Relationships between equine airway reactivity measured by flowmetric plethysmography and specific indicators of airway inflammation in horses with suspected inflammatory airway disease

Summary Reasons for performing study Agreement between airway reactivity measured by flowmetric plethysmography and histamine bronchoprovocation, and lower airway inflammation measured by bronchoalveolar lavage ( BAL ) cytology, has not been studied in horses with suspected inflammatory airway disease ( IAD ). Objectives We tested the hypothesis that airway reactivity is associated with BAL cytology in horses presenting for unexplained poor performance and/or chronic cough. Study design Prospective clinical study. Methods Forty‐five horses, predominantly young Standardbred racehorses, presenting for unexplained poor performance or chronic cough, underwent endoscopic evaluation, tracheal wash, flowmetric plethysmography with histamine bronchoprovocation and BAL . Histamine response was measured by calculating PC 35, the concentration of nebulised histamine eliciting an increase in Δ flow of 35%. Results In this population, there was no significant correlation between histamine response and cell populations in BAL cytology. When airway hyperreactivity ( AHR ) was defined as ≥35% increase in Δ flow at a histamine concentration of <6 mg/ml, 24 of the 45 horses (53%) were determined to have AHR . Thirty‐three (73%) had either abnormal BAL cytology or AHR , and were diagnosed with IAD on this basis. Of horses diagnosed with IAD , 9 (27%) had an abnormal BAL , 11 (33%) had AHR and 13 (39%) had both. Conclusions Airway reactivity and BAL cytology did not show concordance in this population of horses presenting for unexplained poor performance and/or chronic cough. Failure to include tests of airway reactivity may lead to underdiagnosis of IAD in young Standardbred racehorses that present with clinical signs suggestive of IAD .