Avian leukosis virus (ALV) is a retrovirus which causes the development of several different kinds of benign and malignant neoplasms (tumors) in chickens worldwide. Although there are seven different subgroups of the virus (A through J), subgroup J is the most significant cause of outbreaks among flocks. It is the most pathogenic and prone to mutate than the other subgroups.
The types of neoplasms caused by ALV-J vary and can be influenced by the specific strain of ALV-J that has infected the bird. Some of the most common neoplasms associated with ALV-J include:
- Hemangiomas: Hemangiomas are vascular tumors found in the skin or visceral organs that originate from endothelial cells that line blood vessels.
- Lymphoid leukosis: Lymphoid leukosis is caused by an avian retrovirus that induces a lymphoblastic lymphosarcoma in susceptible chickens, four months of age or older.
- Erythroblastosis (Erythroid leukosis): Erythroblastosis an intravascular erythroblastic leukemia which occurs in chickens between 3 and 6 months of age. Infected chickens are often anemic, with muscle hemorrhages and occasionally abdominal hemorrhage and ruptured liver.
- Myeloblastosis (myeloblastic myeloid leukosis): Myeloblastosis is sporadically reported in adult chickens.
- Myelocytomatosis (myelocytic myeloid leukosis): Myelocytomatosis causes tumors to develop on the surface of the bones in association with the periosteum and near cartilage, although any tissue or organ can be affected. They may also less commonly develop in the chicken’s oral cavity, on the trachea, and in and around the eye. The tumors are usually nodular and multiple, with a soft, friable consistency and of creamy color.
- Fibrosarcomas: Fibrosarcomas are locally invasive tumors, with irregular and indistinct borders, and are frequently ulcerated with skin attached to the mass.
- Histiocytic sarcomas: Histiocytic sarcomass are characterized by the proliferation of malignant cells that have the morphological and immunohistochemical characteristics of mature tissue histiocytes.
- Intestinal adenocarcinoma: A type of cancer that starts in the mucus-producing glandular cells of the chicken's intestine.
Since ALV is widespread among chickens, virus isolation and the demonstration of antigen or antibody have limited or no value in diagnosing field cases of lymphomas. However, assays for the detection of ALV are useful in the identification and classification of new isolates, safety testing of vaccines, and testing pathogen-free and other breeder flocks for freedom of the infection. Virus isolation is generally the ideal detection method (so-called gold standard). Samples most commonly used for detection of ALV include blood, plasma, serum, meconium, cloacal and vaginal swabs, oral washings, egg albumen, embryos, and tumors.
Chickens can become infected vertically (through the egg from parents to chick), or horizontally (through contamination of the environment with feces or skin dander, direct contact with respiratory secretions or skin). Chickens can also become infected by receiving the Marek's disease virus vaccine or fowl pox vaccine, which are frequently contaminated.
The incubation period from time of infection with ALV to time of clinical signs ranges from a few weeks to several months.