Relationship between endoscopic findings and cellular populations of bronchoalveolar lavage fluid (BALF) in 273 standardbred racehorses with inflammatory airway disease (IAD)

Introduction. Inflammatory airway disease (IAD) is a respiratory syndrome characterized by mild lower airway inflammation and excess mucus, that is commonly encountered in young athletic horses. Affected horses may present coughing episodes, increased respiratory secretions and poor performance. Diagnosis is achieved by endoscopic detection of tracheal mucus and bronchoalveolar lavage fluid (BALF) cytological evidence of nonseptic inflammation (Couëtil et al, 2007). Recently, a new definition of “Equine Asthma” to regroup all the inflammatory diseases involving lower airways of the horse has been proposed (Bullone and Lavoie, 2015; Couëtil et al, 2016). Currently, there are only few reports associating endoscopic findings (tracheal mucus and edema of tracheal septum) and BALF cytology in IAD (Koblinger et al, 2011; Wysocka and Klucinski, 2014). No association between IAD and upper airway inflammation has been described so far. The aim of the present study was to evaluate the relationship between endoscopic findings and variations of the cellular populations of BALF in horses affected with IAD. Materials and used methods. Data were compiled retrospectively from a cohort of hospitalized patients from 2003 to 2015 examined for poor performance, which fulfilled the following inclusion criteria: being racing Standardbred racehorses in training; being subjected to endoscopic examination and BAL of the airways; having differential count of neutrophils > 5% and/or mastocytes > 2% and/or eosinophils > 1% on cytological evaluation of BALF. Endoscopic examination of upper and lower airways was performed with a 2 m videoendoscope (Pv-g 34-325, Storz, Germany). Pharyngeal lymphoid hyperplasia (PLH) was scored 0 to 4 according to Baker, 1987. Tracheal mucus was scored 0 to 5 according to Gerber et al, 2004. Edema of tracheal septum was scored 0 to 4 according to Ferrucci et al, 2008. During endoscopic examination, BALF sampling was carried out. Samples were processed and differential count was performed according to Ferrucci et al, 2001. Data were statistically analyzed by linear regression with categorical variables, evaluating the dependence of BALF leucocytes differential count on endoscopic scoring. Statistical significance was set at p<0.05. Outcomes. Records of 273 horses matched the inclusion criteria. The age of the horses was 3.7±1.5 yrs. Most horses showed a PLH score between 1 and 3 (90.4%), tracheal mucus scores between 1 and 2 (72.2%), and tracheal septum edema scores between 0 and 2 (85%). Statistical analysis showed a highly significant association (p<0,01) between the increase in endoscopic scores and variation of BALF cell populations. Particularly, tracheal mucus score was associated with an increase in BALF neutrophils and macrophages differential counts; carina edema scores with an increase in lymphocytes differential count and PLH scores with an in increase eosinophils differential count. Conclusions. Average age and endoscopic scores recorded are consistent with that reported for IAD affected horses (Couëtil et al, 2007). The association between macrophages, neutrophils and tracheal mucus are in agreement with findings previously reported by other workers (Gerber et al, 2004; Koblinger et al, 2011); it may reflect the chemotactic action of macrophages in recruiting neutrophils in bronchial lumen, and thus increasing the amount of mucopus present in the tracheal lumen (Lunn et al 2007). Relation between edema of the carina and lymphocytes could be due to their role in recruiting the other leucocytes that are able to release cytokine with vasoactive effects, although further studies are needed to evaluate the exact role of lymphocytes during IAD (Lunn et al 2007). Finally, this is the first study that reports an association between IAD and upper airways inflammation. The highly significant association between eosinophils and PLH could be compared to what happens in human asthma. In fact, asthmatic children, whose BALF is characterized by increased eosinophils differential count, do often present hyperplasia of lymphoid tissues of the upper airways (tonsils, adenoids), possibly due to a higher sensitivity to environmental antigens (Kaditis et al, 2010). A similar situation could be hypothesized in young racehorses with IAD. Therefore, this finding could support the recently reviewed definition of “Equine Asthma”.