M. Bakhshesh - Razi Vaccine and Serum Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran
S. Masoudi - Razi Vaccine and Serum Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran
M. Esmailizad - Razi Vaccine and Serum Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran
B. Khalesi - Razi Vaccine and Serum Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran

Avian infectious bronchitis is an acute and highly contagious disease that mainly causes respiratory symptoms in poultry. A number of serotypes and variants of the viral agent with poor cross-protection are the major problem to achieve desired immunity from vaccination. The S۱ subunit of S glycoprotein (spike) is the major determinant of IBV so that a minor change in amino acid sequence of this protein, alters the virus strain. Therefore, characterization of the sequence of S۱ gene is necessary to identify virus strains and their similarities with the vaccinal strains. In this research, the S۱ sequence of H۵۲ and H۱۲۰ vaccinal strains of Razi Institute was fully characterized, and also the effect of serial passages in embryonated - eggs (۵ passages beyond the master seed) on the S۱ gene was investigated. The results showed that H۱۲۰ and H۵۲ strains of Razi Institute are ۱۰۰% identical to the reference vaccine strains available in the GenBank. In addition, the H۵۲ strain showed one amino acid substitution from the ۳rd passage in which Glycine (G) was replaced by Valine (V) at position ۱۱۸ making these passages exactly identical to the H۱۲۰ strain while no change occurred for the H۱۲۰ strain during these passages. Analysis of the original vaccinal strains which are widely administered in Iran, is definitely useful for prevention and control strategies against the circulating viruses. To identify the genetic change(s) responsible for attenuation of these strains during passages in embryonated-egg, characterization of other genes, especially those involved in replication is recommended.

IBV, Vaccinal strain, S۱ gene characterization, egg- passaged