Persistent lymphocytosis in cattle: its cause, nature and relation to lymphosarcoma.

Studies in well-characterized cattle populations strongly support the view that the bovine leukemia virus (BLV) is the causative agent of the adult (enzootic) form of bovine lymphosarcoma and persistent lymphocytosis (PL), and that host genetic factors play an important role in the development of these two conditions. On the other hand, the available information indicates that the genetic factors controlling the development of PL are frequently independent of those controlling the development of lymphosarcoma. There is no evidence that clinically normal cattle with PL harbor malignant cells or have any other clinical abnormality. In these animals lymphocytosis results from the expansion of two distinct subpopulations of non-neoplastic B lymphocytes, one of which is free of BLV. Long-term studies have shown that the large majority of cattle with PL do not develop lymphosarcoma even when kept to advanced age. These data indicate that PL is not a disease nor a preclinical stage of lymphosarcoma. Rather PL should be considered as a benign response to BLV infection which, although frequently associated with lymphosarcoma, is independant of it.