A modified live Mycoplasma gallisepticum vaccine to protect chickens from respiratory disease.

The aim of this study was to assess the efficacy of a modified live Mycoplasma gallisepticum vaccine (GT5) for the protection of chickens against infection and respiratory disease. GT5 was constructed by the reconstitution of the avirulent high passage R (R(high)) strain with the gene encoding the major cytadhesin GapA. GT5 expressed GapA on its surface yet retained the phenotypic characteristics of the parental R(high) strain. Birds vaccinated with GT5 were protected upon challenge with the virulent low passage R (R(low)) strain as evidenced by a complete absence of tracheal lesions 2 and 4 weeks post-challenge, in contrast to sham immunized/challenged control birds. Modest amounts of IgG, and little, if any secretory IgA or IgM anti-M. gallisepticum were found in tracheal washings following vaccination. However, copious amounts of specific IgA were found following challenge, especially in sham immunized birds. This suggests that the tracheal IgG elicited by GT5 vaccination may have been responsible for blocking the initial colonization of R(low), thereby resulting in protection.