantation screening and research, not veterinary diagnostics -- no swine diagnostic laboratory offers a PLHV panel, because there is no clinical disease to test for -- which is the level 3 label: a validated assay exists, but at reference or research level. THE CASE FOR LEVEL 4 is that no assay has been evaluated for diagnostic sensitivity in the way C4 asks about, and the chapter itself warns that the routinely available sample type, blood, is the least sensitive one.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
Basis. ch31 31.4.5, first sentence: "A CLINICAL DISEASE ASSOCIATED WITH PLHV INFECTION UNDER FIELD CONDITIONS IS NOT KNOWN." The one demonstrated disease is experimental and iatrogenic: "immunosuppressed miniature swine subjected to allogeneic hematopoietic stem cell transplantation. A HIGH INCIDENCE OF POST-PTLD WAS OBSERVED, B-cell proliferation occurred, and the majority of the animals died", with high PLHV1 genome copy numbers and PLHV1 transcripts present in affected animals and absent in healthy ones. The chapter records one caveat on its own negative: PLHVs "might have a similar pathogenic potential, either in foreign hosts or in their natural host. THE LATTER MAY BE DIFFICULT TO OBSERVE SINCE MOST COMMERCIAL PIGS ARE SLAUGHTERED AT APPROXIMATELY 6 MONTHS OF AGE."
How this level was decided
Level 1, and this is an examined negative rather than a silence, which is the distinction that separates a supported level 1 from an unassessed cell. These viruses have been looked for in domestic pigs and wild boar across three continents for twenty-five years, are found in half to 95% of them, and no field disease has ever been associated with them. Disease appears only when a pig is deliberately immunosuppressed for a transplant experiment. No measurable effect on cost of production. NOTE THE CHAPTER'S OWN CAVEAT, recorded because it is the one thing that could overturn this: a slowly developing lymphoproliferative disease might be invisible in a population slaughtered at six months.
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 impact on markets"
Basis. Viruses carried by a majority of pigs worldwide with no known field disease, and no trade measure, movement control or consumer response recorded anywhere in chapter 31.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact, agents already present in US pigs with no market move on record are level 1. Nothing that causes no recognised disease reaches pork supply or demand.
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. No treatment for PLHV infection is described anywhere in chapter 31. 31.4.5: "A clinical disease associated with PLHV infection under field conditions is not known." 31.4.8 discusses control solely in terms of producing PLHV-free donor animals for xenotransplantation, by caesarean derivation and barrier rearing.
How this level was decided
Level 1 on the second clause -- animals recover acceptably without treatment, because on the evidence held they do not become ill. Eric's 2026-09-02 rule forbids the lazy route to this answer, that nobody treats viral disease, and it is not the route taken here: C7 asks whether outcomes would meaningfully improve if a treatment existed, and for an infection with no recognised field outcome there is nothing to improve. This cell moves with C5 if a field disease is ever described.
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. No vaccine for any PLHV is mentioned anywhere in chapter 31. 31.4.8 gives the only control objective the chapter recognises and it is not a veterinary one: "Concerns regarding the safety of pig-to-human xenotransplantation make PLHV-FREE DONOR PIGS A DESIRABLE GOAL", achieved by caesarean derivation and barrier rearing rather than by vaccination.
How this level was decided
Level 1 on the "not needed" clause. There is no vaccine, so the question is whether the disease would justify one on the agent's own clinical and economic weight, and an infection with no known field disease does not. NOTE WHERE THE ONLY DEMAND SITS, and that the criterion cannot see it: xenotransplantation wants PLHV-free donor herds, and that demand is met by how the pigs are derived, not by immunising them. It belongs to human medicine rather than to pig production, which is what this MCDA prioritises.
Levels and evidence are generated from data/assignments/porcine_lymphotropic_herpesviruses.yml; the overview is authored in data/overviews/porcine_lymphotropic_herpesviruses.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.