Histologic characterization of the involuting bursa of Fabricius in single-comb white Leghorn chickens.

Bursas from specific-pathogen-free white leghorn chickens of both sexes were examined at several intervals from hatching to 28 weeks of age. No histologic alterations other than scattered atrophic or cystic follicles were observed through 20 weeks. Obvious involution, first noted at 24 weeks, was in early stages in females and quite advanced in males. Involution was essentially complete by 26 weeks, and only cicatrized vestiges of bursas were present at 28 weeks of age. Gross manifestations included bursal atrophy, variable yellowish discoloration of the mucosa, and matting or total loss of identity of the mucosal plicae. Histologic characteristics of involution are summarized by the following approximate sequence: atrophy and exfoliation of plica epithelium; subepithelial stromal fibrosis; fusion and ultimate collapse of plicae; liquefactive necrosis of first medullary then cortical elements of follicles, which seemed to progress from basal to apical portions of the plicae; progressive proliferation of stromal connective tissue and infiltration of macrophages into areas occupied by necrotic follicles; and, finally, complete fibrous organization of luminal debris, leaving a firm nodule formed by a contracted muscularis surrounding the cicatrized remains of the mucosa.

Eradication of subgroups A and B lymphoid leukosis virus from commercial poultry breeding flocks.

A specific-pathogen-free pedigreed breeding population of White Leghorns was determined to be free of lymphoid leukosis subgroups (LL-A and LL-B) on the basis of the standard resistance-inducing factor (RIF) test of sera of individual birds. The fourth pedigreed generation (G-4) was the first to test free of viruses of both sub-groups. In three subsequent pedigreed generations (G-5 through G-7) no viremic birds were found. The RIF test of sera was also used to detect carriers infected with LL-A and LL-B in sample populations of commercial broiler, brown-egg layer, and other White Leghorn breeding lines. When only known nonviremic dams were used for reproduction, in only one instance were viremic birds detected in samples of their progeny in succeeding generations. Some implications for a virus-free breeding program are discussed.