The influence of clinical severity and topical antimicrobial treatment on bacteriological culture and the microbiota of equine pastern dermatitis

Background Equine pastern dermatitis (EPD) is a common dermatological problem in horses, yet its aetiology and pathogenesis are poorly understood. Objectives This study aimed to investigate the effects of lesion severity and topical antimicrobial treatment on bacterial flora of EPD‐affected skin. Animals Sixteen horses with EPD were investigated. Methods and materials An observational study was conducted by assigning a clinical severity score ranging from 0 (macroscopically nonlesional) to 21 (severe), and sampling the most and least severely affected limbs of 16 horses (32 limbs) for bacteriological culture and 16S rRNA sequencing. Topical antimicrobial treatment in the month before sampling was recorded. The limbs were allocated to a nonlesional or mildly affected group (Group A, score 0–3) and a moderate to severely affected group (Group B, score 4–21). Results The most commonly cultured bacterial species was Staphylococcus aureus (one of 15 Group A versus nine of 17 Group B). Within Group B, S. aureus was found in three of six limbs treated with topical antimicrobials and in six of 11 untreated limbs. β‐haemolytic streptococci (three of 32) and Trueperella pyogenes (two of 32) also were cultured exclusively in the untreated limbs of Group B. Staphylococci and streptococci were found more often by 16S rRNA sequencing than in culture. Limbs with higher lesion severity and topical antimicrobial treatment appeared to have a lower alpha diversity and different beta diversity compared to milder and untreated lesions. Conclusions and clinical importance Observed differences in microbiota of equine skin are likely to be linked to the presence and severity of EPD and topical antimicrobial treatment. Further research is needed to establish causal bacteria. Résumé Contexte La dermatite des paturons équine (EPD) est un problème dermatologique fréquent chez le cheval bien que son étiologie et sa pathogénie soient peu connues. Objectifs Cette étude a pour but d’étudier les effets de la sévérité des lésions et un traitement antibiotique sur la flore bactérienne de la peau atteinte d’EPD. Sujets Seize chevaux atteints d’EPD ont été étudiés. Méthodes Une étude d’observation a été conduite en attribuant un score de sévérité clinique allant de 0 (macroscopiquement non lésionnel) à 21 (sévère), et en prélevant les membres les plus et les moins sévèrement atteints de 16 chevaux (32 membres) pour culture bactériologique et séquençage d’ARNr16S. Le traitement antibiotique du