Demonstration of a virulent subpopulation in a prototype live attenuated turkey rhinotracheitis vaccine

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