Respiratory disorders are responsible for poor performance in racehorses and for overall significant economic problems in the equine industry (1). Mycoplasma (M.) spp. have been regularly reported in up to 15% of clinical specimens in different countries, but their clinical contribution remains unclear. To assess their role, a detection / isolation protocol combining culture and post-enrichment PCRs was optimized and used on 1,948 respiratory samples collected from 1,764 horses in France over the 2020-2022 period (2). The prevalence of mycoplasmas was refined to 16.1%, with a predominance of M. equirhinis species (85.3%). We further used a clinical scoring, a risk factor analysis as well as a test for association with other bacteria and viruses to improve our understanding of the clinical role of M. equirhinis. In contrast to the pattern of primary pathogens, the M. equirhinis prevalence remained steady whatever the clinical score. Nonetheless, in presence of Streptococcus equi subsp. zooepidemicus (S. zoo) or viruses like EHV-5 (known pathogenic or opportunistic agents) the prevalence of M. equirhinis was significantly increased. Co-infection with S. zoo was associated with, on average, a significantly higher load of S. zoo. These results indicate that M. equirhinis is not a primary pathogen but could contribute to clinical signs in association with other microbes. In bronchoalveolar lavages, the detection of M. equirhinis was associated with neutrophil-mediated inflammation, suggesting it could participate in an inappropriate immune response, similar to that observed in equine asthma. This is consistent with the fact that M. equirhinis was more frequently isolated in racehorses and stabled horses, two favouring factors for asthma. Through comparison of 24 genomes, we further showed that M. equirhinis is a highly homogeneous species, with only a few mobile genetic elements like the M. arginini-like ICE and a M. arthritidis-like prophage. The repertoire of genes putatively associated to virulence was rather limited to cytoadherence and immune escape, similarly to its closest phylogenetic neighbour, M. hominis. In conclusion, M. equirhinis could play a part in the equine respiratory disease complex, in association with other microbes, essentially through dysregulation of the host immune response, as is well known for other mycoplasma species in other respiratory complexes.