Vaccination of 1-day-old broilers against infectious bronchitis by eye drop application or coarse droplet spray and the effect of revaccination by spray.

Broilers hatched with maternal antibodies against infectious bronchitis virus (IBV) had developed protective immunity by 3 weeks after vaccination by the conjunctival route with H120 vaccine virus at 1-day-old. Immunity was still present when birds were 7 weeks old. Following vaccination of similar 1-day-old chicks by the same route by a coarse droplet spray, immunity developed more slowly; 50% of the 3-week-old birds developed clinical and/or pathological changes typical of infectious bronchitis following challenge, and at 5 weeks of age some birds were still susceptible to the virulent IBV. While this retarded development of protection coincided with delayed lymphocytic infiltration and follicle formation in the Harderian gland, protection was not associated with circulating antibody levels against IBV, nor was it after eye drop application of the vaccine. Birds first vaccinated at 3 weeks by spray had developed protective immunity by 6 weeks, which was complete by 8 weeks of age. Revaccination by spray at 3 weeks of age of birds first vaccinated by spray at 1-day-old caused a depression of the protective immunity for approximately 1-2 weeks. This coincided with the presence of a few plasma cells and their destruction in the Harderian gland. Immunity in revaccinated birds first vaccinated at 1-day-old by eye drop was neither depressed nor increased. Formation of circulating antibody could not be demonstrated in the birds 2 weeks after they were vaccinated for the first time at 3 weeks of age. Revaccination at 3 weeks of age of their mates first vaccinated at 1-day-old resulted in a great decrease of circulating antibody. In contrast, application of virulent virus by eye drop at 3 weeks of age caused increasing titres. These different reactions are discussed.