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Proposed classification system to improve M. hyo control tested in commercial setting

By Lucina Galina Pantoja, DVM, PhD
Director, swine technical services


Enzootic pneumonia caused by Mycoplasma hyopneumoniae (M. hyo) has been a well-known feature of swine production for decades. Most pork producers are familiar with the dry, non-productive cough and labored breathing that characterizes the disease.

M. hyo -associated enzootic pneumonia has been shown to reduce growth rate by 16% and feed conversion by 14%.1 It’s been estimated that for every 10% increase in swine lung tissue affected by pneumonia, there’s a 41-gram decrease in average daily gain and a 16.7 increase in days to market weight.2 On M. hyo.-positive sites coinfected with swine influenza or porcine reproductive and respiratory syndrome (PRRS) virus — a common scenario — the cost of MH was found to be approximately $10 per pig.3

Perhaps the most serious consequence of M. hyo is its ability to colonize the respiratory cilia, enabling the infection to persist in the airways for as long as 30 weeks after exposure. The result is a population of asymptomatic but infected animals that continually expose other pigs.

It’s clear the swine industry needs a better way to manage M. hyo. One important step forward would be a classification system with standardized terminology that defines the M. hyo status of breeding herds. Such a system could help facilitate communication among swine producers, veterinarians, diagnosticians and breeding-stock companies. It could help with the evaluation of M. hyo-control strategies and support regional control and elimination efforts.

Toward this end, Zoetis sought input from 20 US swine veterinarians and proposed a herd classification system that incorporates comprehensible diagnostic criteria based on biologically relevant features. The system was introduced at the 2016 American Association of Swine Veterinarians conference and recently “pressure tested” in a commercial setting (more on the results later).

Focus on breeding herd

The proposed system focuses on the breeding herd, a population of animals that includes breeding animals and their offspring as well as gilts if there’s a gilt-development unit on the premises. The status of the entire herd is determined by the group of animals with the poorest M. hyo health status.

Table 1. Four major herd-status categories are proposed for breeding herds


Positive unstable (I)

These are breeding herds with M. hyo-associated clinical signs. M. hyo is detected within the respiratory tract of weaning-age pigs. The animals may or may not be serologically positive, as might occur when there’s a recent acute outbreak. Herds that haven’t been tested are considered category I, even if they don’t have clinical signs.

Positive stable (II)

Breeding herds don’t have obvious M. hyo-associated clinical signs and pulmonary lesions, but they are positive on serology.

There’s a reduced prevalence of M. hyo in the upper respiratory tract of weaning-age pigs (<10% of positive laryngeal swabs) for at least 90 days based on four consecutive samplings of 45 laryngeal swabs taken every 30 days.

Provisionally negative (III)

Breeding herds don’t show M. hyo-associated clinical signs and M. hyo is not detected, but the population remains serologically positive.

To qualify for category III, herds should have at least 90 days with no positive results from wean-age pigs in samples from the respiratory tract based on four consecutive samplings of 30 laryngeal swabs taken every 30 days.

Herds undergoing elimination efforts such as herd closure fall into this category.

Negative but vaccinated herds are also considered category III. Commercial farms are not infected but have implemented continued use of an M. hyo vaccine in replacement gilts that entered the breeding herd to prevent the economic consequences of an acute outbreak on a naïve farm.

Negative (IV)

Breeding herds don’t show M. hyo-associated clinical signs. M. hyo isn’t detected in any type of sample, and the population is serologically negative.

Herds undergoing elimination efforts should have been in category III for at least 1 year before they roll into category IV.

Naïve, newly established herds and those that went through complete depopulation and repopulation efforts fall into category IV. There have been no positive serology results for at least 60 days after animal placement. This should be based on a minimum of 30 sow samples (estimated prevalence 10%) and, ideally, 57 samples (estimated prevalence of 5%) collected to detect one positive.

Diagnostic criteria

M. hyo-associated clinical signs with disease are defined as a dry, non-productive cough that gets worse with physical exertion. Other clinical signs that may be present are fever, decreased appetite and labored breathing. Clinical signs, however, don’t prove M. hyo infection is present, and other respiratory pathogens must be ruled out.

Microscopic lesions have a lobular distribution of peribronchiolar and perivascular lymphocytic cuffing. Alveoli and airways may contain serous fluid with a few macrophages and neutrophils. The airway epithelium is intact and sometimes slightly hyperplastic. Microscopic lesions are nonspecific and can be similar to those observed with viral agents.

The best way to test for M. hyo lesions in the lungs or upper respiratory tract is with polymerase chain reaction (PCR). Other testing methods are immunohistochemistry, fluorescent antibody and in situ hybridization. Culturing can confirm the diagnosis, but it’s not often used because it’s time consuming and costly.

Sampling selection site

The optimal sampling site in live pigs can be affected by the timing of infection. For example, recently infected pigs might harbor M. hyo in the nasal cavity while non-acute pigs are more likely to harbor M. hyo in the larynx, trachea or deeper sections of the respiratory tract. Current literature suggests that laryngeal swabs are preferable to nasal swabs and tracheo-bronchial lavage.4

Sample-size calculations

For sample-size calculations, several factors must be considered: the assumed true prevalence, the level of precision, the sensitivity and specificity of the diagnostic test and the population size.

For instance, if it’s assumed the herd is positive and stable — Category II — at least 45 laryngeal samples should be collected from a population of 1,000 pigs to detect a 10% prevalence of M. hyo infection in weaning-age pigs.5

On the other hand, if it’s assumed the herd is provisionally negative or negative, the objective is to detect at least one positive; at least 30 samples should be collected — assuming the prevalence of M. hyo is 10% (i.e., provisionally negative) — or 57 samples if a 5% (i.e., negative) prevalence is assumed.

Both of these calculations assume nearly perfect tests and, as we all know, the diagnostic M. hyo tests available are not perfect. Therefore, veterinarians should recognize these limitations when considering sample sizes and interpreting diagnostic results.

Definition of stability

A positive correlation has been reported between the presence of M. hyo in the upper respiratory tract at weaning and the extension of pulmonary lesions at marketing.6

Based on those findings, reasonable goals for control should be to produce litters that are either negative or that have a low M. hyo prevalence at weaning. This classification has adopted the suggested cutoff point of 10% of infected weaning-age pigs to determine if a population has reached stability. This herd-status classification can be further refined when we have more information about the prevalence at weaning and its impact on pigs at the finisher stage.

The classification for PRRS describes stable herds as those that are weaning PRRS-virus-negative pigs. Under the M. hyo classification, stable herds are considered those that are weaning fewer than 10% of the pigs positive for M. hyo. This distinction was made because the M. hyo transmission rate is low compared to PRRS. We might, however, eventually revise this definition if it needs to be more consistent with other classifications used throughout the swine industry.

Role of replacement gilts critical

Introduction of replacement gilts in the breeding herd plays a critical role in M. hyo herd stability. When positive replacement gilts are introduced into a negative breeding herd, the breeding herd is likely to become positive unstable (Category I) or stable (Category II). When positive or negative replacement gilts are introduced into a positive breeding herd, the breeding herd is likely to be positive unstable (Category I) or stable (Category II).

Finally, provisionally negative or negative breeding herds that acquire negative replacement gilts are likely to maintain their provisionally negative (Category III) or negative (Category IV) status.

Pressure-testing results

We recently tested the ability of the classification system to characterize the M. hyo status in a commercial setting.

The trial involved eight farms in southeastern Iowa with 1,200 to 5,600 sows. We took 45 laryngeal swabs from the oldest piglets on each farm. The samples were collected at 0-, 30-, 60- and 90-day intervals from May to August 2016 and were then tested for M. hyo using real-time PCR performed at Iowa State University’s diagnostic lab. Two of the eight farms were found to be provisionally negative and six were positive stable.7

The trial demonstrated that the classification system worked well in a commercial setting. Using the sample size of 45 pigs and laryngeal swabs as proposed, it was possible to detect positive-M. hyo samples.


This proposed classification system is just that — a proposal — and there are still gaps of knowledge. For instance, we need an industry consensus on what makes a herd M. hyo-stable or M. hyo-unstable. We need to determine a threshold for M. hyo-colonization prevalence in weaned pigs as an indicator for disease at the finishing sites, validated in different commercial environments.

Nevertheless, it’s my sincere hope this classification system initiates collaborative efforts that identify worthwhile areas for further research and that it ultimately helps the industry identify areas of improvement.

We welcome ideas for improving this classification system.

For more information on M. hyo, including risk management, diagnostics and intervention, see A Contemporary Review of Mycoplasma Hyopneumoniae Control Strategies, authored by a distinguished panel from academia, industry and clinical practice.





1. Pointon AM, et al. Effect of enzootic pneumonia of pigs on growth performance. Aust Vet J 1985;62:13-18.
2. Hill M, et al. Association between growth indicators and volume of lesions in lungs from pigs at slaughter. Am J Vet Res 1992;53:2221-2223.
3. Haden DC, et al. Assessing production parameters and economic impact of swine influenza, PRRS and Mycoplasma hyopneumoniae on finishing pigs in a large production system. In: Proceedings 43rd Annual Meeting Am Assoc Swine Veterinarians. Denver, Colorado. 2012:75-76.
4. Pieters M, et al. Comparison of various sample types for detection of Mycolasma hyopneumoniae in recently infected pigs. Proceedings Allen D. Leman Swine Conference, St. Paul, MN. 2013;75-76.
5. EpiTools epidemiological calculators. Accessed December 2015. http;//epitools.ausvet.com/au/content.php?page=PrevalenceSS.
6. Fano e, et al. Effect of Mycoplasma hyopneumoniae colonization at weaning on disease severity in growing pigs. Can J Vet Res 2007;71:195-200.
7. Spronk E, et al. Determining Mycoplasma hyopneumoniae status in commercial breeding herds. 48th Annual Meeting of the American Association of Swine Veterinarians. (February 25-28, 2017).




Editor’s note: The opinions and recommendations presented in this article are the author’s and are not necessarily shared by the editors of Pig Health Today or its sponsor.


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Mycoplasma hyopneumoniae (M. hyo) has been a well-known feature of swine production for decades. Veterinarian Lucina Galina Pantoja explains a classification system that could help manage outbreaks.

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Posted on May 19, 2017

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