These specialized organs are an integral component of the defense system that protects the animal against infection by producing antibodies and specialized cells which attack bacteria or viruses. Lymphoid cells are also found in other organ systems and circulating in the blood. Enlargement of the external or superficial lymph nodes is common, but internal nodes may also be enlarged in the absence of external involvement.
Tumors often invade the gastrointestinal tract, particularly the abomasum.
Ulcers or GI obstructions may result from abomasal tumors, leading to reduced feed intake, weight loss and poor milk production. Signs of heart failure may result from tumors invading heart muscle. 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.