our expertise: VET COLUMN
Exercise Intolerance: Part 2
The Lower Airway
Tom Hutchins, DVM, DABVP
Dr. Tom Hutchins
A
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s mentioned in the previous installment, second to
musculoskeletal disease, respiratory disease is the
most common cause of poor performance, interruption in
training, and premature retirement among performance
horses. Inflammatory airway disease (IAD) and exercise-
induced pulmonary hemorrhage (EIPH) are the most
common conditions originating from the lower respiratory
tract associated with poor race performance. Low-grade
chronic obstructive pulmonary disease (COPD) is a
common source of exercise intolerance in middle-aged
performance horses.
The young racehorse population is frequently exposed
to viral respiratory pathogens that impair pulmonary
defense by damaging mucociliary clearance mechanisms,
destroying bronchial-associated lymphoid tissue, and
impairing pulmonary macrophage function. Regeneration
of the mucociliary apparatus requires approximately
three weeks after recovery from viral respiratory disease,
and racehorses are rarely permitted sufficient time for
convalescence after overt disease. Strenuous exercise and
long distance transport impair pulmonary immunity and
promote deep inhalation of dust particles. The presence of
blood within the lung tissue, as occurs with EIPH, initiates
an inflammatory response resulting in bronchiolitis and
airway inflammation. Inflamed airways are fragile and
are predisposed to further hemorrhage with exercise.
Therefore, repeated episodes of EIPH appear to create
self-perpetuating lower respiratory tract inflammation
and hemorrhage. Impaired pulmonary defense, irritant
exposure, and episodes of pulmonary hemorrhage not
only predispose racehorses to develop chronic IAD,
but promote development of fulminant pneumonia and
pleuropneumonia which may result in prolonged recovery,
permanent pulmonary damage, premature retirement, or
even death.
Thoracic auscultation with the rebreathing procedure
should be performed in all horses with poor performance
or exercise intolerance. The increased depth of repiration
caused by the rebreathing bag accentuates abnormal lung
sounds, and the rebreathing procedure allows the clinician
the subjectively evaluate the time period for induction
of and recovery from labored breathing. Listening to
the lungs and rebreathing facilitate identification of the
abnormal lung sounds characteristic of bronchopneumonia
(crackles), pleuropneumonia (silence in the lower lung
fields), and COPD (end expiratory wheeze). Horses with
IAD and EIPH rarely have abnormalities that can be heard
even with rebreathing, but the procedure may induce
coughing.
Diagnostic investigation of poor performance should
routinely include endoscopic examination of the upper and
lower respiratory tract. Endoscopic examination allows for
direct visualization of inflammatory exudate or blood in
the airway, and eliminates upper respiratory tract disorders
as the source of poor performance. Bronchoalveolar lavage
(BAL) or a transtracheal wash (TTW) is indicated in horses
with endoscopic evidence of lower respiratory tract disease
to confirm the source of poor exercise performance,
characterize the inflammatory process, and/or quantitate
the severity of pulmonary hemorrhage.
Inflammatory airway disease (IAD) occurs in 22%
- 50% of thoroughbreds and Standardbred racehorses
and is a common cause of impaired performance and
interruption of training. Chronic cough and mucoid
to mucopurulent nasal discharge are common clinical
findings in racehorses with low-grade airway inflammation.
Horses with IAD show poor exercise tolerance at race
speeds and perform several seconds slower than previous
performances. Proposed causes of lower airway disease
in racehorses include recurrent pulmonary stress, deep
inhalation of particulate matter, exposure to noxious