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 previousl y 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 m onth, the 10 oldest chickens were removed from each pen and replaced w ith 10 vaccinated chickens to return the total number of chickens in e ach 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 stra in of all MG isolates was determined by rapid amplified polymorphic DN A. AU vaccine strains were isolated from tracheas prior to contact cha llenge, but colonization by the 6/85 vaccine strain was inconsistent. Beginning with the group placed in contact 3 mo after the initiation o f 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 th at pen. However, neither strain ts-ll nor 6/85 was able to displace th e R strain under the conditions of this study.