Further studies on the development of a live attenuated vaccine against turkey rhinotracheitis.

Summary Turkey rhinotracheitis (TRT) virus attenuated by passaging in Vero cells was tested at two different passage levels (15 or 25 passages) and two dose levels [103 or 104 TCID50 (50% tissue culture infectious doses) per bird] to determine the efficacy in protecting turkey poults against experimental challenge with virulent TRT virus. Following administration by the eyedrop route at 10 days of age, all four preparations proved successful in providing protection against clinical disease and establishment of challenge virus in the trachea when challenged with virulent virus 3 weeks later. Twelve‐day‐old poults given the 25th Vero passage TRT virus at a dose of 103.5 TCID50 per bird were protected against experimental challenge with virulent virus for at least 22 weeks post‐primary inoculation. The 25th passage virus was tested for safety by administering ten times the dose (104.5 TCID50 per bird) used in the previous trial to a group of 10‐day‐old turkey poults. None of the birds showed any clinical sig...