Some Characteristics of Isolates of Infectious Bronchitis Virus from Commercial Vaccines

Abstract

SUMMARY Six isolates of infectious bronchitis virus (IBV) from commercial vaccines were studied and compared by respiratory signs incited in chickens, by protection from IBV challenge with homologous and heterologous serotypes, and by virus-neutralizing (VN) antibodies stimulated. Five of the isolates were Massachusetts-type, and one was Connecticut-type. Three of the vaccine viruses induced moderate to severe respiratory signs. Three isolates (two Massachusetts and one Connecticut) induced only mild signs. When heterologous viruses were employed for immunity challenge, all the Massachusetts-type vaccines proved superior to the Connecticut-type. Responses to challenge differed among the Massachusetts-type vaccines themselves, however, and were not necessarily correlated with the intensity of respiratory reaction in the immediate postvaccination period. Variations within the Massachusetts-type were clearly evident from the results of immunity challenge. Furthermore, some serological variations in results were seen from VN tests with homologous and heterologous viruses and the serum antibodies detected. VN test results correlated little with resistance to immunity challenge with the heterologous IBV types employed. The need for more complete characterization of IBV isolates, and types, for vaccine use is discussed with reference to improved predictability of field performance, purity of the seed virus, and a clearer description of the individual IB vaccine when marketed.