Reticuloendotheliosis virus (REV)
Reticuloendotheliosis viruses (REVs) consist of several retroviruses that have been isolated from birds, which originated in mammals. Antibodies to REV are widespread among poultry flocks, however outbreaks occur sporadically and considered to be somewhat rare. Infection is associated with a number of different disease syndromes which include immunosuppression, neoplasia, stunted growth, feather abnormalities ("nakanuke"), and anemia.
The fowlpox virus and Marek's disease vaccines have caused outbreaks of REV in flocks on numerous occasions, due to contamination of the vaccines.