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Avian leukemia virus (ALV) is an RNA virus belonging to the Alphatrovirus genus in the family Retroviridae. It has reverse transcription properties and can integrate genetic material into the host genome. Virus particles are temperature sensitive and remain stable within the pH range of 5-9. This virus is native to China and can be divided into 10 subgroups, including A to J, based on the host range. Among them, subgroups A, B, and J are common exogenous viruses, while subgroup E is an endogenous virus. The virus integrates the host genome through long terminal repeat (LTR) sequences to induce tumors, mainly causing diseases such as avian lymphocytic leukemia and hemangioma, with a latent period of up to 14 weeks or more. The virus spreads vertically through hatching eggs and horizontally during the incubation period, and the semen of roosters can also transmit pathogens. Diagnosis is performed using ELISA to detect p27 antigen, PCR to detect viral nucleic acid, and immunofluorescence test.
Figure 1. Structure of Avian Leukosis Virus. (Feng, Min,et al, 2016)
Avian leukemia is a collective term for avian neoplastic diseases caused by avian C-type retroviruses, and belongs to three types of infectious diseases. Mainly including lymphocytic leukemia, erythroblastic leukemia, and myeloid leukemia, it can also cause myeloma, connective tissue tumors, epithelial tumors, and endothelial tumors, with the vast majority of cases presenting as hematopoietic system tumors. Among them, J subgroup avian leukemia is classified as a second-class animal disease. The pathogen is avian leukosis virus (ALV), which is mainly transmitted naturally in chickens through vertical transmission. Different virus subgroups (such as A, B, J) have specific host ranges and epidemiological characteristics. The virus can reproduce in chicken embryo fibroblasts, clinically manifested as hepatosplenomegaly, myeloma, and other visceral tumors. Diagnostic criteria include epidemiological characteristics, pathological examination, and laboratory testing techniques (such as virus isolation, ELISA antibody detection).
The table below summarizes key host proto-oncogenes targeted by ALV insertional mutagenesis and the associated diseases:
| ALV Subgroup / Strain | Target Host Gene / Pathway | Mechanism of Activation | Resulting Neoplasm / Disease |
| Slowly Transforming ALVs (e.g., ALV-A, B, J) | c-myc (Avian Myelocytomatosis Virus oncogene cellular homologue) | Promoter Insertion: Viral LTR drives constitutive, high-level expression of c-myc, a transcription factor regulating cell growth and proliferation. | B-cell Lymphomas (Lymphoid Leukosis) – The most common ALV-induced tumor. |
| Acutely Transforming ALV Strains (carry v-onc) | v-src (Rous Sarcoma Virus) | Encodes a constitutively active tyrosine kinase (pp60v-src), hijacking multiple signaling pathways (MAPK, PI3K). | Sarcomas. |
| ALV-J (Subgroup J) | *c-*Jun (AP-1 transcription factor) | Integration and activation of this proto-oncogene is associated with the unique pathogenesis of ALV-J. | Myeloid Leukosis – A distinctive tumor type predominant in meat-type chickens caused by ALV-J. |
Avian leukemia virus belongs to the avian C-type retrovirus group of the retrovirus family. Avian leukemia virus is closely related to sarcoma virus, and is therefore collectively referred to as avian leukemia/sarcoma virus. The core of this group of viruses has a high electron density with an internal diameter of about 35-45nm, and is surrounded by a middle and outer membrane. The entire virus has a diameter of 80-120nm, with an average of 90nm. Most strains of avian leukemia virus can grow well in 11-12-day-old chicken embryos and can produce proliferative acne spots on the trophoblast membrane. Vaccination of susceptible chicks aged 1-14 days through intraperitoneal or other routes can cause disease in chickens. Most avian leukemia viruses can grow in chicken embryo fibroblast culture and usually do not produce any obvious cellular lesions, but the presence of the virus can be checked using resistance inducible factor assay (RIF).
Avian leukemia can only be infected in chickens under natural conditions. Rous sarcoma virus has the widest host range, and artificial inoculation in wild chickens, guinea pigs, pigeons, quails, turkeys, and partridges can also cause tumors. The resistance of different breeds or strains of chickens to viral infections and tumor development varies greatly. The susceptibility of hens is higher than that of roosters, and it mostly occurs in chickens over 18 weeks of age, presenting as a chronic process with a mortality rate of 5% to 6%.
| Diagnosis | Details |
| Animal transmission | The sources of infection are diseased chickens and infected chickens. Hens with viremia have virus reproduction throughout their reproductive system, with the highest concentration of virus in the fallopian tubes, especially in the protein secretion part. Therefore, the eggs they produce often carry the virus, and the hatched chicks also carry the virus. Chickens with congenital infections often exhibit immune tolerance, as they do not produce anti-tumor virus antibodies and carry the virus for a long time to detoxify, becoming an important source of infection. |
| Natural transmission | Under natural conditions, this disease mainly spreads vertically and can also spread horizontally, but it is relatively slow, and in most cases, contact transmission is considered unimportant. Although the infection of this disease is widespread, the incidence of clinical cases is quite low, usually sporadic. Factors such as vitamin deficiency and endocrine disorders in feed can promote the occurrence of this disease. |
Avian leukemia has different symptoms and pathological characteristics due to the different strains of infection.
1. Lymphocytic leukemia
Lymphocytic leukemia is the most common type of disease. Chickens under 14 weeks of age are extremely rare, and they begin to get sick after 14 weeks of age, with the highest incidence rate at the sexual maturity stage. The sick chicken is mentally lethargic, with overall weakness, progressive emaciation and anemia. The comb and beard of the chicken are pale, wrinkled, and occasionally cyanotic. Sick chickens experience reduced or absent appetite, diarrhea, and cessation of egg production. The abdomen often shows obvious swelling, and the swollen liver can be felt by pressing it with the hand. Finally, the sick chicken dies of exhaustion.
2. Erythroblastic leukemia
Erythroblastic leukemia is relatively rare. Usually occurs in high-yielding chickens over 6 weeks of age. Clinically, there are two types of diseases: proliferative and anemic. Hyperplasia type is more common, characterized by the presence of a large number of red blood cells in the blood, while anemia type has only a small number of immature cells in the blood.
3. Myeloid leukemia
This type rarely occurs naturally. Its clinical manifestations include drowsiness, anemia, weight loss, hair follicle bleeding, and a longer course of disease than erythrocytic leukemia. During the autopsy, the bone marrow was found to be solid, ranging from red gray to gray in color. Occasionally, gray diffuse tumors can also occur in other internal organs of the liver. Histological examination showed a large accumulation of myeloid cells inside and outside the blood vessels. A large number of myeloid cells often appear in peripheral blood, accounting for 75% of all blood tissue.
In practical diagnosis, the diagnosis is often made by combining hematological examination and pathological features with pathogen and antibody testing. Red blood cell leukemia can be seen in peripheral blood, liver, and bone marrow smears with a large number of red blood cells, and the liver and bone marrow appear cherry red. Myeloid leukemia accumulates myeloid cells both inside and outside blood vessels, with the liver appearing pale red and the bone marrow appearing white. Lymphocytic leukemia should be distinguished from Marek's disease (see Marek's disease for details). But the isolation of pathogens and detection of antibodies are important means to establish a population of leukemia free chickens.
Reference
| Target | Cat. No. | Product Name | Host | |
| Avian Leukosis Virus | DAG-WT1122 | Recombinant Avian Leukosis Virus p27 | E. coli | Inquiry |
| DAGC532 | Avian Leukosis Virus Subgroup J (Hc-1) Tissue Culture | N/A | Inquiry |
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