Background: Recent evidence suggests that an altered intestinal microbiota, specifically a reduction of bacterial diversity or a shift in microbial composition, is associated with the development of hypersensitivity disorders in humans, but this is unknown for horses. Objectives: In this study we hypothesized that horses affected by either Culicoides hypersensitivity or severe equine asthma or both show a decreased diversity of their intestinal microbiota. We also investigated environmental effects. Methods: Rectal swab samples of a total of 140 horses were collected and the owners completed a detailed questionnaire about their horse. For each allergic horse, a healthy peer from the same stable was equally sampled as an environmentally matched control. Microbiota in the swabs was determined by assessing the V4 region of the bacterial 16S rRNA gene. Structures of bacterial communities were investigated by means of alpha and beta diversity indices. Results: Group wise comparisons between healthy and allergic horses showed no significant differences regarding alpha (p ¼ 0.9) and beta diversity (p ¼ 0.5). However, the microbial structure was associated with environmental factors such as the type of stable (p ¼ 0.001), access to pasture (p ¼ 0.001) or the type of feeding (p ¼ 0.003). There was also a strong location effect meaning that the microbiota was more similar within the same as compared between farms within this study. Conclusion: Our observations suggest that hypersensitivity disorders in adult horses are not associated with an alteration of the intestinal microbiota, but environmental and/or location factors strongly influence these bacteria.ABBREVIATIONS: ASV: amplicon sequence variant; bp: base pair; CH: Culicoides hypersensitivity; HOARSI: horse owner assessed respiratory signs index; NMDS: nonmetric multidimensional scaling; OTU: operational taxonomic unit; SEA: severe equine asthma; V4: variable region 4 (of the bacterial 16Sr RNA gene) ARTICLE HISTORY

Background Equine pastern dermatitis (EPD), a multifactorial syndrome, manifests as skin lesions of variable severity in the pastern area. Despite the widespread use of antibacterial therapy for treating this condition, little is known about the contributing bacteria. Hypothesis/Objectives To investigate the bacterial skin microbiota in EPD‐affected and unaffected (control) pasterns. Animals Case‐control study with 80 client‐owned horses; each with at least one EPD‐affected and one control pastern. Methods and materials Horses were grouped by the form of EPD (mild, exudative or proliferative), the assigned severity grade and type of pretreatment (disinfectant, topical antibacterial or no antibacterial pretreatment). Skin swabs were obtained, and the microbiota composition was compared between the groups. Results Bacterial alpha diversity was reduced in affected pasterns (P < 0.001) and this reduction was significantly associated with the EPD forms (P < 0.001), and not with the type of pretreatment (P > 0.14). Analyses of beta‐diversity confirmed a disordering of the skin microbiota (P = 0.004) in affected versus control pasterns, that was particularly profound in more severe lesions. The type of pretreatment was not significantly associated with this disordering. Four differentially abundant families were detected, of which Staphylococcaceae was the most distinct. The relative abundance of staphylococci was significantly increased in affected pasterns (P = 0.011), particularly in those that had received antibacterial treatment previously. Conclusions and clinical relevance Changes in the microbiota are associated with the EPD form or severity of lesions. The role of bacteria in the pathogenesis of EPD as well as the propriety and consequences of antibacterial treatment should therefore be further investigated.