Thermostability profile of Newcastle disease virus (strain I-2) following serial passages without heat selection

I–2 is an avirulent strain of Newcastle disease virus. During establishment of the I-2 strain master vaccine seed, a series of selection procedures was carried out at 56^°C in order to enhance heat resistance. This master seed is used to produce a working seed, which is then employed to produce the vaccine. These two passages are done without further heat selection; however, it is not known how rapidly and to what extent thermostable variants would be lost during further passage. The study was therefore conducted to determine the effect of passage on thermostability of strain I-2. The virus was serially passaged and at various passage levels samples were subjected to heat treatment at 56^°C for 120 min. The inactivation rates for infectivity and haemagglutinin (HA) titres were assayed by use of chicken embryonated eggs and HA test, respectively. Thermostability of HA and infectivity of I-2 virus were reduced after 10 and 5 passages, respectively, without heat selection at 56^°C. These results suggest that 5 more passages could be carried out between the working seed and vaccine levels without excessive loss of thermostability. This would result in increased vaccine production from a single batch of a working seed.