Immunosuppressive effect of infectious bursal disease virus on vaccination against infectious bronchitis.

Groups of broiler chicks hatched with parental antibodies to infectious bursal disease virus (IBDV) and infectious bronchitis virus (IBV) were vaccinated against IBV at 1 day of age via the oculonasal routes and inoculated with virulent IBDV at 1, 5, 10, 15 or 20 days of age. While the non-IBDV inoculated birds were solidly immune against IBV challenge at an age of 29 days, immunity in the IBDV infected birds was depressed. This depression, which was most serious in the birds IBDV inoculated at 1 or 5 days of age, coincided with a delayed infiltration of the Harderian gland by lymphocytes and immunoglobulin-bearing cells. In the groups inoculated at older ages infiltration did not seem to be delayed but moderate destruction of plasma cells was observed 7 to 14 days later. The neutralisation index to IBV, which was significantly higher in the non-IBDV infected than in the infected birds at the day of challenge, increased sharply after challenge in the IBDV infected but not in the non-infected birds. All IBDV-inoculated birds developed typical lesions when 17 to 26 days old whereas precipitins reappeared when birds were 29 to 33 days old.