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 disease, caused by a herpesvirus, and avian leukosis/sarcoma and reticuloendotheliosis, 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), multicentric histiocytosis, and adenocarcinoma are discussed here.