Retrobulbar tumors, masses behind the eye but within the orbit, occur frequently and cause the eye to protrude. Bovine leukemia virus can be identified in many cattle herds throughout the United States. The percentage of infected cattle within herds ranged from 0 to more than 50 percent for dairy and from 0 to 20 percent for beef. The percentage of herds infected with BLV varies from state to state. Bovine leukemia virus targets lymphatic tissues.
Lymphocytes make up one of the classes of white blood cells.
Less than 1 percent of BLV-infected cattle will develop lymphosarcoma. Approximately one third of cattle with BLV infection develop persistent lymphocytosis, an increase in the number of lymphocytes in circulation lasting from months to years. Animals with lymphocytosis and most animals that become infected with BLV do not develop clinical illness. In these animals, milk production and fertility are not adversely affected. Whether or not EBL results from BLV infection, this virus is maintained in lymphocytes for the life of an infected animal.