Summary Background Horses with bacterial sinusitis frequently undergo empirical treatment with antimicrobials, however, in some cases bacterial culture of the affected sinus is used to direct therapy. Data regarding which organisms are part of the commensal microbiota of the equine sinus are lacking making it difficult to interpret culture results and guide empiric antimicrobial selection. Objectives Our objectives were to describe the bacterial and fungal microbiota of the paranasal sinuses in clinically normal horses using culture‐dependent and independent approaches and to compare the bacterial culture and susceptibility patterns of normal horses with those from horses affected with primary and secondary sinusitis. Study design Experimental study and descriptive retrospective review of case records. Methods Sinus washes were collected from 23 healthy horses. Washes were submitted for routine culture and susceptibility testing and DNA was isolated for next generation sequencing of bacterial and fungal marker genes. For clinical cases of sinusitis, medical records from 2010 to 2017 were reviewed and horses diagnosed with primary and/or secondary sinusitis were included. Results The paranasal sinus cavity hosts multiple bacterial and fungal organisms. The bacterial microbiota of healthy horses consists largely of uncultivable, aerobic bacteria. While few anaerobes were isolated from normal horses, the majority of clinical cases resulted in growth of anaerobic organisms with no difference in the proportion of aerobic and anaerobic bacteria isolated from clinical cases. Main limitations Small sample size in both populations of horses and heterogeneity of the population prevent a more in‐depth analysis. Conclusions The microbiota of the paranasal sinuses of horses consists primarily of aerobic bacteria and fungal organisms, the majority of which are uncultivable via common clinical methods. Anaerobic bacteria are found in the majority of horses with clinical sinusitis. These findings suggest anaerobic bacteria are associated with sinusitis and their presence should be considered when treating horses with sinusitis.

In this ex vivo study, both closure techniques and both suture types appeared to be acceptable for laparoscopic closure of the urinary bladder in adult horses.