Mycoplasma gallisepticum strain ts-304 is a safe and effective live attenuated vaccine for use in chickens.

Vaxsafe MG (strain ts-11) is a live attenuated vaccine against the important poultry pathogen Mycoplasma gallisepticum that has been used globally to improve poultry health. However, the majority of the bacterial cells in Vaxsafe MG do not express the GapA cytadhesin, reducing their capacity to colonise the respiratory tract. Vaxsafe MG (strain ts-304) is a GapA positive clone recovered from Vaxsafe MG (strain ts-11) that has been shown to be safe and efficacious in turkeys, and preliminary studies have suggested that Vaxsafe MG (strain ts-304) may have greater efficacy in chickens than Vaxsafe MG (ts-11). The studies described here aimed to meet the international regulatory requirements for safety and efficacy in chickens. The vaccine colonised the trachea of 3-week-old chickens without inducing signs of respiratory disease or significant lesions in the respiratory tract, and was safe at a tenfold overdose and after repeated administration. It was transmissible from vaccinated to naïve chickens with no evidence of reversion to virulence following multiple in vivo passages. Finally, the superiority of Vaxsafe MG (strain ts-304) was demonstrated by its capacity to induce similar protection against infection with wild type M. gallisepticum at a 40 fold lower dose than the end of shelf life titre dose of Vaxsafe MG (ts-11). The lower effective dose of Vaxsafe MG (strain ts-304) allows it to be freeze-dried, enhancing its stability, making it easier to transport and store the vaccine and increasing its shelf life. Vaxsafe MG (strain ts-304) is, therefore, a highly efficacious and promising live attenuated vaccine candidate suitable for use in chickens.