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.