Pen trial studies on the use of live vaccines to displace virulent Mycoplasma gallisepticum in chickens.

Groups of 10 8-wk-old chickens that had been vaccinated 4 wk previously with the F strain, ts-11, or 6/85 strain of Mycoplasma gallisepticum (MG) were challenged by placing them in contact with 20 chickens that had been previously infected with the virulent R strain of MG. Each month, the 10 oldest chickens were removed from each pen and replaced with 10 vaccinated chickens to return the total number of chickens in each pen to 30. Chickens were bled and cultured for MG prior to contact challenge and at the time of removal from the challenge pen. The strain of all MG isolates was determined by rapid amplified polymorphic DNA. All vaccine strains were isolated from tracheas prior to contact challenge, but colonization by the 6/85 vaccine strain was inconsistent. Beginning with the group placed in contact 3 mo after the initiation of the study, F strain had begun to displace the R challenge strain. By the eight month, F strain had completely displaced the R strain in that pen. However, neither strain ts-11 nor 6/85 was able to displace the R strain under the conditions of this study.