Avian sarcoma leukosis virus

Avian leukosis virus (ALV) is a retrovirus that infects chickens and causes a B-cell lymphoma. Some forms are present as an endogenous retrovirus and can spread vertically (parent-to-chick) while others only spread horizontally. Lymphoid leukosis is the most common form of this disease and with typical presentation of gradual onset, persistent low mortality, and neoplasia of the bursa. The disease is also characterized by an enlarged liver due to infiltration of cancerous lymphoid cells. In addition, other abdominal organs and the bursa of Fabricius are often infected. Experimentally it can infect other species of birds and mammals. Cladistically it includes viruses that naturally infect species other than chickens.

It is closely related to the Rous sarcoma virus (RSV), the two being able to recombine with each other and swap genes. The main difference between these two is that RSV carries an src oncogene, granting it the ability to turn fibroblasts malignant, while ALV does not. (This is the species demarcation criterion used by ICTV.) In addition, many strains of RSV lack a env gene and depend on using the env protein from ALV (or an endogenous element derived from ALV env) to reproduce. The two are sometimes lumped together under the name avian sarcoma leukosis virus (ASLV), avian leukosis-sarcoma virus (AL-SV), or loosely the "leukosis/sarcoma group".

ASLV replicates in chicken embryo fibroblasts, the cells that contribute to the formation of connective tissues. Different forms of the disease exist, including lymphoblastic, erythroblastic, and osteopetrotic.