Co-Occurrence of Equine Asthma and Pharyngeal Lymphoid Hyperplasia in Pleasure Horses

With the increasing awareness of the “united airway disease” theory, more horses, not only sport horses but also pleasure horses, undergo detailed examinations of the respiratory tract. Using endoscopy, equine asthma (EA) is most commonly diagnosed in the lower airway, while pharyngeal lymphoid hyperplasia (PLH) is common in the upper airway. Grading EA as mild–moderate (MEA) and severe (SEA), this study aims to compare the co-occurrence and investigate the possible relationship between the clinical symptoms and endoscopic signs of MEA/SEA and PLH in pleasure horses. In this retrospective study, 80 out of 93 pleasure horses suspected of EA were enrolled and underwent a standardized protocol for a complete airway examination, including resting endoscopy with mucus accumulation assessment and cytology. The obtained results were scored and analyzed. In the studied pleasure horses, PLH co-occurred more frequently in horses with EA than without (p < 0.0001) and more in horses with SEA than with MEA (p = 0.025). However, when EA and PLH co-occurred, the severity of the clinical symptoms of EA did not increase (p > 0.05). In both EA and PLH, the amount of tracheal and nasopharyngeal mucus increased with the severity of the disease; however, it was positively correlated (ρ = 0.33; p = 0.02) only in SEA horses. In conclusion, it is likely that EA is often accompanied by PLH; however, PLH did not play a role in increasing the severity of EA’s clinical symptoms. The role of the severity of accumulated mucus in the lower and upper airways when EA/PLH co-occur requires further research to confirm the morphological and functional unity of the respiratory tract, aligning with the concept of “united airways disease”.