Avian leukosis is a group of leukemia-like, tumor-forming diseases caused by avian leukosis virus (ALV). There are 10 identified subgroups of ALV (A–J), of which six affect chickens. Subgroups A and B are the most common, particularly in egg-laying hens, followed by subgroup J, which is seen in both broilers and layers. Subgroup E is widespread in non-commercial flocks but is not considered oncogenic (tumor-causing).
Avian leukosis can present in several forms:
- Lymphoid leukosis: This is the most common form of cancer associated with ALV and typically affects chickens older than four months of age. Tumors most often develop in the liver, spleen, and bursa of Fabricius, though they may also occur in the kidneys, lungs, gonads, heart, mesentery, and bone marrow. Tumors may appear nodular, diffuse, or miliary, and the bursa is almost always involved. Microscopically, tumors are composed of uniform large lymphocytes (lymphoblasts), with characteristic involvement of the bursa and nodular growth in organs such as the liver and spleen.
- Erythroblastosis (Erythroid leukosis): This form is an intravascular leukemia affecting red blood cell precursors and can occur in birds younger than four months, including chicks as young as five weeks. Affected birds are often anemic and may show muscle hemorrhages, as well as occasional abdominal bleeding or liver rupture.
- Myeloid leukosis: Most commonly associated with subgroup J, this form includes two overlapping types: myeloblastosis and myelocytomatosis. Myelocytomatosis is characterized by multiple tumor masses (myelocytomas), often found on the shanks, head, oral cavity, trachea, and around the eyes. These tumors are typically soft, nodular, and creamy in appearance. While most common in adult birds, cases can occur in chickens as young as five weeks. Both forms frequently involve leukemia, with the bone marrow replaced by neoplastic myeloid cells.
- Avian Osteopetrosis (marble bone disease): This condition affects bone development by altering osteoblast function, leading to thickening of the long bones, particularly in the legs and, less commonly, the wings.
In addition to these forms, ALV has been associated with a variety of other tumors, including hemangiomas, histiocytic sarcomas, nephroblastomas, fibrosarcomas, chondromas, osteomas, myxomas, adenocarcinomas, and mesotheliomas. These may occur alone or alongside leukosis.
Clinical Signs
Clinical signs vary depending on the form of the disease and the location of tumors but are often nonspecific. Common signs include decreased appetite, diarrhea, dehydration, weight loss, and weakness. Some birds may develop abdominal enlargement or exhibit a pale, shrunken, or occasionally bluish (cyanotic) comb. Once clinical signs appear, the disease typically progresses rapidly, with death occurring within a few weeks.
Marek's disease is another common tumor-causing viral disease in chickens that can resemble avian leukosis. However, Marek’s disease usually affects birds younger than four months, while avian leukosis is more commonly seen in older chickens.
Transmission
Avian leukosis virus is spread both horizontally (through direct or indirect contact between birds) and vertically (from hen to chick through the egg). Infected birds shed the virus in feces, saliva, skin, and feather debris, which can infect other birds through close contact. The virus does not survive long outside the host.
Diagnosis
Definitive diagnosis is typically achieved through virus isolation, considered the gold standard. Common sample types include blood, plasma, serum, cloacal or vaginal swabs, oral washings, egg albumen, embryos, and tumor tissue. ELISA testing is also widely used for detection.
While the gross appearance of tumors may suggest leukosis, histopathological examination is essential for a definitive diagnosis. Key tissues for evaluation include the liver, spleen, bursa of Fabricius, bone marrow, and peripheral nerves.
