Chronological observations of feather pulp lesions in chickens inoculated with Marek's disease virus.

Observations of the feather pulp from chickens inoculated with Marek's disease (MD) virus revealed diverse lesions similar to those in the peripheral nerves of chickens infected with MD. These lesions were categorized as follows: R1-type lesions = from minimal perivascular to diffuse infiltration of small lymphocytes mixed with a few medium lymphocytes or blast cells; R2-type lesion = edema and cellular infiltration consisting of plasma cells and small lymphocytes; and T-type lesion = tumorous proliferation of lymphoid cells predominantly composed of medium lymphocytes or blast cells. Chronological observations of the feather pulp biopsies taken at 2-week intervals from individual chickens revealed the lesion progression R1-type leads to T-type, or R1-type leads to R2-type. The former progression was usually seen in birds showing evidence of both persistent nuclear-inclusion (NI) formation in the feather-follicle epithelium (FFE) and development of lymphoid tumors in the viscera. The latter progression was seen exclusively in chickens that showed transient NI formation in the FFE, regardless of the incidence of MD.