Differentiation of avian infectious bronchitis virus isolates by thermal sensitivity

Six isolates of avian infectious bronchitis virus (IBV) at different passage levels in the chicken embryo were tested for sensitivity at 56 C, with the following results: 1) isolates differed in thermal sensitivity; 2) virus was inactivated in a first-order exponential-kinetics 2-component fashion indicative of a heterogeneous population with a preponderance, 98% or more, of thermal-sensitive (S) virions over thermal-resistant (R) virons; 3) R virions 2-component populations were inactivated; 4) recovered R virions could be maintained in continuous passage in the chicken embryo as a one-component population by the limiting-dilution technique, although there was a progressive parallel increase in thermal sensitivity associated directly with the continuous passage; 5) growth curves of isolated one-component populations of R virions were quantitatively and in time sequence similar to curves of 2-component populations of continuous-passage virus.