Epidemiology and pathogenicity of M. equirhinis in equine respiratory disorders.

Mycoplasmas are pathogens involved in respiratory disorders of various animal hosts. In horses, Mycoplasma (M.) equirhinis is the species most frequently detected in clinical respiratory specimens, with a prevalence of 12-16%, but its clinical implication in equine respiratory disorders remains unclear. Here we screened 1948 clinical specimens for the presence of M. equirhinis. The samples were both tracheal washes (TW) and bronchoalveolar lavages (BAL) collected by veterinarians in France in day-to-day work between 2020 and 2022. The samples were associated with a standardized form that served to collect key general and clinical information, such as horse age, breed, and living environment. M. equirhinis was detected using a combination of culture and post-enrichment PCR. Other diagnostic data included virology and bacteriology as well as neutrophil counts, when available. Prevalence of M. equirhinis was examined as a function of a clinical score based on four significant clinical signs (nasal discharge, cough, dyspnoea, and hyperthermia). Multivariate logistic regression analysis was run to identify risk factors for the presence of M. equirhinis, and comparative prevalence analysis was used to test for association with other bacteria and viruses. TW and BAL were analysed independently, as we found that TW samples were associated with a higher prevalence of M. equirhinis. As prevalence remained steady whatever the clinical score, M. equirhinis cannot be considered a primary pathogen. M. equirhinis was more frequently isolated in thoroughbreds and trotters and in horses living exclusively stabled compared to other horses or other living environments. M. equirhinis was never detected in BAL specimens with a 'normal' neutrophil count, i.e. 5%, suggesting it could be associated with an inflammatory response, similar to that observed in equine asthma. Prevalence of M. equirhinis was shown to increase in the presence of other bacteria such as Streptococcus equi subsp. zooepidemicus (S. zoo) or viruses, and S. zoo load was higher in M. equirhinis-positive samples, suggesting a potential increase of clinical signs in the event of co-infection.

Detection of Mycoplasma spp. in horses with respiratory disorders.

BACKGROUND: Bacteria belonging to the genus Mycoplasma are small-sized, have no cell walls and small genomes. They commonly cause respiratory disorders in their animal hosts. Three species have been found in the respiratory tract of horses worldwide, that is., Mycoplasma (M.) equirhinis, M. pulmonis and M. felis, but their role in clinical cases remains unclear. OBJECTIVES: The aim of this study was to i) develop and validate tools to detect, isolate and identify different Mycoplasma spp. strains in clinical equine respiratory-tract specimens and ii) subsequently define the prevalence of the three species in France depending on sample types and horse characteristics (age, breed, sex). STUDY DESIGN: Validation of a workflow for mycoplasma diagnosis and subsequent prevalence study. METHODS: Mycoplasma-free tracheal wash samples spiked with numerated strains and DNA dilutions were used to validate the culture methods and real-time PCR (rt-PCR) assay. Isolated strains were identified by 16S rRNA gene sequencing. Prevalences were determined on a population of 616 horses with respiratory disorders, sampled in France in 2020. RESULTS: In total, 104 horses (16.9%) were found to be positive for Mycoplasma spp. by at least one method. M. equirhinis was the predominant circulating species, accounting for 85% of the rt-PCR-positive samples and 98% of the 40 cultured strains. MAIN LIMITATION: The proposed pre-enrichment procedure improves the sensitivity of detection but hinders the quantification of the initial mycoplasma load in the clinical specimens. CONCLUSIONS: Prevalence of mycoplasma varied with age, breed, and type of sample.