Effects of two strains of infectious bronchitis virus on unvaccinated laying hens

Preliminary studies were conducted on unvaccinated laying hens to investigate the effects of two strains of infectious bronchitis virus (T strain and N1/88 strain) on unprotected birds during lay. Clinical symptoms associated with the respiratory system were observed in the T-strain and N-strain groups but not the control and some birds in the T-strain group had enlarged kidneys. Feed intake tended to be depressed in the challenged birds at one week post-challenge and production was reduced in the T-strain group at 3 weeks post-challenge. However, there were relatively few effects of challenge on egg quality except that yolk colour was depressed in the T-strain group. There were no significant effects on excreta moisture for any group. IBV antibody titres increased in response to challenge in the T-strain group but only some challenged birds produced antibodies in the N-strain group within 3 weeks post-challenge. Virus was re-isolated from the kidneys of T-strain birds at all sampling times post-challenge but only at 6-16 days for the N-strain group. Sequential histopathological changes revealed that IBV multiplies initially in the Harderian gland, then in the tracheal mucosa and simultaneously in the kidney and oviduct. The severity and persistence of lesions were greater in the shell gland and kidney of T infected birds whereas, in the trachea and Harderian gland, the effects of the two IBV strains were similar. N1/88 seemed to be more pathogenic for the magnum region of the oviduct.