A novel spontaneous cytotoxic T-cell lymphoma in chickens

Chicken lymphomas are generally classified into three virus-induced tumors: Marek's disease (MD), lymphoid leukosis, and reticuloendotheliosis, with MD being the most common T-cell lymphoma in commercial poultry. In this study, we describe 23 cases of a novel cytotoxic T-cell lymphoma affecting layer chickens aged >1 year in Japan that is distinct from previously known chicken lymphomas. These cases were initially misidentified as MD during routine poultry inspection but were later confirmed as a distinct and novel form of lymphoma. The average incidence of this lymphoma was 0.016%. Histological analysis revealed that the tumors comprised small uniformly round cells that were CD3⁺, CD4^-, and CD8⁺, indicating a cytotoxic T-cell origin. Investigations using immunohistochemistry, <i>in situ</i> hybridization, and polymerase chain reaction ruled out the involvement of known tumor-inducing viruses in the development of these novel lymphomas. These findings confirm the existence of a novel lymphoma type in chickens and provide key histopathological and epidemiological data to aid in definitive diagnosis.