DEMONSTRATION OF A VIRULENT SUBPOPULATION IN A PROTOTYPE LIVE ATTENUATED TURKEY RHINOTRACHEITIS VACCINE

A prototype live attenuated turkey rhinotracheitis (TRT) vaccine, whic h was known to cause occasional disease in young poults following mult iple back passage, was tested for the presence of a virulent subpopula tion by a novel combined in vitro and in vivo screening technique. Whe n vaccine was inoculated at high titres into chick embryo tracheal org an cultures, 17% of aliquots were found to cause ciliostasis, and when these aliquots, in turn, were inoculated into 1-day-old poults, appro ximately one-quarter caused clinical disease. Removal of the subpopula tion by plaque purification led to viruses which had reduced tendency to revert to virulence but remained protective. The technique proved v aluable in identifying virulent subpopulations in specific prototype T RT vaccines. The principle may have more general application.