Outbreaks of neoplastic disease defined as haemangiosarcoma occurred among layer flocks of chickens in Israel. The disease caused bleeding tumours in the skin and internal organs of young layers, followed by anaemia, cessation of egg production and high mortality up to 20%.

Avian leukosis virus was isolated from tumour cells which contain several viral DNA copies integrated in the cell genome. The isolated virus induced haemangiosarcomas in more than 30% of birds inoculated on the day of hatching. Congenital transmission of viruses from tumour-bearing hens to their offspring was observed.