Avian infectious bronchitis virus is a gammacoronavirus containing the largest known viral RNA genome. The RNA forms a single strand and single segment. IBV causes an acute highly contagious viral respiratory disease of chickens which is characterized by respiratory rales, coughing and sneezing. IBV is able to spread very rapidly in non-protected birds. It is shed via both the respiratory tract and the faeces, and can persist in the birds and the intestinal tract for several weeks or months. Infectious bronchitis virus variant strain 4-91 is a isolate used for produce a live attenuated viral vaccine.