Efficacy of chlortetracycline against Mycoplasma synoviae isolated in two periods.

Isolates obtained in 1954-55 were generally more pathogenic for the synovial membranes than those obtained in 1969-73. All isolates were capable of producing synovitis when inoculated into the foot pad. Chlortetracycline was effective in controlling signs of infectious synovitis at the 100-g/ton concentration. The 50-g concentration prevented mortality and reduced morbidity. All concentrations of chlortetracycline used (50, 100, or 200 g/ton of feed) reduced immune response, positive agglutination tests, and isolation of MS below that of inoculated untreated birds. Chlortetracycline at all concentrations prevented the spread of MS in contact controls. Chlortetracycline was more effective against the 1954-55 isolates than the 1969-73 isolates when measured by gross lesions, development of agglutinins, and isolation of mycoplasma. No difference in efficacy was found in mortality, weight gain, and feed efficiency.