Backgrounder on Swine Mycoplasmal Pneumonia

*Mycoplasma hyopneumoniae* is an important and highly prevalent
bacterial respiratory tract pathogen of swine. *M. hyopneumoniae* is
transmitted through direct contact between pigs. Weaned ("nursery"
pigs), and grower-finisher pigs are particularly vulnerable. The Swine
2000 Survey conducted by the USDA's National Health Monitoring System
(NAHMS) showed that 39.1% of deaths in grower-finisher pigs were due to
respiratory diseases.[^1] Of these respiratory diseases, the survey
listed Mycoplasmal pneumonia as **the** most common respiratory disease
in grower-finishers.[^2]

Cases of pure Mycoplasmal pneumonia in swine are rare. Most commonly,
Mycoplasmal pneumonia occurs as part of "Porcine Respiratory Disease
Complex" (PRDC). When *M. hyopneumoniae* infects a pig in concert with
other pathogens, disease virulence increases. Clinical signs of PRDC
include death, lethargy, respiratory distress, cough, and decreased
average daily gain (ADG.); conversely, clinical signs of "pure"
Mycoplasmal pneumonia are subtle and mild. PRDC is a mixed viral and
bacterial infection of *M. hyopneumoniae,* *Actinobacillus
pleuropneumoniae*, *Streptococcus suis, Pasteurella multocida*,
Pseudorabies virus, Porcine Reproductive and Respiratory Syndrome Virus
(PRRSV), or Swine Influenza Virus (SIV). At Iowa State University's
laboratories, the three most common pathogens isolated in PRDC cases are
*M. hyopneumoniae,* PRRSV, and SIV*.*[^3] Clinical presentation of PRDC
varies in severity, with uneven growth rates among littermates, cough,
respiratory, and reduced feed efficiency. Clinical signs of "pure"
Mycoplasmal pneumonia are subtle and mild, with dry non-productive
cough.

Though *M. hyopneumoniae* is highly prevalent in swine pneumonias, *M.
hyopneumoniae* infections are hard to diagnose. Diagnostic methods are
serology, culture, polymerase chain reaction (PCR), gross pathology and
histopathology. Of these methods, culture is still considered the
diagnostic "gold standard." However, *M. hyopneumoniae* is a fastidious
and slow-growing organism, and culture is not very sensitive for
detection of the organism. PCR methods are used for detecting *M.
hyopneumoniae* in broncho-alveolor lavage (BAL) fluids, on nasal and
bronchial swabs, and differentiating it from other Mycoplasma
species.[^4] [^5] Researchers found correlation between detection of
*M. hyopneumoniae* by PCR and presence of lung lesions.[^6] PCR is very
sensitive and does not differentiate from dead and live organism DNA.
Compounding the diagnostic challenge is that there are no specific lung
lesions for *M. hyopneumoniae*, making the lesions hard to differentiate
between other bacteria or viruses. In addition, lung lesions may take up
to 4 weeks to develop in growing pigs with experimentally induced *M.
hyopneumoniae* infections.[^7]

Several commercial vaccines are available in the US for *M.
hyopneumoniae,* and one drug, lincomycin, is approved in the US for use
in pigs to treat pneumonia due to *M. hyopneumoniae*. In spite of the
availability of commercial vaccines, an important management tool*,*
pneumonia associated with *M. hyopneumoniae* continues to be a problem
in the US.

[^1]: [Part I: Reference of Swine Health and Management in the US,
    2000,]{.underline} National Animal Health Monitoring System (NAHMS)
    Swine Survey 2000, USDA-APHIS-CEAH

[^2]: All Mycoplasma spp., NAHMS Part II preliminary data, Swine Survey
    2000, USDA-APHIS-CEAH)

[^3]: Dr. Eileen Thacker, June 11, 2001 presentation to ONADE.

[^4]: Baumeister AK, Runge M, Ganter M, et al., "Detection of Mycoplasma
    hyopneumoniae in Bronchoalveolar Lavage fluids by PCR", J Clinical
    Microbiology, July 1998, P 1984-8

[^5]: Calsamiglia M, Collins JE, and Pijoan C, "Correlation between the
    presence of enzootic pneumonia lesions and detection of Mycoplasma
    hyopneumoniae in bronchial swabs by PCR", Vet Microbiology 76(2000),
    p. 299-303

[^6]: Calsamiglia M, Collins JE, and Pijoan C, "Correlation between the
    presence of enzootic pneumonia lesions and detection of Mycoplasma
    hyopneumoniae in bronchial swabs by PCR", Vet Microbiology 76(2000),
    p. 299-303

[^7]: Thacker EL, Halbur PG et al., J Clinical Microbiology, March 1999,
    pages 620-7