Evaluation of attenuated strains of Mycoplasma gallisepticum as vaccines in young chickens.

Five trials were conducted to evaluate the virulence and the vaccination efficacy of the F, R, S6, and A5969 strains of Mycoplasma gallisepticum (MG) at different in vitro passage levels. Vaccination was done by eye-drop or aerosol, and efficacy was evaluated in terms of air-sac lesions after aerosol challenge with the R strain of MG. Continuing medium passage of these strains of MG resulted in gradual attenuation. Aerosol vaccination with highly attenuated MG at 21 days of age was more effective in stimulating the development of immunity than was eye-drop vaccination at 7 days, but aerosol vaccination induced more air-sac lesions. Aerosol vaccination with high-passaged F or S6 strain provided good immunity against MG challenge. Back-passage in chickens of the F strain of over 300 medium passages did not substantially increase the virulence, but back-passage of the S6 strain at the 258th passage and F strain at the 11th and 252nd passage did result in increased virulence. Both low- and high-passaged F and S6 strains were virulent for turkey poults, but only the high-passage F strain did not significantly (P less than 0.05) affect body weight.