Depending on whether the etiologic agent is known, neoplasms of poultry are divided into two main categories: virus-induced neoplasms and neoplasms of unknown etiology. There are three economically important virus-induced neoplastic diseases of poultry: Marek's disease Marek's Disease in Poultry Marek disease is a highly contagious viral disease of poultry characterized by T-cell lymphomas and peripheral nerve enlargement. Standard criteria used for diagnosis include history, clinical... read more 
, caused by a herpesvirus, and avian leukosis/sarcoma Lymphoid Leukosis in Poultry Lymphoid leukosis is a neoplastic disease of poultry caused by avian leukosis virus. The disease is characterized by B-cell lymphoma, occurring in chickens approximately 16 weeks of age and... read more 
and reticuloendotheliosis Reticuloendotheliosis in Poultry Reticuloendotheliosis is a neoplastic disease of poultry caused by reticuloendotheliosis virus. Reticuloendotheliosis bursal lymphomas are virtually identical to lymphoid leukosis B-cell lymphomas... read more , caused by retroviruses. Although these neoplastic diseases cause economic losses from tumor mortality and poor performance, some of them have served as highly suitable models to study neoplasia.
A rare neoplastic disease of turkeys known as lymphoproliferative disease that has been reported in Europe and Israel is induced by a retrovirus that is distinct from both the leukosis/sarcoma and reticuloendotheliosis viruses. Although reports suggest that lymphoproliferative disease has recently been detected by PCR in a small number of wild turkeys in the USA, incidence of the disease has always been sporadic and therefore it is not discussed here.
Neoplasms of unknown etiology are classified according to their morphologic characteristics; they include a wide variety of benign and malignant neoplasms. Of these tumors, only dermal squamous cell carcinoma (avian keratoacanthoma Avian Keratoacanthoma Of the numerous tumors of unknown etiology in poultry, dermal squamous cell carcinoma (avian keratoacanthoma), multicentric histiocytosis, and adenocarcinomas are the most common but appear... read more ), multicentric histiocytosis Multicentric Histiocytosis Of the numerous tumors of unknown etiology in poultry, dermal squamous cell carcinoma (avian keratoacanthoma), multicentric histiocytosis, and adenocarcinomas are the most common but appear... read more , and adenocarcinoma Adenocarcinomas Of the numerous tumors of unknown etiology in poultry, dermal squamous cell carcinoma (avian keratoacanthoma), multicentric histiocytosis, and adenocarcinomas are the most common but appear... read more are discussed here.
