Morphologic changes in the bursa of fabricius of chickens after inoculation with infectious bursal disease virus.

Sequential morphologic changes in the bursa of Fabricius were studied after oral inoculation of 1-day-old chicks with infectious bursal disease virus (IBDV). The epithelial surface morphology was studied by scanning electron microscopy, whereas the IBDV replication was sequentially followed by immunofluorescence and transmission electron microscopy. The earliest detectable changes in the bursal epithelium were evident at postinoculation hour (PIH) 48. They were characterized by reduction in numbers and size of microvilli on the epithelial cells accompanied by gradual involution of the button-like bursal follicles. At PIH 96 some specimens showed localized surface erosions due to loss of epithelial cells. As the damage progressed, the infolding of the buttomlike follicles became more pronounced and the surface erosions became more extensive. Loss of surface epithelium exposed the underlying damaged bursal follicles which appeared to be bounded by columnar epithelium. Some follicles had lost almost all the lymphocytes and macrophages and appeared as empty craters. Intrafollicular replication of IBDV was detectable as early as PIH 24 by immunofluorescence technique. Viral replication primarily took place in the lymphoid follicles. Regeneration of the follicles was not seen up to postinoculation day 12, suggesting that the IBDV-induced bursal damage could be permanent.