INTRODUCTION
Asthma in horses is a common cause of respiratory disease and poor
performance. This term was proposed to regroup two distinctive
phenotypes of inflammatory respiratory diseases: mild-moderate asthma
(MA) – previously known as inflammatory airway disease (IAD), and
severe asthma (SA) – previously known as recurrent airway obstruction
(RAO).1 Prevalence of SA has been estimated between 14
and 20% within the northern hemisphere,2,3 and MA was
identified in up to 66% of Standardbred racehorses presented for poor
performance in France.4
Equine asthma of all severities has common clinical presentations
(chronic cough, excess mucus, poor performance) but a wide heterogeneity
in terms of triggering factors, severity, and pathologic
characteristics.5 However, strong evidence supports
the role of exposure to environmental dust in the pathophysiology of
both mild-moderate and severe equine asthma.1 Exposure
to inhaled airborne agents, predominantly via stabling and
provision of hay feeding, was identified as a risk factor for
SA.6 Controlled challenges and exposure studies have
demonstrated an association between the severity of airway neutrophilic
response in SA horses and exposure to β-glucan (marker of mold
exposure), or with the concentration of mold
spores.7,8 In MA, β-glucan contents of respirable dust
have been associated with increased proportions of mast cells in
bronchoalveolar lavage fluid (BALF).9 Moreover, a
recent publication showed that horses with fungal elements detected in
the tracheal wash (TW) cytology were twice likely to have MA than horses
with no fungi.10 However, a retrospective study on 886
samples showed that cytological evidence of fungal elements in BALF was
not a risk factor for respiratory inflammation.11
For clinical purposes, questions remain about the most relevant sampling
site and methodology for detection of fungal elements in equine airways.
Laboratory analytical methods for fungal detection include either
cytology or fungal culture of respiratory tract fluids. Poor agreement
between those two techniques has previously been
reported.10,12 In TW samples, fungal detection is
common, occurring in 55 to 82% of samples depending on
methodology,10,12 rendering questionable the clinical
significance of the presence of fungi in TW. Fungal detection in BALF is
less common (estimated at 37% in a recent study12),
with a poor agreement between the two sampling
sites.12 One rationale for performing fungal detection
on TW samples is that this sample reflects more accurately the overall
exposure of the equine airways to these particles.10However, as equine asthma is a disease of the lower respiratory
tract,1 investigating fungal detection in BALF may be
a more accurate sampling site regarding the association of mold and
development of equine asthma.
With the hypothesis that exposure to fungal elements are a risk factor
for equine asthma, the aim of the study was to investigate different
combinations of analytical methods/sampling sites in relation to the
clinical status. To do so, the objectives were to: 1) determine the
prevalence of fungal elements in different respiratory samples (TW or
BALF) and methods (cytology or fungal culture), 2) compare the
prevalence of fungal elements between groups (control, MA and SA), and
3) determine any eventual association between fungal detection and
clinical status.