Porcine Endogenous Retroviruses
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_endogenous_retroviruses |
| Type | virus |
| Scientific name | Porcine endogenous retrovirus |
| NCBI taxid | 61673 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Porcine endogenous retroviruses are not an infection in the ordinary sense: they are retroviral sequences written into the DNA of every pig, inherited like any other gene. Most European breeds and wild boars carry around 60 copies of PERV-A and PERV-B; PERV-C is present in some animals but not all. Because they are part of the genome they cannot be removed by treatment, vaccination, caesarean delivery, colostrum deprivation, early weaning or embryo transfer, and there are no data showing one pig transmitting them to another. They cause no disease in pigs. They are on this register because of xenotransplantation. PERVs are related to the murine, feline and koala leukaemia viruses, which cause lymphoma, leukaemia and immunodeficiency in their hosts, and PERV particles suppress human immune cells in the test tube — so the theoretical risk of transmitting them to a human organ recipient has been taken seriously. The record so far is reassuring: no transmission was seen in pig-to-primate transplantation experiments with or without immunosuppression, none in the early human clinical trials, and none in two larger islet-cell trials. There is no evidence they can be transmitted to people by food or direct contact.
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 41.5.1: "There is no evidence that PERVs can be transmitted to humans by food or direct contact." "no PERV transmission was observed in pig-to-small animal or pig-to-nonhuman primate transplantation experiments, either with or without pharmaceutical immunosuppression. The same was true for the first clinical trials in humans." In two larger islet-cell trials "no PERV transmission was observed".
How this level was decided
Level 1, and the negative is unusually well tested. PERV infects human cells in culture, so the question was asked seriously and repeatedly -- across food exposure, direct contact, animal transplantation models, and human clinical trials including encapsulated islet grafts -- and no transmission has been observed in any of them. The residual concern is confined to vascularised organ xenotransplantation into severely immunosuppressed recipients, which is not a route by which a pig transmits an agent to a person in the sense C1 measures.
C2 Zoonotic impact
Severity of human illness caused by the agent
No human illness: Agent does not cause illness in people
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c2l1, these agents are a persistent worry but only as they relate to a potential risk during xenotransplantation. There is no evidence of transmission to humans at this time."
Basis. No human PERV infection has been documented, so no human illness exists to describe. ch41 41.5.1 notes that PERVs are related to murine, feline and koala leukaemia viruses, which "induce lymphoma, leukemia, and immunodeficiencies in their infected hosts", and that PERV particles are immunosuppressive in vitro against human PBMCs.
How this level was decided
INFERRED. C2 scores level 1 whenever the agent does not cause disease in people, and no person has been shown to be infected. FLAGGED as the weakest kind of level 1: the folder describes a plausible mechanism for serious disease -- relatives that cause leukaemia and immunodeficiency, and demonstrated immunosuppressive activity against human cells -- and scores level 1 only because the infection has never occurred. The general rule forbids scoring hypothetical futures, which is what holds it here.
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. ch41 41.5.4: PERVs "cannot be eliminated by treatment, vaccination, cesarean delivery, colostrum deprivation, early weaning, or embryo transfer because they are integrated into the genome of every pig". 41.5.1: PERV-A and PERV-B "are present in the genome of all pigs", with most European breeds and wild boars carrying around 60 copies. 41.5.2: "there are no data showing transmission of PERV from one pig to another".
How this level was decided
The most absolute level 1 in the register. This is not an agent that is present in every herd -- it is written into the germ line of every pig, at roughly sixty copies, and has been for something like seven million years. There is no introduction event to prevent because there is no introduction, and the chapter lists every conventional exclusion method and says none of them touches it.
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 41.5.3 lists PCR, nested PCR, qPCR, droplet digital PCR, Southern blot, RT-PCR, immunoperoxidase, immunofluorescence, Western blot, IHC, immunogold TEM, reverse transcriptase assays, and infectivity assays on specific target cells -- all research methods, several requiring "specific antibodies against viral proteins". Detecting PERV in a recipient additionally "requires exclusion of the presence of pig cells (so-called microchimerism) to avoid false positive results".
How this level was decided
Level 3. The methods are numerous, well characterised and validated in the xenotransplantation literature, so this is not level 4 -- but every one of them is a research assay, and the field that uses them is transplant safety rather than veterinary diagnostics. No local or regional laboratory offers PERV work, and nothing in a pig would prompt anyone to ask.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
Basis. ch41 41.5.1: "it is still unknown whether PERVs cause clinical disease in swine". 41.5.2: "there is no evidence suggesting that these melanomas were caused by PERV", and of PERV isolated from lymphoma and leukaemia cells, "there is no evidence indicating an association with the disease". The one correlation reported runs the other way: pigs in units with higher mortality showed higher PERV expression, "probably due to a higher PERV expression in lymphoid organs as the result of a more vigorous immune response against the endemic infections".
How this level was decided
Level 1. The chapter examines the disease question directly and finds nothing, and it disposes of the one apparent correlation by reversing it -- PERV expression rises in response to other endemic disease rather than causing it. No measurable effect on cost of production. This is scored rather than left unassessed because causation was investigated and rejected, not merely unaddressed.
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 agent is widespread in the US and has no impact on the current market"
Basis. Section 41.5 records no trade measure, movement control or consumer response. PERV is in the genome of every pig that has ever been traded.
How this level was decided
INFERRED. An agent integrated into the germ line of the entire species cannot disrupt a market that has always been trading it. 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. ch41 41.5.4: PERVs "cannot be eliminated by treatment, vaccination, cesarean delivery, colostrum deprivation, early weaning, or embryo transfer". 41.5.1: whether PERVs cause clinical disease in swine "is still unknown".
How this level was decided
Level 1 on the second clause. There is no established adverse outcome in pigs for a treatment to improve, and the chapter says outright that treatment cannot remove the virus in any case. NOTE: the antiretroviral drugs and neutralising-antibody vaccines the chapter describes are aimed at protecting a human xenotransplant recipient, not at treating a pig.
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. ch41 41.5.4 describes "vaccines based on neutralizing antibodies against the envelope proteins of PERV" among the tools to prevent transmission to xenotransplant recipients, and states that PERVs cannot be eliminated by vaccination because they are integrated into the genome of every pig.
How this level was decided
Level 1 on the second clause of the reworded criterion. Vaccine work exists, but it is directed at protecting human transplant recipients rather than at any porcine disease, and the chapter says vaccination cannot eliminate an integrated provirus anyway. There is no unmet need in the pig to record.
Levels and evidence are generated from data/assignments/porcine_endogenous_retroviruses.yml; the overview is authored in data/overviews/porcine_endogenous_retroviruses.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.