Safety and efficacy of a Mycoplasma gallisepticum oppD knockout mutant as a vaccine candidate.

Control of the important poultry pathogen Mycoplasma gallisepticum is highly dependent on safe and efficacious attenuated vaccines. In order to assess a novel vaccine candidate we evaluated the safety and efficacy of the M. gallisepticum mutant 26-1. The oppD gene in this mutant has been interrupted by a signature-tagged transposon and previous studies have shown that it can colonise the respiratory tract of chickens without inducing significant disease. The capacity of the oppD mutant to induce protective immunity in the respiratory tract after vaccination by eye-drop was assessed by challenging vaccinated birds with an aerosol of the virulent M. gallisepticum strain Ap3AS. Vaccination with the oppD mutant was shown to fully protect against the lesions caused by pathogenic M. gallisepticum in the air sacs and tracheas. It also protected against the effect of infection on weight gain, and partially protected against colonisation of the trachea by virulent M. gallisepticum. These results indicate that a M. gallisepticum mutant with the oppD gene knocked out could be used as a live attenuated vaccine as it is both safe and efficacious when administered by eyedrop to chickens.