[Mycoplasma hyopneumoinae Associated Respiratory Disease on the
Farm](../02N-0027_ts00003_02_vol2.ppt)

Brad J. Thacker, DVM, PhD

*Mycoplasma hyopneumoniae* is an obligatory pathogen of the respiratory
tract of pigs. Although infection is limited to the airways and *M.
hyo.* by itself causes little if any reduction in performance, the
disease is recognized as either a serious inititator or potentiator of
more serious respiratory diseases caused by other pathogens. Whether or
not *M. hyo.* potentiates the impact of bioreactive contaminants such as
endotoxins and glucans is unknown. *M. hyo .*is present in most swine
herds and accordingly, these herds are at greater risk for developing
respiratory disease.

Historically, the clinical presentation of swine respiratory disease was
primarily a chronic, low-to-moderate grade pneumonia with variable
morbidity, reduced growth rate, and low mortality in finishing pigs,
starting soon after leaving the nursery. This presentation was
characterized as "enzootic pneumonia." *M. hyo*. was believed to be the
primary cause and opportunistic bacteria, especially *Pasteurella
multocida* (Pm) were believed to be "secondary invaders." The actual
contributing role of each bacteria was difficult to ascertain but Pm was
generally not a concern in *M. hyo.* free herds. Occasional explosive
outbreaks of respiratory disease were attributed to swine influenza
virus (SIV), *Actinobacillus pleuropneumoniae* (App) or a "hot"
Pasteurella. Prior to the use of mycoplasma vaccines, control of
enzootic pneumonia was based on adoption of all-in-all-out pig flow and
periodic medication for the secondary invaders.

Recently, the characteristics of swine respiratory disease have evolved
towards more explosive, but repeated, outbreaks in somewhat older pigs
(16-20 weeks of age), resulting in high morbidity and variable
mortality. This type of pattern has been characterized as the "porcine
respiratory disease complex" (PRDC). Common organisms isolated from PRDC
cases besides *M. hyo*. include PRRS virus, SIV, Pm, circovirus and
other bacteria such as App, *Salmonella cholerasuis*, *Hemophilus
parasuis* and *Streptococcus suis*. The main difference between enzootic
pneumonia and PRDC is the viral component, especially PRRS virus and in
some cases, endemic SIV. The role of circovirus in PRDC is unclear at
this time. *M. hyo*. often plays a significant role in the underlying
severity of PRDC. Production practices that have changed include
increased herd size, reduced weaning age and all-in-all-out production
by room, building or site. The impact of these factors on the change in
respiratory disease pattern is largely unknown but some impact is
expected.

Although identifying all of the organisms present in a given situation
is essential, the clinician's challenge is sorting out the relative role
that each organism plays. It is becoming increasingly clear that gross
lesions are of little diagnostic value except in App cases. The lesions
of *M. hyo.,* SIV and in a recent case of PRRSV/circovirus are the same.
Clinical signs such as fever, cough, increased respiration rate and
pronounced breathing pattern are not characteristic of any one disease
or disease combinations. In the days of enzootic pneumonia, our view of
respiratory disease was often limited to necropsy lesions with very
little laboratory testing. Were our assessments correct? Did we truly
understand the role of the various organisms involved? Likely not and
the same holds true with PRDC. Laboratory testing is essential. With
regard to slaughter health checks, the severity of pneumonia observed at
slaughter may have little relationship to the impact of pneumonia on the
pig's health status and growth performance throughout the pig's entire
life, especially with viral infections such as SIV that resolve quickly.
Furthermore, few production systems allow for precise, day-to-day
monitoring of feed and water intake in order to pinpoint when disease is
affecting performance. Hence, our ability to detect short time periods
of reduced performance that significantly contribute to reduced
close-out performance is often lacking.

In summary, we still have a lot to learn about the relative
contributions of the myriad of organisms and environmental stressors
with regard to PRDC. Because every herd situation is somewhat unique,
the ultimate assessment and remedial actions need to be made on a
farm-by-farm basis. Laboratory research and diagnostic testing are
essential for framing the questions and perhaps defining the limits of
the clinician's approach to each situation, but the final answer
ultimately lies with the attending clinician.