| Avian infectious bronchitis is an economically important viral disease of poultry resulting in reduced performance, reduced egg quality and quantity, significantly increased susceptibility to concurrent infections and a costly vaccination program. The disease is caused by infectious bronchitis virus (IBV) which is in the family Coronaviridae. IBV is highly infectious and spreads rapidly through horizontal transmission by aerosol or ingestion. In an earlier study sentinel chickens were employed to determine the incidence of IBV in Ontario layers. IBV isolates recovered from the sentinel birds were not vaccine related as shown by genetic analysis of the S1 region. Five of these IBV field isolates were selected for further characterization.; Analysis of tracheal lesion scores for each isolate revealed statistically significant differences (p<0.01) between infected groups and negative control group. Moreover, statistically significant differences (p<0.01) were also observed among; particular isolates. Birds exposed to IBV-ON1 and IBV-ON2 had 2.6 and 2.2 grams, respectively, lower average daily weight gains than birds in the negative control group (p<0.01) during the first 11 days of the trial, but not over the entire study period. The protection provided by a commonly used vaccine, MILDVAC-Ma5, was studied by challenging immunized chickens with the respiratory (IBV-ON1) and nephropathogenic (IBV-ON4) viruses. The mean vaccine efficacy for IBV-ON1 was 66.7% indicating that a Massachusetts serotype vaccine would provide some protection against IBV field isolates. (Abstract shortened by UMI.)... |
