Avian leukosis is a group of tumor-forming diseases in chickens caused by the avian leukosis virus (ALV), a retrovirus. ALV is classified as either endogenous or exogenous based on its mode of transmission and is divided into 11 subgroups (A–K). Chickens are primarily affected by subgroups A–E, J, and K. Most subgroups are exogenous, meaning they are transmitted between birds, while ALV-E is endogenous and typically has little to no pathogenic effect.
Different subgroups are associated with specific disease manifestations. ALV-A and ALV-B are commonly linked to lymphoid tumors, ALV-K is mainly associated with immunosuppression, and ALV-J causes malignant proliferation of hematopoietic cells, leading to myeloid leukosis, hemangiomas, and severe immunosuppression.
Avian leukosis can present in several forms:
- Lymphoid leukosis: The most common form, typically affecting chickens older than four months. Tumors most frequently develop in the liver, spleen, and bursa of Fabricius, though other organs such as the kidneys, lungs, heart, gonads, mesentery, and bone marrow may also be involved. Tumors may appear nodular, diffuse, or miliary. Microscopically, they consist of a uniform population of large lymphocytes (lymphoblasts), with consistent involvement of the bursa.
- Erythroblastosis (erythroid leukosis): An intravascular leukemia affecting red blood cell precursors. It can occur in young birds, including chicks as young as five weeks. Affected birds are often anemic and may exhibit muscle hemorrhages, internal bleeding, or liver rupture.
- Myeloid leukosis: Most often associated with subgroup J, this form includes myeloblastosis and myelocytomatosis. Myelocytomatosis is characterized by soft, nodular, cream-colored tumors (myelocytomas) that commonly appear on the shanks, head, oral cavity, trachea, and around the eyes. Although more common in adults, it can occur in young birds. Both forms frequently involve bone marrow replacement by neoplastic myeloid cells.
- Avian osteopetrosis (marble bone disease): A condition affecting bone development, resulting in thickened long bones, particularly in the legs and occasionally the wings, due to abnormal osteoblast activity.
In addition to these forms, ALV has been associated with a wide range of tumors, including hemangiomas, histiocytic sarcomas, nephroblastomas, fibrosarcomas, chondromas, osteomas, myxomas, adenocarcinomas, and mesotheliomas. These may occur alone or alongside leukosis.
Transmission
ALV is transmitted both vertically (from hen to chick through the egg) and horizontally (through direct or indirect contact). Infected birds shed the virus in feces, saliva, skin, and feather debris, allowing spread through close contact or contaminated environments. The virus does not survive long outside the host. Hens may also pass both the virus and protective antibodies to their offspring.
Clinical Signs
Clinical signs are often nonspecific and vary depending on the form and tumor location. Common signs include decreased appetite, weight loss, diarrhea, dehydration, weakness, and reduced egg production. Birds may appear pale due to anemia, with a shrunken or occasionally bluish comb. Abdominal enlargement may also occur. Once signs develop, the disease often progresses rapidly, with death occurring within weeks. Avian leukosis can resemble
Marek's disease, another tumor-causing viral condition in chickens. However, Marek’s disease typically affects birds younger than four months, whereas avian leukosis is more common in older chickens.
Diagnosis
ALV can be detected using serologic tests and molecular methods, with virus isolation considered the gold standard. Common sample types include blood, serum, plasma, cloacal or vaginal swabs, oral washings, egg albumen, embryos, and tumor tissues. ELISA testing is also widely used.
While gross lesions may suggest leukosis, definitive diagnosis requires histopathologic examination. Key tissues for evaluation include the liver, spleen, bursa of Fabricius, bone marrow, and peripheral nerves.
Treatment
There is no cure for avian leukosis. Management is primarily supportive and focused on flock health. Some supplements, such as vitamin E, ashwagandha (Withania somnifera), and baicalin, have been studied for their potential immune-supportive effects.
Experimental evidence suggests that lamivudine, an antiviral drug used in human medicine, may reduce viral load and mortality in infected chickens, though its practical use in poultry remains limited.
