Immunity following inoculation of the H120 and H52 vaccine strains of infectious bronchitis virus into the crop of the domestic fowl

Abstract

The serological responses and protection associated with inoculation of the HI20 and H52 vaccine strains of infectious bronchitis virus (IBV) into the trachea and directly into the crop of chickens were compared. Intratracheal inoculation at 3 weeks of age with HI20 resulted in the moderate log2 haemagglutination inhibition (HI) titres of 6, previously found to be characteristic of this highly attenuated vaccine strain of IBV. More than 80% of these birds gave a positive reaction to the agar gel precipitin (AGP) test. These precipitin responses had waned in most birds by the time they were re-vaccinated with H52 at 11 weeks of age, when moderate but somewhat higher HI titres of 7 and 8 were obtained and 70% of the birds were AGP-positive. At 15 weeks of age, the inoculates and susceptible controls were challenged intravenously with H52. The intratracheally inoculated birds were fully protected against this challenge which severely damaged the kidneys of 80% of the controls. Crop inoculation of HI20 and H52 failed to produce circulating antibody as measured by the HI and AGP tests. Intravenous challenge, however, showed that the kidneys of 50% of the crop inoculates resisted the challenge which severely damaged the kidneys of 80% of the controls, demonstrating a degree of protection in the absence of circulating antibody.