Studies on Marek's disease. II. Pathogenesis.

The development of Marek's disease (fowl paralysis; neurolymphomatosis) has been studied by examination of peripheral nerves and other tissues at different times after infection of young chicks with the HPRS-B14 strain of Marek's disease. Three types of nerve lesions were found: 1) A-type, characterized by proliferation of lymphoid cells, presence of Marek's disease cells, and sometimes demyelination and Schwann cell proliferation; 2) B-type, characterized by diffuse infiltration by plasma cells and mainly small lymphocytes, usually interneuritic edema, sometimes demyelination and Schwann cell proliferation; 3) C-type, characterized by light infiltration by plasma cells and small lymphocytes. A mixed A- and B-type lesion was also found. Serial killing experiments and grouping of lesions from transmission experiments according to the time elapsed since infection indicate that the nerve lesion follows the progression: A-type--> mixed A- and B-type-->B-type. The C-type lesion is believed to be a mild form of the B-type. The study indicates that Marek's disease is characterized by a neoplastic-like proliferation of lymphoid cells in the nerves and in other organs, notably the ovary. In some birds the proliferation is progressive and they succumb early in the course of the disease with tumor-like infiltration of the nerves and often other organs. Demyelination and other nerve tissue changes appear to be secondary to the lymphoid proliferation. In other birds the proliferation of lymphoid cells in the nerves is arrested, and the lesion changes into one of a more inflammatory appearance.