Cutaneous lesions in broiler chickens spontaneously affected with Marek's disease

A total of 91 8- to 9-week-old broiler chickens with Marek's disease (MD) skin tumours ("skin leukosis"), collected from 15 farms at the processing plants, were examined pathologically. Grossly, the skin lesions comprised various sizes and numbers of feather follicular nodules, which tended to fuse with each other as their size increased. Histologically, the lesions were classified into five types: type A was small lymphoid cell aggregates (LCA) consisting mostly of small lymphocytes with a few lymphoblasts and very rare mitotic figures; type B had large LCA consisting mainly of small lymphocytes with considerable numbers of lymphoblasts and very rare mitotic figures; type C was characterized by large coalesced LCA consisting almost equally of small lymphocytes and lymphoblasts with infrequent mitotic figures; type D exhibited very large coalesced LCA consisting mainly of lymphoblasts with some small lymphocytes and occasional mitotic figures; type E had very large coalesced LCA consisting almost completely of lymphoblasts with frequent mitotic figures. The histological MD cutaneous lesions were related to the size of gross skin nodules, the small feather follicular nodules consisted mainly of types B and C with type A, whereas large fused feather follicular nodules were composed mainly of types D and E. Nuclear inclusions were frequently found in the feather follicular epithelium in all skin nodules except for the largest fused ones. MD visceral lesions were more pronounced in birds having marked skin leukotic lesions. Feather pulp lesions (FPL) were more related to the visceral than the skin lesions; the constituent cells of FPL were compatible with those of the former lesions.