Herd-status classification is first step to Mycoplasma control[Sassy_Social_Share]
By Lucina Galina Pantoja, DVM, PhD
Director, US Pork Technical Services, Zoetis
Mycoplasma hyopneumoniae (M. hyo) continues to plague US swine herds. The pathogen is especially troublesome in cases of co-infection with other diseases on a farm. The first step to controlling and eliminating M. hyo is to classify each herd’s status. Herd-status classification focuses on the breeding herd, a population of animals that includes the breeding animals and their offspring. If a gilt development unit (GDU) is located within the premises, the GDU will be considered part of the breeding herd, and animals with the lowest M. hyo health status on the premises will determine the status of the entire site.
The benefits of a clear and concise classification system for M. hyo are manifold. They include:
- Facilitating communication between swine producers, veterinarians, diagnosticians and breeding stock companies; and evaluating strategies for disease control.
- Characterize the health status of farms and set realistic goals for control or elimination and improve pig flow management
- Supporting regional control and elimination efforts.
- Establish contractual arrangements that could include premiums for weaned pigs from certain categories of the classification.
- Identify knowledge gaps that recognize areas that warrant further research
In 2016, standardized terminology describing the M. hyo status of breeding herds was proposed in the first edition of a M. hyo manual produced by Zoetis1 and at the 2016 AASV Conference. In 2019, a revised classification was created with input from a M. hyo Status Definitions Committee composed of a diverse, distinguished group of swine veterinarians representing industry, academia, and trade organizations.
This classification was then reviewed and approved by the American Association of Swine Veterinarians (AASV) Committee for Transboundary and Emerging Diseases.
The authors followed similar terminology for classifying herds by porcine reproductive and respiratory syndrome virus infection status as defined by Holtkamp and collaborators.
The new M. hyo herd-status classification system provides an updated standardized system for classifying the M. hyopneumoniae status of swine breeding herds by defining a set of diagnostic guidelines to determine the exposure and shedding status of herds.
Four major herd-status categories were proposed for breeding herds:
- Category 1: Positive uncontrolled — Breeding herds in this category meet at least one of the two diagnostic criteria. The following herds fall into Category 1: a) breeding herds going through an hyo outbreak, and b) herds that have not performed the necessary testing described below and the status is unknown.
- Category 2: Positive controlled — In these herds, the agent is not present in first-parity (P1) sows and the herd is serologically positive. For the purpose of classifying herds, P1 sows are those that have farrowed their first litter. Herds in this category are assumed to have an ongoing hyo gilt-acclimation program where gilts are exposed at an early age; however, this is not a requirement.
This status will be considered the end goal for those herds that do not wish to pursue elimination and decide to only control M. hyo. To classify into this category, four consecutive negative monthly samplings of 30 tracheal swabs, tested by polymerase chain reaction (PCR), from P1 sows up to 30 days post-weaning should be obtained, suggesting successful early acclimation and cessation of shedding by the end of the first parity.
It’s important to note this status cannot rule out the possibility that there is continued M. hyo transmission in the herd. It is presumed that Category 2 herds have a low level of infection in piglets at weaning and thus improvements on pig flow (i.e., by minimizing commingling from multiple sources) and/or medications (i.e., stopping antimicrobial treatments at weaning) are possible.
- Category 3: Provisionally negative — In these herds, the agent is not detected within the breeding herd population, but the population may be serologically positive. Category 3 is subdivided into two subcategories:
- Subcategory 3a: Provisional negative unvaccinated — These herds have completed a whole-herd elimination program. To be classified as 3a, herds need to meet one of two diagnostic requirements: a) Two consecutive negative samplings of 60 tracheal swabs of the last population exposed prior to introducing negative replacement gilts, or b) two consecutive monthly negative samplings of 30 serum samples, tested by enzyme-linked immunoassay (ELISA) or 30 tracheal swabs, tested by PCR, from naive replacement gilts after a minimum of 120 days post-entry, to allow sufficient time for hyo to be detected if still present in the herd. These herds are not routinely vaccinated for M. hyo.
- Subcategory 3b: Provisional negative vaccinated — Herds in this subcategory have: a) completed a whole-herd hyo elimination program and have fulfilled the diagnostic requirements for Subcategory 3a but continue to vaccinate naive breeding females for M. hyo or b) herds that have been stocked negative but implement M. hyo vaccination. Herds may decide to continue vaccinating and remain in Subcategory 3b indefinitely. Clinical signs and lesions suggestive of M. hyo in the breeding herd would trigger a diagnostic investigation.
- Category 4: Negative — In these herds, the agent is not detected in any type of sample so the population is considered serologically negative. Herds undergoing elimination efforts should have been Subcategory 3a, and the breeding herd has been completely rolled over to fall into Category 4. Newly established hyo-negative herds and those that went through complete depopulation and repopulation efforts fall within Category 4. To maintain negative status, a minimum of 30 monthly negative ELISA test results from various parity sows should be obtained.
Diagnostic criteria considerations
M. hyo-associated clinical signs are defined as a dry, non-productive cough exacerbated by physical exertion. Other clinical signs may also be present, including fever, decreased appetite and labored breathing. Clinical signs are not pathognomonic for M. hyo, and other respiratory pathogens must be ruled out.2
Microscopic lesions consist of lobular distribution of peribronchiolar and perivascular lymphocytic cuffing. Alveoli and airways may contain serous fluid with a few macrophages and neutrophils. Airway epithelium is intact and sometimes slightly hyperplastic.
Microscopic lesions are non-specific and can be similar to those observed with viral agents. Neither clinical signs nor microscopic lesions were used as diagnostic criteria for the new M. hyo breeding-herd classification. However, two diagnostic criteria — antigen and antibody detection — were used for classification.
Detection of antigen within lung lesions or the upper respiratory tract can be achieved using several tests by diagnostic labs; however, PCR is the recommended test method. Although culturing is confirmatory, it is less practical than the other methods and not frequently used.
To determine M. hyo exposure, the ELISA test is used for antibody detection in serum. The selection of the optimal sampling site in live animals will likely be affected by timing of infection. For instance, it is possible that recently infected pigs harbor M. hyo in the nasal cavity, whereas 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 tracheal and laryngeal swabs are the preferred sample type for M. hyo over nasal swabs and tracheo-bronchial lavage, with recent investigations showing a higher sensitivity with tracheal swabs.3,4
Seroconversion within a population can take several weeks to be detected by ELISA, and therefore, timing should be considered.5,6
For the proposed classification, to detect a 10% prevalence of M. hyo infection in weaning-age pigs (i.e., for a positive stable herd), at least 45 samples should be collected from a population of 1,000 pigs.7
On the other hand, when 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 5% (i.e., negative) prevalence is assumed.
Both of these calculations assume nearly perfect tests, but diagnostic tests available today for M. hyo 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 extent of pulmonary lesions at slaughter. Based on those findings, reasonable goals for control should be to produce litters that are either negative or with low M. hyo prevalence at weaning.
Introduction of replacement gilts in the breeding herd plays a critical role in the M. hyo stability of a herd. Negative breeding herds that introduce positive replacement gilts are likely to become positive uncontrolled (Category 1) or controlled (Category 2). Positive breeding herds that introduce positive or negative replacements are likely to be positive uncontrolled or controlled. Provisionally negative or negative herds that introduce negative replacement gilts are likely to remain provisionally negative (Category 3) or negative (Category 4).
The herd-status classification can be further refined as more information about prevalence at weaning and disease in the finisher stages becomes available.
Further collaboration a goal
The M. hyo herd-status classification was created to initiate collaboration efforts, to identify areas of improvement and to recognize areas that warrant further research.
Additional research will increase knowledge on M. hyo and will facilitate improvements in control and eradication of this costly disease.
For more details regarding M. hyo classification or to download a copy of the updated M. Hyo manual released in 2020, visit www.pgtstaging.wpengine.com/m-hyo-manual. More details can also be found at the AASV website8 (requires member login).
Editor’s note: The opinions and advice presented in this article belong to the author and, as such, are presented here as points of view, not specific recommendations by Pig Health Today.
1 Galina L, Clavijo M. Establishing Mycoplasma hyopneumoniae herd status classification criteria for breeding herds. Proceeding of the AASV 2016. Establishing Mycoplasma hyopneumoniae herd status classification criteria for breeding herds.
2 Thacker E. Mycoplasmal diseases. In: Straw BE, Zimmerman JJ, D’Allaire S, et al, eds. Diseases of Swine, 9th ed. Oxford, UK: Blackwell Publishing Ltd; 2004:701-717.
3 Pieters M, Rovira A. Comparison of various sample types for detection of Mycoplasma hyopneumoniae in recently infected pigs. Proceedings Allen D. Leman Swine Conference, St. Paul, MN. 2013;75-76.
4 Johnson C, Farkas A, Cano JP, Clavijo MJ. What Happens when M. hyopneumoniae enters a herd? Longitudinal assessment of M. hyopneumoniae natural infection in gilts. In: Proceedings of Iowa State University James D. McKean Swine Disease Conference, 2017 Nov 2-3;103-105.
5 Pieters M, Rovira A. Comparison of various sample types for detection of Mycoplasma Hyopneumoniae in recently infected pigs.
6 Sibila M, et al. Chronological study of Mycoplasma hyopneumoniae infection, seroconversion and associated lung lesions in vaccinated and nonvaccinated pigs. Vet Microbiol. 2007;122:97-107.
7 EpiTools epidemiological calculators. http://epitools.ausvet.com/au/ content.php?page=PrevalenceSS.
8 Clavijo MJ, Galina Pantoja L, Holtkamp D, Yeske P, Becton L, Snelson H, Main R, McDowell E, Johnson C, Sprague M, Fano E, Painter T, Glowzenski L, Baumert D. Establishing Mycoplasma hyopneumoniae herd status classification criteria for breeding herds. https://www.aasv.org/aasv%20website/Resources/Diseases/SwineDiseases.php