Diseases Caused by Replication-Competent Avian Retroviruses
Lymphoid leukosis
Lymphoid leukosis (synonymous with visceral lymphomatosis) is the most common form of avian leukosis and occurs in chickens approximately 14–30 weeks of age. Clinical signs are non-specific, but the comb may be pale, shriveled, and occasionally cyanotic, and affected birds may exhibit inappetence, emaciation, weakness, and abdominal swelling. Tumors may be present for some time before clinical illness is recognized, although the course may be rapid from the onset of disease. Often, tumor formation is only detected after slaughter at poultry processing plants, but results in condemnation of the carcass. Hematological changes are variable in infected birds, with overt leukemia rarely observed. Tumors, usually as discrete, nodular lesions, occur first in the cloacal bursa (bursa of Fabricius), with subsequent metastasis to liver, spleen, and other internal organs. These multicentric tumors consist of aggregates of lymphoblasts that express B lymphocyte markers. These cells may secrete large amounts of immunoglobulin M (IgM), but their capacity to differentiate into IgG-, IgA- or IgE-producing cells is arrested. Bursectomy (removal of the cloacal bursa), even up to 5 months of age, abrogates the development of lymphoid leukosis. Tumor induction is typically caused by virus activation and overexpression of the c-myc oncogene, with contributions from Blym-1 and c-bis.