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.