Background: Comprehensive endoscopic scoring of the upper and lower airways for inflammation has not been critically assessed among a large population of horses. The relationship between upper and lower airways described in humans by the "one airway, one disease" concept might also apply to horses.Hypothesis/Objectives: To evaluate if an association exists between endoscopic inflammatory scores and mucus scores of upper and lower airways and to investigate if endoscopic findings correlate with the lower airway inflammation measured by bronchoalveolar lavage (BAL) cytology.Methods: Prospective field study. Pharyngitis, pharyngeal mucus, tracheal mucus, tracheal septum thickness, and bronchial mucus were scored using new and previously described scoring systems on a convenience sample of 128 horses with and without lung inflammation. Based on BAL fluid cytology, horses were categorized as having normal, moderate, or severe inflammation of the lower airways.Results: All 5 endoscopy scores showed excellent interobserver agreement. Tracheal mucus (P < .001), tracheal septum thickness (P = .036), and bronchial mucus (P = .037) were significantly increased in horses with severe inflammation BALs and were correlated among themselves but not with upper airways scores. BAL neutrophils percentage was correlated with tracheal mucus (r s = 0.41, P < .001), bronchial mucus (r s = 0.27, P = .003), and had a weak negative correlation with pharyngitis (r s = À0.25, P = .004).Conclusions and Clinical Importance: Lower airway endoscopy scores are reflective of lower airway inflammation; however, upper and lower airways are independent in terms of severity of inflammation. Therefore, observing upper airway inflammation is not an indication to test for lower airway inflammation.

During a standardised BAL procedure, clinicians can expect lung inflammation in horses that have bronchial collapse and bronchial collapse in horses with lung inflammation. Lung inflammation may be a contributing factor in the mechanism of bronchial collapse during BAL in horses.