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.