A newly developed temperature-sensitive <i>Mycoplasma synoviae</i> live attenuated strain prevents pathological lesions of the respiratory and reproductive tracts in chickens caused by a wild-type <i>M. synoviae</i> strain

A field isolate of Mycoplasma synoviae, designated D14-383, caused pathogenic lesions in the air sac, trachea, lung, and ovary and induced a decline or stop in egg production and eggshell apex abnormalities in chickens after intratracheal inoculation. A live M. synoviae vaccine candidate was developed after in vitro passaging at 32°C from a wild-type M. synoviae strain originally isolated from the trachea of a 5-month-old asymptomatic layer chicken. In this study, the vaccine efficacy of the attenuated vaccine candidate strain against M. synoviae infection caused by the D14-383 strain was investigated. Eye-drop vaccination of 4-week-old chickens with 10^5.9 colony-forming units (CFU)/dose of the attenuated strain induced high levels of anti-M. synoviae antibodies, which were tested by serum plate agglutination, and a 95% seroconversion rate was maintained in the vaccinated birds for 108 weeks after vaccination. In the vaccinated birds, the air sac lesion score was 0.93 versus 4.0 in the unvaccinated group, as revealed by postmortem examination 7 days after challenge infection with the D14-383 strain. Moreover, the percentage of ovarian lesions was 15.9% (7/44 birds) and 73.3% (11/15 birds) in the vaccinated and unvaccinated groups, respectively. During the observation period after vaccination, egg production stopped in the unvaccinated birds but not in the vaccinated birds. Thus, the attenuated strain was proven to induce 2 years of protective immunity against M. synoviae infection in chickens.