Cross-protection studies with vaccine strain H-120 of infectious bronchitis virus using ciliary activity.

Forty 3-day-old chickens were immunized intratracheally and another 40 intranasally-intraocularly with vaccine strain H-120 of infectious bronchitis virus (IBV). The chickens were divided into groups of five and each group was challenged intratracheally or intranasally-intraocularly with one of 8 different heterologous strains of IBV 4 weeks after vaccination. The vaccinated chickens were protected against challenge with three heterologous strains (Massachusetts M41, B and AEG), showing 89, 86 and 89% ciliary activity, respectively, but were not protected against challenge with strains D-3896 and D-3128. Clinical signs and/or a reduction in ciliary activity was observed neither for the vaccinated nor for the unvaccinated groups challenged with strains Sz, D-207 and D-212. This suggests that these isolates have no primary pathogenic role in broiler flocks. The remaining 5 strains (M41, B, AEG, D-3896 and D-3128) caused mild respiratory signs and ciliostasis in birds of the unvaccinated groups.