Introduction and History

Abstract

Retroviruses have long been known to cause a variety of naturally occurring disorders in several animal species (Gross 1970). A synopsis of the most important of these diseases is given in Table 1. They can be categorized into (a) diseases characterized by uncontrolled growth of cells of various types and origins (leukemias, lymphomas, erythroblastosis, sarcomas, carcinomas); (b) diseases characterized by the loss of certain cell types (immunodeficiences, anemias, chronic degenerative disorders of the central nervous system, CNS); and (c) disorders in which signs and symptoms of inflammation and autoimmunity prevail (arthritis, mastitis, pneumonia, encephalitis). Some of these viruses are capable of inducing several well-defined disease entities. Avian leukosis virus (ALV), for example, though most frequently causing leukosis, may also produce erythroblastosis, anemia, or even fibrosarcoma. Murine leukemia virus (MuLV) may induce not only malignant tumors of various cellular origins, but also autoimmune disorders similar to human systemic lupus erythematosus, or CNS disorder. In some cases, the age at the time of infection is crucial for disease manifestation: caprine arthritis-encephalitis virus (CAEV) causes fulminant encephalitis in young kids, in older animals it usually causes chronic arthritis. Some viruses may infect more than one host species. Bovine leukemia virus (BLV) causes B-cell lymphoma (leukosis) in cattle, T-cell lymphoma in experimentally infected sheep, and immunodeficiency in experimentally infected rabbits (A. Burny, personal communication).