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

The study investigates the role of Mycoplasma (M.) equirhinis, a bacteria commonly found in the respiratory tracts of horses, in equine respiratory disorders. Using various diagnostic methods, the study does not find M. equirhinis as a primary cause of respiratory illness, though its prevalence differs among breeds and living conditions, and often coincides with other bacteria and viruses.

Screening of Clinical Specimens

• The researchers screened 1948 clinical samples, tracheal washes (TW) and bronchoalveolar lavages (BAL), collected by veterinarians from 2020 to 2022 in France.
• The collection was accompanied by a form detailing critical data like horse age, breed, and living conditions.
• The presence of M. equirhinis was detected through a combination of culture and post-enrichment PCR methods.
• Other diagnostic data such as virology, bacteriology, and neutrophil counts were also evaluated, when available.

Analysis and Findings

• The researchers used statistical models to study the prevalence of M. equirhinis based on clinical scores derived from four principal symptoms of respiratory illness (nasal discharge, cough, dyspnoea, and hyperthermia).
• Multivariate logistic regression was used to single out the risk factors associated with M. equirhinis infection. A comparative prevalence analysis was performed to investigate associations with other bacteria and viruses.
• The study determined that TW samples had a higher prevalence of M. equirhinis compared to BAL.
• The M. equirhinis bacterium was frequently found in specific horse breeds – thoroughbreds and trotters, as well as horses living in contained stable conditions compared to other horses or living situations.

Interpretation and Implications

• With the prevalence of M. equirhinis remaining steady despite varied clinical scores, the researchers concluded that the bacterium could not be regarded as a primary pathogen in respiratory illness in horses.
• The study suggests that M. equirhinis might be involved in an inflammatory response, similar to that seen in equine asthma, as it was never detected in BAL samples with ‘normal’ neutrophil counts.
• An increase in M. equirhinis prevalence was observed when other bacteria such as Streptococcus equi subsp. zooepidemicus (S. zoo) or viruses are present, indicating potential severity of symptoms in co-infection cases.