Introduction
Equine Asthma (EA) is a highly prevalent disease worldwide (Allen et al., 2006; Courouce-Malblanc et al., 2010; Robinson et al., 2006), and can be further divided into two phenotypes; severe equine asthma (SEA), affecting primarily older horses, and mild-moderate equine asthma (MEA) affecting horses of all ages (Couetil et al., 2016). Horses diagnosed with EA express different severities of coughing, poor performance and increased respiratory effort, either at exercise and/or at rest (Couetil et al., 2020; Koblinger et al., 2011; Robinson et al., 2003). A further subdivision of the MEA phenotype has been proposed to include mild EA for high performance horses with clinically unrecognized airway disease, as well as moderate EA of horses with clinical signs such as coughing and poor performance (Couetil et al., 2020).
A relationship between lower airway inflammation, poor performance, Tracheal mucus score (TMS) and coughing has been noted in both small and large-scale studies (Bedenice et al., 2008; Christley et al., 2001; Fogarty and Buckley, 1991; Holcombe et al., 2006; Lavoie et al., 2011; Widmer et al., 2009). Other, large-scale studies are warranted to investigate associations between clinical signs and diagnostics.
Horse owner questionnaires have shown to be reliable for identifying horses with SEA, whereby the horse owner assessed respiratory signs index (HOARSI) has been validated (Hotchkiss et al., 2006) and used in EA populations (Laumen et al., 2010). Besides owner reported clinical signs, endoscopic examination including scoring of tracheal mucus (Gerber et al., 2004b) and bronchoalveolar lavage (BAL) is considered the gold standard in EA diagnostics (Bond et al., 2018; Couetil et al., 2016). For example, tracheal mucus score is reported to increase with both MEA and SEA cases, poor performance and coughing, and is thus a reliable, important parameter of the endoscopic examination (Almeida et al., 2015; Cardwell et al., 2011; Gerber et al., 2004a; Holcombe et al., 2006; Koblinger et al., 2011; Pirie et al., 2016; Widmer et al., 2009). Tracheal mucus score between MEA subtypes has been evaluated sparsely, and an increased amount of mucus has been found associated only with the mixed MEA subtype (Nolen-Walston et al., 2013).
The amount of BAL fluid obtained after instilling a volume of 250-500 ml, and manually aspiration, is expected to be between 50-70 percent of the instilled fluid volume (Couetil et al., 2016). The volume aspirated has been found to be lower in horses diagnosed with SEA compared to healthy horses (Couëtil et al., 2005; Jean et al., 2011), but no difference in volume aspirated has been found between MEA subtypes (Richard et al., 2014). This present study investigated the associations between stabling factors, clinical signs of EA, TMS and BAL cytology, including a large group of horses. It was hypothesized that 1) Clinical signs of coughing and poor performance would be associated with BAL neutrophilia, BAL mastocytosis and an increased TMS. 2) The TMS score will increase with an increase in BAL neutrophils and severity of EA diagnosis, but not with an increase in BAL mast cells.3) Volume of BAL aspirated in percentage of BAL instilled would decrease with EA disease severity and lastly, 4) Stabling environment and roughage fed would be associated with the EA diagnosis.