Observations on the pathology of non-alimentary lymphomas in the horse.

A retrospective study was carried out on post-mortem and biopsy tissue samples from 26 horses with non-alimentary lymphoma. On the basis of their histopathology and cytology, the cases were grouped into several categories: (1) Lymphoblastic lymphomas (6 cases) and primary lymphoblastic leukaemia (2 cases). (2) Histiolymphocytic lymphomas involving the skin (6 cases) or lymph nodes (3 cases). (3) Lymphomas showing follicular development (4 cases). (4) Plasmacytic lymphomas (4 cases). (5) Lymphocytic lymphoma (1 case). Most affected horses were middle-aged or old but lymphoblastic neoplasms tended to affect younger horses than other forms. The course of the disease was highly variable. The most rapidly progressive forms were of lymphoblastic cytology, whereas some cases with histiolymphocytic skin nodules appeared to be only slowly progressive. Further information is needed on the pathology and clinical behaviour of some types of lymphoma, particularly those manifesting in the skin. Elucidation of the histogenetic relationships of the different cytological types of lymphoma will involve the development of immunohistochemical and other techniques which are specifically applicable to the horse.