Vaccination against infectious bronchitis and the immunosuppressive effects of infectious bursal disease.

An immunosuppressive effect was demonstrated in chickens which were infected with infectious bursal disease virus (IBDV) early in life and prior to or shortly after vaccination with infectious bronchitis virus (IBV). This effect was evident by an increased susceptibility to respiratory tract infection with IBV and reduced virus-serum neutralizing antibody levels. Chickens which were hatched from dams susceptible to infectious bursal disease (IBD) were less responsive to IBV immunization attempts, if exposed to IBDV, than were those individuals hatched from IBD immune dams. However, in some cases, chickens from IBD immune dams were also more susceptible to IBV challenge when they had been exposed to IBDV and when compared to birds unexposed to IBDV but vaccinated against IB. An effect of cyclophosphamide on the bursa of Fabricius also had an immunosuppressive action on IBV immunity which was similar to the results from IBDV exposure. The data engendered from these trials may explain the unsatisfactory immunity sometimes observed under field conditions when broilers and replacement pullets are vaccinated at an early age.