An epizootic of subcutaneous tumors associated with subgroup A avian leukosis/sarcoma virus in young layer chickens.

An outbreak of subcutaneous tumors in young layer chickens in a flock in Japan was investigated. Tumors appeared as extensive swelling or bulbous protrusions of the integument and were observed in the head or wing of chickens approximately 9 wk old, with a prevalence of 0.4% (157 of 42,000) in the affected flock. Histologically, two types of tumor were observed: myxoma containing abundant hyaluronic acid and neurofibroma with hyperplasia of the Herbst corpuscles. Ultrastructurally, type C retroviruses, such as viral particles, were found in the tumors. The tumors were specifically stained by immunohistochemistry using monoclonal antibodies against the subgroup A avian leukosis/sarcoma virus (ALSV) and yielded a positive reaction to primers specific for subgroup A ALSV by reverse transcriptase-polymerase chain reaction assay. The virus was isolated from the tumors. Seventeen of 20 clinically normal chickens in the affected flock showed antibodies against ALSV. These results suggest that subcutaneous tumors are associated with subgroup A ALSV infection.