Enzootic Pneumonia
LEVELS: Highly unlikely; No human illness; Routine biosecurity keeps it out; Laboratory-dependent; Moderate; Negligible; Some; Available but inconsistent
| Register id | mycoplasma_hyopneumoniae |
| Type | bacteria |
| Scientific name | Mesomycoplasma hyopneumoniae |
| NCBI taxid | 2099 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Mycoplasma hyopneumoniae causes enzootic pneumonia and is the foundation of the porcine respiratory disease complex. It infects only pigs. Once inhaled it colonises the ciliated lining of the airways for months — seven to eight is typical — stripping away the mucociliary escalator that normally clears the lung and blunting the immune response. The direct effect is a chronic cough with high morbidity and low mortality; the larger effect is that it opens the door for everything else, worsening Pasteurella multocida, Streptococcus suis and Glaesserella parasuis and potentiating PRRSV and PCV2. The cost is almost all in performance: 6–20% less daily gain, worse feed conversion and more days to market. Spread is slow and needs close nose-to-nose contact — roughly one or two new pigs infected per infected pig over four to six weeks — but airborne spread between farms has been documented over both short and long distances. It is present wherever pigs are kept apart from Switzerland and Norway, which have eradicated it nationally. Vaccines improve performance but do not prevent colonisation or much reduce transmission, so herd-level elimination requires depopulation, herd closure with medication, or partial depopulation.
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. "c1l1, this is a better rating than i had initially proposed as an example, should be considered unlikely as i can't find any reference to human infetion"
Basis. ch53 53.2, first paragraph: "THE PATHOGEN INFECTS ONLY PIGS and induces long-term colonization of the ciliated respiratory epithelium." The chapter gives a Public Health section to one species only, M. hyorhinis at 53.3.2, and says nothing about human infection with M. hyopneumoniae anywhere.
How this level was decided
Level 1, and this is a stated host restriction rather than a silence. The chapter opens the section by saying the organism infects only pigs, and it demonstrates elsewhere that its authors write about public health where they think there is a question -- M. hyorhinis has its own numbered section for exactly that. The absence of one here is informative. FLAGGED: THIS SCORE DISAGREES WITH THE CRITERIA FILE, and validate_criteria.py reports it. criteria_levels.yml names Mycoplasma hyopneumoniae as a worked example at C1 LEVEL 2, beside pseudorabies virus. The chapter says the pathogen infects only pigs, which is a stated host restriction and cannot support level 2 -- level 2 requires that human infection from pigs be plausible. The comparison with its co-example is instructive: pseudorabies belongs at level 2 because human PRV encephalitis has been reported, rarely; nothing equivalent exists for this organism. THE PASS TREATS THIS AS A BADLY CHOSEN EXAMPLE RATHER THAN A WRONG SCORE and asks Eric to confirm. If he agrees, the example should be moved to C1 level 1 or dropped, which is the same shape of edit he made when Cystoisospora suis left C3 level 4.
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 M. hyopneumoniae is described anywhere in chapter 53. The organism "infects only pigs" (53.2).
How this level was decided
Level 1. C2 as rescoped grades illness acquired FROM PIGS, and an organism that infects only pigs is not a source of any. This is the straightforward case, not the one where the label has to be read carefully.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Routine biosecurity keeps it out: Quarantine of incoming stock, transport and fomite control, cleaning and disinfection, and the usual monitoring reliably exclude it
Corrected by Eric from L4 Extraordinary biosecurity required. Reason: "c3l3, in the past we did not think this was eradicable and now we know it is. In all but the most rare instances, L3 will keep it out of herds. There is a small amount of evidence that the infection can aerosolize over a few 100m or perhaps a bit further but this is really the exception rather than the rule"
Basis. ch53 53.2.2: "Infections caused by M. hyopneumoniae are present in virtually every country where pigs are raised, except Switzerland and Norway... Once M. hyopneumoniae is introduced to a swine herd, the herd typically remains endemically infected." On the routes: "M. hyopneumoniae is most commonly introduced into naive populations by direct transmission through the introduction of infected pigs. However, AIRBORNE TRANSMISSION of M. hyopneumoniae has been suspected for decades and has been shown over short (Fano et al. 2005) and LONG DISTANCES (Otake et al. 2010). Several studies of the Danish specific-pathogen-free systems found that HERD REINFECTIONS with M. hyopneumoniae often occurred in the autumn and winter when atmospheric conditions favored aerosol transmission." On finding carriers: colonisation "persists for long periods, typically 7-8 months", shedding varies over time, "seroconversion does not typically occur in all infected animals" and "antibodies are not detectable during the entire duration of the infection". 53.2.11: eradication by partial depopulation, or herd closure and medication, is practised widely in North America, and "diagnostic data from M. hyopneumoniae-negative flows raised in pig-dense areas have instilled trust in the likelihood of success", but "reservations on elimination decisions are mostly based on the perceived risk for reinfection".
How this level was decided
Level 4, and FLAGGED AS THE JUDGMENT MOST WORTH ERIC'S EYE IN THIS BATCH, because a case can be made for level 1 and for level 3 and the three answers are far apart. THE CASE FOR L1 is the chapter's own sentence that most herds are endemically infected, which is the level 1 label. It is rejected because there plainly IS an introduction event to prevent here: US producers run eradication programmes, maintain negative flows and pay to keep the organism out, which is not something anyone does about an agent with no introduction to prevent. THE CASE FOR L3 is that the commonest route is an infected pig, and quarantine of incoming stock is the routine measure that addresses it. It is rejected on two facts the chapter states: carriers are hard to find -- colonisation runs 7 to 8 months with intermittent shedding, and serology misses infected animals entirely -- so finding them is the "repeated costly testing to find carriers" that the L4 label names; and aerosol spread over long distances is documented, with Danish SPF herds reinfecting seasonally when the weather favoured it, which no quarantine protocol touches. AIR FILTRATION AND COSTLY CARRIER TESTING ARE BOTH NAMED IN THE L4 LABEL and both are what this organism actually requires. It is not L5, because the reservoir is other pig herds rather than something outside the industry -- the Swiss and Norwegian wild boar note is the only exception the chapter raises and it applies to countries that have already eradicated.
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 53.2.7: "Mycoplasma hyopneumoniae does not cause pathognomonic clinical signs or lesions... diagnosis of M. hyopneumoniae infection should not be based only on the presence of clinical respiratory signs, as animals can be subclinically infected." Gross lesions "are not pathognomonic... as other organisms like influenza A virus or the combination of P. multocida and Aujeszky's disease virus may produce similar lesions." Culture is the traditional gold standard but "slow growth, frequent overgrowth by other bacteria, high cost, and low sensitivity of this method limit its usefulness", and 53.2.1 says outright that "culture is not used as a routine diagnostic method". Against that: "Various PCR assays have been developed and VALIDATED, including gel-based, nested, real-time, and digital PCR... High accuracy, fast turnaround, high throughput, and utility for use with tissue samples collected post mortem or clinical samples collected from live pigs have made real-time PCR the most-used confirmatory test." Commercial ELISAs are available and their accuracy has been compared without significant differences being found.
How this level was decided
Level 2 on both halves of the label. The presentation does not point here -- a nonproductive cough has many causes, the lesions are shared with influenza A and with Pasteurella plus Aujeszky's, and animals can be infected with no signs at all. But a validated assay exists and runs in ordinary diagnostic laboratories: real-time PCR is described as the most-used confirmatory test, with high accuracy and fast turnaround, on samples that can be taken from live pigs. Under C4 as reworded the axis is whether a validated assay exists and where it runs, and this one is validated and everywhere. NOTE THE INTERPRETIVE TRAPS the chapter lists, which are limits on reading the result rather than on getting one: PCR detects viable and nonviable cells alike and cross-contamination is possible; serology is a population tool because maternal, infection and vaccination antibodies are indistinguishable; and M. flocculare, a nonpathogenic commensal sharing 78% of coding sequences, causes false positive ELISA results.
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
Basis. ch53 53.2: "EP is characterized by chronic bronchopneumonia, HIGH MORBIDITY, LOW MORTALITY, and decreased production performance. THE ECONOMIC IMPACT OF EP IS SIGNIFICANT and primarily related to decreased average daily gain, increased feed conversion ratio, and increased number of days to reach market weight." The size of the effect is given indirectly through what removing it is worth: 53.2.10, "vaccination improves daily weight gain (2-8%), feed conversion ratio (2-5%), and sometimes mortality rate. Additionally, a shorter time to reach slaughter weight, reduced clinical signs and lung lesions, and lower treatment costs are observed." 53.2.3: "Infection with M. hyopneumoniae often has its greatest economic significance due to interaction with other respiratory pathogens", and it is a major component of PRDC alongside PRRSV and PCV2.
How this level was decided
Level 3, and it reads on the label's own words -- losses that measurably increase cost of production but remain manageable within normal farm operations, "whether acute, chronic, or associated with endemic disease". This is the endemic case in its purest form: high morbidity, low mortality, and a continuous tax on feed conversion and days to market rather than an event. The vaccination figures put a number on it, because 2-8% of daily gain and 2-5% of feed conversion is what the herd is losing when it is not vaccinated. It is not level 4, because nothing here is unsustainable -- the disease is present in most of the world's pigs and they are still farmed profitably -- and it is not level 2, because the losses are neither small nor short-lived: coughing can persist through the whole finishing period and the interaction with PRRSV and PCV2 is where the chapter says the money actually goes.
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, old disease with no impact on the market"
Basis. ch53 53.2.2: present "in virtually every country where pigs are raised" bar two. No trade measure, movement control or consumer response is recorded anywhere in chapter 53.
How this level was decided
INFERRED. Under the standing note SCORING/market_impact, an agent endemic in most US herds and diagnosed continuously for decades, with no market move ever recorded, is level 1. This is the endemic case the note was written for.
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, there are labelled meds available but their efficacy is not consistent"
Basis. ch53 53.2.8: "Potentially active antimicrobials against M. hyopneumoniae include tetracyclines, macrolides, lincosamides, pleuromutilins, amphenicols, aminoglycosides, aminocyclitols, and fluoroquinolones... most agents belonging to the former antimicrobial classes improved clinical parameters (clinical signs, lung lesions) and reduced performance losses." Tiamulin and valnemulin had the lowest MIC values in pan-European monitoring. AGAINST THAT: "Antimicrobials may not prevent infection or eradicate the bacterium from the respiratory tract but reduce the number of M. hyopneumoniae organisms"; "The results of antimicrobial susceptibility testing are difficult to interpret in terms of treatment outcome, AS NO CLINICAL BREAKPOINTS HAVE BEEN DEFINED for M. hyopneumoniae"; and "although treatment may lead to clinical improvement, CLINICAL SIGNS OF EP AND SHEDDING OF M. HYOPNEUMONIAE TYPICALLY REAPPEAR AFTER CESSATION OF THERAPY."
How this level was decided
Level 2, and it reaches that level twice over. The label test is the first route: several US products are used against mycoplasmal pneumonia, but the label wording is not in the folder and the species question is exactly the one Eric reserved to himself. 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 SECOND ROUTE DOES NOT DEPEND ON THE LABEL AT ALL, which is why level 2 is scored rather than held: the C7 level 1 label asks for treatment that is available and WORKS, and the chapter says treatment does not clear the organism and that signs and shedding typically return once medication stops. That is "works only in some circumstances" in the level 2 label. It is not level 3, because effective drugs plainly exist and improve clinical signs, lung lesions and performance. If Eric rules that a US label names M. hyopneumoniae, the cell still sits at level 2 on the relapse finding -- the label ruling would change the reasoning rather than the level.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c8l2, under perfect circumstances (usually related to vaccine timing) the vaccines are pretty good but really only reduce clinical signs and generally would not prevent the clincial disease or lesions completely. Close to an L1 but we will make L2."
Basis. ch53 53.2.10: "Vaccination is widely applied worldwide to control M. hyopneumoniae infections. Commercial vaccines mostly consist of inactivated, adjuvanted whole-cell preparations that are administered intramuscularly... Commercial vaccines are licensed for either single or double vaccination, and combinations with other pathogens (e.g. PCV2) are commonly available." AND THE LIMITS, in the same section: "Although protection against clinical pneumonia is OFTEN INCOMPLETE and vaccination DOES NOT PREVENT COLONIZATION, currently used vaccines reduce the number of organisms in the respiratory tract... However, vaccination DOES NOT SIGNIFICANTLY REDUCE THE TRANSMISSION of M. hyopneumoniae." "Although vaccination confers beneficial effects in most infected herds, THE EFFECTS ARE VARIABLE BETWEEN THEM." 53.2.9 adds the operational consequence: gilt acclimation "is challenging since vaccination does not prevent infection or shedding".
How this level was decided
Level 2, and the label describes this vaccine exactly -- commercial products exist in the US and protection may be inconsistent. Both halves are stated outright. The products are widely available, licensed, commonly sold combined with PCV2 antigen, and they deliver measurable gain and feed conversion benefits; and they do not prevent colonisation, do not meaningfully reduce transmission, and vary in effect between herds. It is not level 1, because "available and effective" overstates a vaccine the chapter says confers often incomplete protection, and it is not level 3, because a commercial product plainly exists and is worth buying. FLAGGED: THIS SCORE DISAGREES WITH THE CRITERIA FILE, and validate_criteria.py reports it. criteria_levels.yml names Mycoplasma hyopneumoniae as a worked example at C8 LEVEL 1, beside neonatal colibacillosis and Lawsonia. THE ARGUMENT FOR LEVEL 1 IS REAL AND IS THE FRAMEWORK'S OWN: M. hyopneumoniae vaccine is among the most widely used products in US swine practice, it is licensed, it is commonly bought combined with PCV2 antigen, and it delivers measurable gain and feed conversion benefit -- "effective vaccines are widely available in the US" describes it fairly. THE ARGUMENT FOR LEVEL 2, WHICH IS SCORED, is that the chapter's own words are the level 2 label almost verbatim: protection is "often incomplete", vaccination "does not prevent colonization", it "does not significantly reduce the transmission", and "the effects are variable between them". This is a genuine disagreement about the entry rather than about the example, and it is Eric's to settle. NOTE THE CONSEQUENCE EITHER WAY: this entry and the three other Mycoplasma entries are the batch's whole C8 spread, and M. hyorhinis sits at level 2 on an autogenous product nobody has evaluated -- if the best-supported commercial vaccine in the chapter also scores level 2, the criterion is not separating them.
Levels and evidence are generated from data/assignments/mycoplasma_hyopneumoniae.yml; the overview is authored in data/overviews/mycoplasma_hyopneumoniae.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.