A complex of several related and unrelated viruses (both C-type retroviruses and herpesviruses) that are collectively responsible for a variety of benign and malignant neoplasms in chickens and, to a lesser extent, in other avian species. Although most neoplasms observed in avian species are induced by viruses, there are some of unknown etiology.
The neoplastic diseases induced by the leukosis-sarcoma group of retroviruses include lymphoid leukosis, myeloid or erythroid leukemias or solid tumors, tumors of connective tissue origin (for example, sarcomas, fibromas, and chondromas), epithelial carcinomas, and endothelial tumors. The many viral strains involved have similar physical and chemical characteristics and share a group-specific antigen; some can cause more than one type of neoplasm. The viruses are about 100 nanometers in diameter; have a core composed of ribonucleic acid; contain a reverse transcriptase; mature by budding from the cell membrane; and are divided into subgroups based on envelope glycoproteins. Some strains carry their own specific oncogenes that induce neoplasms within days or weeks. Others lack an oncogene and cause neoplasms less frequently and only after several months, probably by activating a specific cellular oncogene. Cancer (medicine) Oncology
Lymphoid leukosis is the most important of the leukosis sarcoma diseases. The lymphoid leukosis virus is transmitted vertically from hen to chick through the egg. Infection can result in leukotic neoplasms in various visceral organs following metastasis from primary tumors in the bursa of Fabricius. Large-scale transmission of the lymphoid leukosis virus can be eradicated by eliminating individual infected breeders.
Reticuloendotheliosis virus strains constitute another retrovirus group, unrelated to the lymphoid-sarcoma group, and also may carry a specific oncogene. They can cause a chronic neoplastic form of reticuloendotheliosis or other neoplasms in turkeys, chickens, ducks, geese, quail, and pheasants, and a runting disease has been seen in chickens after accidental contamination of vaccines with reticuloendotheliosis virus.
Marek's disease in chickens is caused by an oncogenic, cell-associated, lymphotrophic, highly contagious herpesvirus. Inhalation of the virus causes an active infection in lymphoid organs; after about 1 week, a latent infection develops in lymphocytes. T-cell lymphomas may develop within a few weeks or months, depending on age, genetic makeup, virus virulence, and other factors. Degenerative, inflammatory and lymphoproliferative lesions occur principally in the peripheral nerves (causing paralysis), lymphoid tissues, visceral organs, muscle, and skin. Eye involvement (gray eye) can cause blindness. The disease is of great economic importance in chickens, and several vaccines, injected at 1 day of age, have been in worldwide use since about 1970. Animal virus Tumor viruses