Full genome sequences comparison of Infectious Bronchitis virus QX L1148 vaccine and its virulent progenitor led to the identification of the changes involved in the attenuation process.

Infectious Bronchitis virus (IBV) is responsible for an important contagious poultry disease and it is distributed worldwide. The control of the disease is mainly achieved by the use of live vaccines. IBV QX vaccine strains are available and currently used in many countries where this genotype is circulating. In the present study the complete genome sequences of a QX vaccine strain L-1148 and its progenitor virulent strain 1148-A were obtained and compared so as to identify the nucleic acid changes occurring during the attenuation process. The comparative sequence analysis of the two strains showed the presence of 28 nucleic acid substitutions, resulting in 15 amino acid changes. Most nucleic acid substitutions are located in the 1a-1ab and S genes. One change each is located in the E, M, and 5a genes and two changes each in the 5’ UTR, 5b gene and 3’UTR. Changes associated with adaptation to chicken embryos were identified, of which some would have resulted in attenuation.