The live attenuated temperature-sensitive (ts ) Mycoplasma synoviae (MS) strain, MS-H, is used as a vaccine in a number of countries to control virulent MS infection in commercial chicken flocks. Nine out of 50 isolates made from flocks vaccinated with MS-H were found to have lost the ts phenotype of the original vaccine strain. In order to examine the influence of the ts− phenotype on virulence of the isolates, four of the ts− isolates, the MS-H vaccine, and the vaccine parent strain 86079/7NS were administered by aerosol in conjunction with infectious bronchitis virus to 3-wk-old specific-pathogen-free chickens. The four ts− clones induced only minimal air sac lesions that were not different in severity from those caused by MS-H vaccine; however, the vaccine parent strain 86079/7NS caused air sac lesions that were significantly greater than those of MS-H and all ts− clones. The vaccine parent strain 86079/7NS and two of the ts− clones were recovered from the air sacs of the respectively infected chickens whereas the MS-H vaccine and two other ts− clones were not. Three of the ts− isolates caused increased tracheal mucosal thicknesses that were significantly greater than those from birds inoculated with MS-H, and one caused increased tracheal mucosal thicknesses that were significantly less than those from birds inoculated with 86079/7NS.
In conclusion, unlike the MS-H vaccine, the MS-H ts− clones were associated with minor changes in tracheal mucosa; however, unlike the vaccine parent strain, they did not induce lesions in the air sacs. These results suggest that factors other than ts phenotype are involved in the attenuation of the MS-H vaccine.