The study aimed to identify some pathological characteristics of the Marek disease in the Ri chicken breed. Ri chickens raised at Lien Ninh breeding chicken farm, Thanh Tri district, Ha Noi city. The studied results showed that the Marek infection age of chicken was from 3.5 to 10 months of age, the most infection age was 6-7 months of age. The disease infection rate was from 0.56 to 5.79 percent of the herd total and from 33.33 to 76.32 percent of the number of necropsy
The low mortality rate was (0.37 to 3.99 percent) and the high mortality was (40 to 68.97 percent). The main clinical symptoms were tumors on the skin, pale crest, limp and thin body. The paralytic symptom was rarely appeared but very typical. The main gross lesions were the appearance of tumors in two forms, diffusion and particle coalescence at the organs: liver (91.36 percent), skin (77.78 percent), spleen (72.84 percent), kidney (58.02 percent), gland stomach (49.38 percent), colon (27.16 percent), heart (18.52 percent), ovary (9.88 percent), lung (8.64 percent), muscle (3.70 percent), and nerves (1.23 percent). The main histological lesions were high infiltration of polymorphism lymphocytes in nervous tissue, skin and internal organs: liver, spleen, kidneys, lungs, heart, gland stomach, muscle stomach, ovary, pancreas, intestine, Fabricius sac and muscles.