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Histological and to some extent additional enzyme-histochemical studies were conducted into 28 cases of Marek's disease with minor, moderate, and intensive lymphoma formation. The investigations were undertaken with a view to differentiating T-lymphocytes and macrophages by differentiated activity on acid unspecific alpha naphthylacetate esterase. Lymphoma specimens were cytologically evaluated under semi-quantitative or quantitative aspects. Evidence was provided to the existence of correlations between intensity of Marek's disease lymphomas and their relative contribution to lymphocytes, lymphoblasts, and macrophages. The amount of lymphoblasts was found to go up along with advancing lymphoma proliferation, while the amounts of lymphocytes and macrophages went down. Moderately pronounced lymphomas contained 16 percent macrophages on average, whereas only about 8 percent were recordable from highly pronounced lymphomas. The high presence of macrophages in less developed lymphomas is considered to reflect unspecific defence reaction against neoplastic lymphatic cells in the phase of low tumour malignancy.

Virus-induced antigens were isolated from MDV and HVT-infected cells by salt extraction with 3M KCl and purified by Con.A chromatography and isoelectric focusing. Electrophoretic analysis of 35S-methionine, 3H-fucose and 32P-orthophosphate labeled antigens revealed 7 different polypeptides, two of them containing labeled carbohydrates, and one phospholipid component. The isolated virus-induced antigens from infected cells were identified as serologically active membrane complexes carrying common antigenic determinants of MDV and HVT. They were able to bind to virus-neutralizing immunglobulins as shown by antibody binding tests. Summarizing the presented analytic and serologic findings it was proposed to classify MDV and HVT as different serotypes of a common Marek's disease virus group.

Enzootic bovine leucosis and Marek's disease of fowl are the most important tumour diseases of farm animals, with the greatest repercussions on the national economy, today. Experimental findings obtained from either disease in recent years are reported in this paper, and an assessment is made of their importance to practice. In the context of enzootic bovine leucosis, particular reference is made to those findings which helped to establish the presence of virus infection, results by which the pathogen could be characterised, and to studies undertaken with the view to devising methods for virologico-serological diagnosis. In the context of Marek's disease, reference is made to virological, epizootiological, and diagnostic aspects, however, with emphasis being laid on possibilities for immunisation and, in that connection, on known fundamental concepts for successful immunoprophylaxis against that tumour disease.

An account is given of the present international position in immunoprophylactic control of Marek's disease. An assessment is made of the method used for immunisation in the GDR. In that context, reference is made to problems for which solutions will have to be found in the long run.

The tumours associated to lymphoid leucosis and Marek's disease are compared with lymphoproliferative diseases of man and other species, by morphological, immunomorphological, and aetiological criteria. Morphological and immunomorphological similarities were found to exist between lymphoid leucosis and malignant lymphomas of the B-cell system in man, more particularly, follicular lymphoma, lamphoblastic lymphosarcoma of the B-cell type, Burkitt's lymphoma, and immunoblastic lymphoma. Equivalent similarities with Marek's disease are relating particularly to Hodgkin's disease, infectious mononucleosis, and malignant lymphomas of the T-cell system. Aetiological relations were found to exist between lymphoid leucosis and the oncorna-virus-dependent lymphatic tumours in mammals, while similarities with Marek's disease were relating to infectious mononucleosis, Burkitt's lymphoma, and herpes-virus-dependent lymphatic tumours in monkey. Reference is made to the model nature of both lymphoid leucosis and Marek's disease and the resulting relevance to tumour research.

Viable spleen lymphocytes of 13 chickens with clinical and histological signs of Marek's disease were transferred by intravenous injection to 46 chickens 4-10 days old, X-rayed 24 hours before inoculation. Histologically Marek-like neural lesions were almost regularly observed in the chickens after 6-14 days. Chickens having received destroyed spleen lymphocytes of Marek-donors were free of nerve infiltrations. There were also no neural lesions detectable within 14 days after application of pathogenic Marek's disease virus alone, or in combination with viable spleen cells from healthy donors immunized against Marek's disease. These results exclude that the formation of neural lesions 6-14 days following the spleen cell transfer is solely induced by the virus.

The reliability of criteria for diagnosis and differential diagnosis of Marek's disease and lymphoid leucosis of fowl, as part of herd diagnosis and on the basis of pathologico-anatomic findings as well as of pathomorphological and statistical tests has been checked for treatment in this paper. Tumorous lesions were macroscopically and histologically recorded from 1,261 hens of various industrialised poultry units. Marek's disease was established in 316 cases, lymphoid leucosis in 828 cases, mixed manifestations in 33 cases, and other neoplasias in 84 cases. These animals, together with 591 hens in which Marek's disease had been experimentally induced, were evaluated by pathologico-anatomic criteria of differential diagnosis. The conclusion was drawn that differential diagnosis of Marek's disease and lymphoid leucosis on the basis of pathologico-anatomic findings, without histological testing, was sufficiently reliable, when undertaken as part of herd diagnosis monitoring. The expected small number of false diagnoses will be unimportant to herd diagnosis. Both diagnosis and differential diagnosis will be conducted by the following criteria: age of animal, alterations caused by Marek's disease to nerves, eyes, and skin, tumour growth in bursa fabricii caused by lymphoid leucosis, atrophy of bursa fabricii caused by Marek's disease, as well as differences between both diseases regarding tumour development in other organs, with due consideration to be given to the number of affected organs, as a whole, and to pathologico-anatomic organic changes caused by tumour growth.

23 chickens affected with Marek's disease were tested by the allergic skin reaction for delayed hypersensitivity against myelin and myelin preparations of peripheral nerves. 19 animals proofed to be positive in the skin test, yet there was no correlation with the intensity of the nerve affections. 4 animals with severe mononuclear neural infiltrations didn't show any allergic skin reaction. It could be demonstrated that in Marek's disease lymphocytes are sensitized against components of the peripheral myelin. Thereafter, Marek's disease stands in analogy to the Guillain-Barré syndrome of man, as well as to experimental allergic neuritis.