High prevalence of Mycoplasma equirhinis in Thoroughbred horses with respiratory symptoms in autumn 2018

Mycoplasma species are often isolated from horses with respiratory symptoms; however, the pathogenicity of Mycoplasma is still unclear. In autumn of 2018, we encountered an increase in cases with respiratory symptoms, mainly coughing, in a group of Thoroughbred racehorses in Japan. We examined tracheal wash samples obtained from 40 of those cases. Bacteria and viruses that commonly cause respiratory symptoms were investigated, and anaerobes were detected in only 5 cases and Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) was detected in only 1 case of 40 cases with loop-mediated isothermal amplification assay. S. zooepidemicus and Streptococcus pneumoniae were isolated at a bacterial count of higher than 1.0 × 10⁴ CFU/ml from 5 and 2 cases of 28 cases cultured, respectively. None of the viruses investigated was detected in 40 cases. Mycoplasma equirhinis (M. equirhinis) was isolated from 40.0% (16/40) of the cases, which was higher than previously reported isolation rates. The rate of M. equirhinis isolation in the cases from 2018 was significantly higher than the isolation rates in the other horses: clinical cases with respiratory symptoms in 2019–2020 (13.6%, 3/22) and healthy horses (13.5%, 5/37) in Japan. In this study, the isolation rate of M. equirhinis from horse group with cough symptoms in 2018 was high and no other common etiological agents were detected. The pathogenesis of M. equirhinis is still unclear, however, M. equirhinis might have been associated with respiratory symptoms in the Thoroughbred horse cases in 2018.