Pathological Studies of Marek's Disease in Laying Hens

Abstract

The results of pathological studies including clinical signs and mortality rate of Marek's disease in laying hens were as follo~vs, the clinical signs of the disease in laying hens were categorised into 2 forms, one had clinical signs of depression and sudden death and some were found dead without clinical signs- The other group ].lad clinical signs of depression, lameness and emaciation and some showed signs of posterior paralysis, drooping wings and alopecia. The mortality rate in this outbreak was 48.5%. Seventeen chickens with nervous signs randomly selected for post-mortem examination showed the gross findings of nerve enlargement and tumourous masses in visceral organs, muscles and skin. Microscopic findings included pleomorphic lymphoid infiltration in nerves and visceral organs and lymphocytic perivascular cuffing in brain. These gross and microscopic findings were identical to classical form of Marek's disease. The chicken with history of sudden death had no gross and microscopic findings in nerves and brain but only tumourous masses containing pleomorphic lymphoid cells could be observed in visceral organs, muscles and skin. This form of the disease was characterised as acute form of Marek's disease. Methyl green-pyronin staining was very helpful guide for differential diagnosis of acute form of Marek's disease and lymphoid leukosis. Ocular form of the disease was not observed in this study.