[Interaction Between M. hyopneumoniae and Other
Pathogens](../02N-0027_ts00008_02_vol2.ppt)

**Eileen L. Thacker, DVM, PhD, DACV**

**Iowa State University**

*Mycoplasma hyopneumoniae*, the causative agent of enzootic pneumonia,
is present in a minimum of 80% of the U.S. swine herds. By itself, *M.
hyopneumoniae* is barely pathogenic inducing only a mild, dry
non-productive cough and minimal-to-no reduction in production
parameters. The pathogenesis of *M. hyopneumoniae* is based on both
physical damage within the respiratory airways and by modulating the
immune system within the respiratory tract. Binding and colonization of
ciliated epithelial cells by the organisms results in loss of cilia and
damage to the mucociliary apparatus. This damage to the mucociliary
apparatus results in the increased secondary infections commonly
associated with *M. hyopneumoniae* infection resulting in enzootic
pneumonia. Classically, enzootic pneumonia is based on the presence of
*M. hyopneumoniae* plus bacterial infections, especially *Pasteurella
multocida*. The concurrent infections between *M. hyopneumoniae* and
secondary bacteria results in an increased pneumonia greater than the
sum of the pneumonia induced by either pathogen. This is especially true
with *P. multocida*, which induces minimal pneumonia on its own. In
addition to physically damaging the cilia and respiratory tract, *M.
hyopneumoniae* infection also induces a mononuclear infiltration of
macrophages and lymphocytes around the airways and blood vessels of the
lungs. The cells attracted by *M. hyopneumoniae* are non-specifically
stimulated resulting in modulation of the respiratory tract immune
system. This change in the responsiveness of the respiratory immune
system by *M. hyopneumoniae* also increases the incidence of secondary
infections within the respiratory tract.

In addition to the pneumonia induced by primary infection, research has
demonstrated that *M. hyopneumoniae*'s interaction with other pathogens
is very important in the induction of respiratory disease in pigs. A
pattern of respiratory disease has emerged in finishing pigs, typically
at 10-20 weeks of age, known as the porcine respiratory disease complex
(PRDC). PRDC is a multifactorial syndrome resulting from infection of
pigs with multiple respiratory pathogens. The resulting respiratory
disease typically consists of fever, anorexia, lethargy, and
significantly reduced production parameters. The three pathogens most
commonly isolated from pigs presented to the Iowa State University
Veterinary Diagnostic Laboratory are porcine reproductive and
respiratory syndrome virus (PRRSV), *M. hyopneumoniae*, and swine
influenza. Isolation of porcine circovirus type 2 (PCV 2) has been
steadily increasing over time.

The presence of *M. hyopneumoniae* increases the severity of pneumonia
induced by PRRSV and SIV. In the case of PRRSV, the presence of *M.
hyopneumoniae*, results in increased severity of pneumonia in addition
to significantly increasing the duration of clinical disease and
pneumonia. While the increase in disease associated with *M.
hyopneumoniae* and SIV infection, although not as severe as observed
with PRRSV, results in significantly increased clinical disease and
pneumonia in pigs. The pneumonia induced by *M. hyopneumoniae* did not
have to be significant in either of these co-infection models with
viruses. *M. hyopneumoniae* only had to be present within the

respiratory tract to result in a significant increase in the viral
pneumonia and disease. Interestingly, the impact of viral infection on
mycoplasmal pneumonia was minimal.

> In summary, The interaction between *M. hyopneumoniae* and other
> respiratory pathogens is of great importance to the swine industry.
> The interaction between *M. hyopneumoniae* and PRRSV creates a
> significant modulation of the respiratory immune system away from an
> efficacious cell mediated immune response toward a less efficient
> humoral-type immune response. This interaction is probably the
> cornerstone upon which the other respiratory pathogens induce
> significant disease. However, *M. hyopneumoniae* also has significant
> interactions with the other pathogens, both viral and bacterial,
> within the respiratory tract. Recent data collected from the NAHMS
> survey on swine respiratory disease found that mycoplasma vaccine is
> the most commonly used vaccine on swine farms. These results indicate
> that control of mycoplasma pneumonia is considered extremely important
> by the industry. The primary importance of *M. hyopneumoniae* is based
> on it's interaction with other pathogens. These interactions must be
> considered in respiratory disease within each production unit.