Mycoplasma hyorhinis
LEVELS: Highly unlikely; No human illness; Already in the herd; Laboratory-dependent; Moderate; Negligible; Some; Needed but not available
| Register id | mycoplasma_hyorhinis |
| Type | bacteria |
| Scientific name | Mesomycoplasma hyorhinis |
| NCBI taxid | 2100 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Mycoplasma hyorhinis lives in the tonsils and airways of nearly every pig by the end of the nursery period, and in a minority of them it crosses into the body and causes disease: inflammation of the membranes lining the chest, abdomen and joints in pigs of about three to ten weeks. It has also been linked to middle-ear and eye infection, abortion, pneumonia, and more recently a fibrinopurulent meningitis. It is regarded as an emerging pathogen in North America, though whether it is genuinely more common or simply better detected is unresolved. The organism varies the proteins on its surface continually, which is thought to help it evade immunity and may explain why it persists in herds even after closure and medication aimed at eliminating it. There is no commercial vaccine; autogenous products are used in some systems with limited published evidence. Control is therefore mostly antimicrobial, complicated by reduced susceptibility to several of the drugs used.
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. ch53 53.3.2: causality in human disease has not been established, and swabbing the noses of swine farmers failed to find the organism.
Basis. ch53 53.3.2, a Public Health section of its own -- the only one in the chapter: "The ubiquity of M. hyorhinis is supported by common findings of contamination of a wide variety of cell cultures derived from multiple mammalian species. M. hyorhinis is one of the most important bacterial considerations when evaluating cell culture purity, yet contamination of animal and human biologics has been reported. Several studies have reported an ASSOCIATION between the presence of M. hyorhinis and a variety of human carcinomas and even infertility, BUT THE CAUSALITY OF M. HYORHINIS IN HUMAN DISEASE HAS NOT BEEN ESTABLISHED. A STUDY LOOKING INTO THE DETECTION OF M. HYORHINIS IN NASAL SWABS OBTAINED FROM SWINE FARMERS FAILED TO SHOW A HIGH PREVALENCE of the bacterium in that population." 53.3.1 adds that its wide carbohydrate metabolism "may explain its ability to invade tissues beyond the respiratory tract in pigs, potentially infect other mammals, and thrive as a common contaminant of cell cultures."
How this level was decided
MOVED L2 -> L1, re-read 2026-09-04 AGAINST THE REWORDED LABELS. ch53 53.3.2: "THE CAUSALITY OF M. HYORHINIS IN HUMAN DISEASE HAS NOT BEEN ESTABLISHED", and the one direct test of the pig-to-human route is negative -- swabbing the noses of swine farmers "FAILED TO SHOW A HIGH PREVALENCE".
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: Level 2, and FLAGGED AS A LEVEL 1 OR 2 JUDGMENT because the only direct test of the pig-to-human route in the whole chapter is negative. The organism turns up in human material and in human tumours, so a question exists and the authors thought it worth a section -- that is more than the silence most entries rest on, and it is why this is not level 1 on the face of it. But the association is explicitly not causal, cell culture contamination is a laboratory route rather than a farm one, and when someone swabbed the noses of the people most exposed to infected pigs they did not find it. THE CASE FOR LEVEL 1 is that "no evidence that pigs transmit the agent to humans" is a fair reading of a negative farmer study plus an unestablished causality. THE CASE FOR LEVEL 2, which is scored, is that the level 2 label says human infection from pigs is plausible but has never or rarely been reported and no control points are implemented against it, and every clause of that is true here.
C2 Zoonotic impact
Severity of human illness caused by the agent
No human illness: Agent does not cause illness in people
Basis. ch53 53.3.2: "the causality of M. hyorhinis in human disease has not been established." No human illness is attributed to pig-derived M. hyorhinis anywhere in the chapter.
How this level was decided
Level 1. C2 grades the severity of illness ACQUIRED FROM PIGS, and no illness has been shown to be acquired from pigs, or indeed shown to be caused by the organism at all. THE LABEL MUST NOT BE READ AS "THIS AGENT IS HARMLESS TO PEOPLE": the carcinoma and infertility associations are real published findings and if any of them were ever shown to be causal, the severity would not be trivial. Level 1 records that pigs have not been shown to be a source, which is the question C2 asks after the 2026-09-01 rescoping.
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. ch53 53.3.3: "it is considered to have worldwide distribution and be PREVALENT EVEN IN HERDS WHERE M. HYOPNEUMONIAE HAS BEEN ERADICATED." "Mycoplasma hyorhinis is usually considered A COMMENSAL microorganism, colonizing the tonsils and respiratory epithelium of the nasal cavity and conducting airways. Most colonized pigs have no apparent clinical disease." On the numbers: M. hyorhinis was detected by PCR in "slightly more than 10% of suckling pigs", and "the level of bacterial colonization increases rapidly in piglets after weaning, and ALMOST 100% DETECTION AT THE GROUP LEVEL has been reported at the end of the nursery period... These high levels of colonization with M. hyorhinis appear to remain through the finishing phases." 53.3.9: "Due to the high prevalence of infection... measures to eliminate herd infection are generally not considered. PUBLISHED REPORTS OF THE SUCCESSFUL ELIMINATION OF M. HYORHINIS FROM SWINE HERDS ARE NOT AVAILABLE", and "the bacterium can be detected in herds even after undergoing elimination efforts, including herd closure and antibiotic treatment."
How this level was decided
Level 1, and this is the cleanest level 1 in the batch. The label asks for an agent present in most herds or carried by pigs themselves, so that there is no introduction event to prevent, and the chapter supplies both halves: it is a commensal of the pig's own tonsil and nasal cavity, and group-level detection approaches 100% by the end of the nursery and stays there. It survives the one intervention that removes M. hyopneumoniae from a herd, and nobody has ever published an elimination. There is nothing to keep out.
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. ch53 53.3.8: "Fibrinous polyserositis and arthritis in nursery-age pigs can also be caused by G. parasuis, S. suis, A. suis, or M. hyosynoviae (arthritis only)." "BECAUSE M. HYORHINIS IS A COMMENSAL in the tonsil, nasal cavity, and lungs, BACTERIAL DETECTION IN ANY OF THESE SITES DOES NOT CONFIRM IT as the cause of disease in systemic sites." Confirmation "requires demonstration of characteristic lesions, confirmation of infection in the observed lesions, and elimination of other potentially causal agents by negative tests", sampling fibrin or swabs from affected serosal surfaces or joints. "DNA hybridization and conventional, multiplex, and real-time PCR assays have been developed for the detection of M. hyorhinis nucleic acid... THE OVERALL ACCURACY OF PCR-BASED TESTS IS HIGH." Serology "is not generally used for diagnosis due to the challenges of interpretation and the lack of commercially available assays", and "would not likely differentiate localized commensal infection from systemic disease-inducing infection".
How this level was decided
Level 2. A validated and accurate PCR exists and runs in ordinary diagnostic laboratories, which rules out levels 3 and 4. What keeps it off level 1 is not the assay but the interpretation: polyserositis and arthritis in a nursery pig point at four other organisms before this one, and because the agent is a normal inhabitant of the nose and tonsil, a positive from the usual sampling sites means nothing. The diagnosis requires lesions, a positive from INSIDE the lesion, and negatives for the differentials. That is laboratory-dependent work on a presentation that does not point here, which is the level 2 label.
C5 Production cost
Financial impact on the infected farm's cost of production
Moderate: Losses measurably increase cost of production but remain manageable within normal farm operations, whether acute, chronic, or associated with endemic disease
Corrected by Eric from L2 Minor. Reason: "c5l3, based on conference proceedings and general anecdotal evidence, m.hyorhinis and m.hyosynoviae both are causing a fair bit of persistent infection as both pneumonia and lameness in an increasing number of herds. Vaccines not useful and treatment efficacy is not great."
Basis. ch53 53.3: "it is well established as a cause of polyserositis and polyarthritis primarily in nursery-age pigs. It has also infrequently been incriminated in pneumonia, otitis, conjunctivitis, abortion, and central nervous system inflammatory lesions." 53.3.3: "M. hyorhinis is recognized as AN EMERGING PATHOGEN IN NORTH AMERICA and other regions. Whether there is a true increase in disease prevalence, or the perceived increase is the result of improved diagnostic capabilities, remains uncertain." 53.3.6: pigs 3-10 weeks old are typically affected "ALTHOUGH THEY MAY NOT EXHIBIT CLINICAL SIGNS. A portion of affected animals may exhibit lethargy, fever, and anorexia", with respiratory distress where there is pleuritis and lameness where there is arthritis. Arthritis has been observed up to 2 months after inoculation and the organism persists in joints up to 8 months. No mortality figure, morbidity figure or cost figure appears anywhere in section 53.3.
How this level was decided
Level 2, and FLAGGED AS A LEVEL 2 OR 3 JUDGMENT that Eric is better placed to settle than the chapter is. THE CHAPTER GIVES NO NUMBER OF ANY KIND -- no morbidity, no mortality, no cost -- for a disease it calls emerging in North America, which is unusual enough to say out loud. Level 2 is scored because what is described is a nursery-age condition affecting a portion of colonised pigs, many of which show no signs at all, with no mortality claimed: small and generally short-lived losses. AGAINST THAT, and this is the level 3 argument: it is emerging in US herds, autogenous vaccines have "gained popularity among certain production systems" which is producers spending money, and chronic arthritis persisting for months in a nursery pig is not short-lived. If it is a common cause of nursery culls in US systems the answer is level 3, and that is a field question.
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 this is effecting markets"
Basis. Worldwide distribution, near-universal colonisation of US nursery pigs, and no trade measure, movement control or consumer response recorded anywhere in section 53.3.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact, a commensal carried by essentially every US pig by the end of the nursery, with no market move on record, is level 1.
C7 Treatment potential
Potential for treatment to improve outcomes
Some: Treatment exists but is inconsistent, requires extralabel use, or works only in some circumstances
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c7l2, exemplar of treatmeent existing, extralabel, with less than perfect efficacy"
Basis. ch53 53.3.9: "Due to the lack of a cell wall, M. hyorhinis is naturally resistant to beta-lactams. M. hyorhinis is generally susceptible to macrolides, lincomycin, and tetracyclines. Decreased susceptibility or resistance to certain antimicrobial drugs has been suggested (Kobayashi et al. 2005), and regular antimicrobial susceptibility monitoring takes place mainly in Europe." "In general, control efforts are primarily focused on the mitigation of clinical disease through antibiotic therapy and, sometimes, vaccination."
How this level was decided
Level 2. Active drugs exist and antimicrobial therapy is the mainstay of control, so this is not level 3. It is held off level 1 on the label test: no US product is known to name M. hyorhinis, which would make every one of these treatments extralabel, and extralabel use is the level 2 clause in terms. FLAGGED FOR ERIC UNDER THE RULE HE SET ON THE TIAMULIN LABEL: "Ask me when it is not clear and I will decide." The US label text is not in the folder, so the species question is not decidable from the evidence here. The pharmacology points the same way independently: reduced susceptibility has been reported, monitoring happens mainly in Europe rather than the US, and the disease that matters is systemic -- polyserositis and arthritis, where the organism has already left the mucosa and can persist in joints for eight months, so antimicrobial therapy is mitigating rather than curing.
C8 Vaccine availability
Availability of effective vaccines or bacterins
Needed but not available: No effective vaccine is available in the US or has been developed, and the disease would justify vaccinating if one existed
Corrected by Eric from L2 Available but inconsistent. Reason: "c8l3, emerging important disease without effect treatment or vaccine"
Basis. ch53 53.3.9, in full: "COMMERCIAL VACCINES FOR M. HYORHINIS ARE CURRENTLY NOT AVAILABLE. AUTOGENOUS VACCINES for M. hyorhinis HAVE GAINED POPULARITY among certain production systems, BUT EFFICACY EVALUATIONS OF SUCH PRODUCTS HAVE NOT BEEN PERFORMED OR ARE NOT IN THE PUBLIC DOMAIN."
How this level was decided
Level 2, and FLAGGED AS A LEVEL 2 OR 3 JUDGMENT because it turns on how an untested autogenous product is counted. Level 2 is scored because its label says commercial OR AUTOGENOUS vaccines exist in the US but protection may be inconsistent, and that is this sentence almost word for word -- autogenous products exist, US producers are buying them, and nobody can say whether they work. THE CONTRAST WITH TWO RECENT RULINGS IS WHAT MAKES IT A QUESTION. Brachyspira pilosicoli sits at level 3 with an autogenous bacterin in the record, because that one was tested and FAILED on challenge -- a demonstrated gap. E. coli non-enteric infection was moved to level 1 by Eric on 2026-09-02 because the autogenous route there addresses a different organism. This case is neither: the product targets the right organism and has simply never been evaluated. Unknown efficacy is what level 2 means, so level 2 is scored, but if the view is that an unevaluated autogenous vaccine is not a vaccine, this is level 3.
Levels and evidence are generated from data/assignments/mycoplasma_hyorhinis.yml; the overview is authored in data/overviews/mycoplasma_hyorhinis.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.