Immunodeficiency in the chicken. IV. An immunological study of infectious bursal disease

Chickens inoculated orally with infectious bursal disease virus (IBDV) 1 day after hatching subsequently showed a 50% incidence of immunodeficiency but little mortality. Antibody responses against IBDV and to immunization with sheep red blood cells (SRBC) or human serum albumin (HSA) were suppressed. Serum IgG concentration was decreased while IgM occurred exclusively in its 7S monomeric form (mIgM). An allotypic marker of chicken IgM (Mla) was lacking in mIgM derived from IBDV-infected birds. The loss of Mla occurred gradually in several birds between 3 and 12 weeks after perinatal infection. Inoculation of IBDV into chickens 3 weeks after hatching resulted in 50% mortality level but little immunodeficiency. Paradoxically, the serum IgG concentration was elevated, in comparison with normal birds. Histology of the bursa showed permanent hypo- or aplasia of follicles irrespective of the age of infection. The results suggest that bursal but not peripheral B cells are targets for IBDV, and immunodeficiency results from impaired peripheral seeding of B cells in infected juvenile chickens.