PROTECTION BY A COMMERCIAL ARKANSAS-TYPE INFECTIOUS-BRONCHITIS VIRUS-VACCINE AGAINST A FIELD ISOLATE OF THE SAME SEROTYPE

A late-breaking infectious bronchitis virus (IBV)-associated respirato ry disease was a chronic problem in Georgia broilers in 1995. The pred ominant virus isolated from diseased birds was the Arkansas (Ark) type of IBV. Because broilers in Georgia are currently vaccinated with the Arkansas serotype, there was concern that a phenotypic and/or genotyp ic change had occurred in the field virus so it could break through im munity conferred by commercial vaccines. The purpose of this study was to determine if a commercially available vaccine for IBV as currently used in the field still protected broilers against those viruses. We obtained 108 1-day-old broilers from a commercial source and assigned them randomly to 12 groups. One-half of the groups of birds were vacci nated at 1 day of age and again at 18 days of age with commercially av ailable B-1/Mass/Ark vaccine. One-half of both vaccinated and nonvacci nated groups of birds were challenged at 35 and 42 days of age with a recent IBV Ark field isolate. Serologic titers were evaluated by enzym e-linked immunosorbent assay at time of challenge and at the end of th e trial. A necropsy was performed on birds at 56 days and pathogenicit y was assessed. Seroconversion was statistically significant in all bi rds exposed to vaccine or challenge by 56 days of age. Gross airsaccul itis was significantly more severe in broilers challenged without prio r exposure to vaccine.