Relationship between several criteria of challenge-immunity and humoral immunity in chickens vaccinated with avian infectious bronchitis vaccines

Several methods were compared for assessing the immunity of chickens vaccinated with avian-infectious bronchitis virus (IBV) by observing tracheal ciliary activity, respiratory signs and virus recovery from several organs following challenge with virus which had not been passaged in embryos. All chickens vaccinated twice with live Kita-1 strain were protected according to all criteria. However, in chickens vaccinated with killed IBV and with H-120 strain 13 weeks earlier, respiratory signs were present and tracheal virus was isolated from the birds with ciliostasis regardless of their having significant levels of haemagglutination inhibiting (HI) antibody. This indicated that only observing the ciliary activity is insufficient for assessing complete immunity and no direct correlation exists between the serum HI titre and the presence of ciliostasis, respiratory signs and virus recovery from tracheas, excluding those from kidneys and genitalia. Regardless of the serum HI titres at challenge, full protection based on the six criteria was found in some birds at 1 week and in most chickens from 2 to 6 weeks after intraocular vaccination with live Kita-1 strain.