Chronological observations of Marek's disease-associated feather-pulp lesions in field chickens

Fourteen test groups consisting of a total of 541 field chickens in six rearing farms, all of which were vaccinated with turkey herpesvirus at one day old, were investigated for chronological changes in Marek's disease (MD)-associated feather-pulp lesions (FPL) and their association with nuclear-inclusion (NI) formation in the feather-follicle epithelium (FFE) and with incidence of MD. The FPL observed were categorized into R1-type (non-tumorous lymphoid lesion), R2-type (inflammatory lesion consisting of plasma cellular infiltration, edema or collagenosis of the pulp, and germinal-center formation) and T-type (tumorous lymphoproliferative lesion). In every test group, the incidence of R1-type lesions was highest at 2-4 weeks of age, in spite of the low incidence of NI formation in the FFE. The initial R1-type lesions were generally transitory and were followed occasionally by R2-type lesions. In each test group, incidence of NI formation in the FFE peaked between 9 and 16 weeks of age. The second peak of incidence of R1-type lesions coincided with the period of peak NI formation. Subsequent skin samples revealed that the incidences of R1-, R2-, and T-type lesions in individual chickens or test groups were closely related to the incidence of NI in the FFE and to the incidence of MD.