Pathogenesis

The causative agent of BSE and other TSEs has not been not fully elucidated. Many believe the TSEs are caused by a prion, an abnormal protein with a conformational change that renders it resistant to protease degradation, resulting in its accumulation within neurons (left). Other theories of the causative agent of the TSEs include an unconventional virus and a virino, or "incomplete" virus composed of nucleic acid covered by host protein. Regardless of its identity, the BSE agent causes no detectable immune or inflammatory response in the host, is smaller than most viral particles, has not been identified microscopically, and is highly resistant to heat, UV light, disinfectants and other substances/processes that inactivate most other infectious agents. Research indicates that cooking and irradiation probably do not kill the BSE agent. Cattle are infected by eating feed contaminated with the BSE agent. There is no evidence that BSE spreads horizontally. Initial replication of the BSE agent occurs in the lymphoreticular tissues, and spreads to the central nervous system by neural pathways. The BSE agent has been identified in neural tissues (brain, spinal cord, ganglia, eyes) and lymphoid tissue (tonsils, bone marrow and gut associated lymphoid tissue of the small intestine). To safeguard against the spread of the BSE agent to humans, these tissues are prohibited from entering the human food supply. Additional strategies include education, changes in feeding procedures with prohibition of feeding most mammalian proteins to cattle, and restriction of importation of cattle from BSE infected countries. Since BSE infects the central nervous system, all cattle are inspected for neurological disease prior to slaughter. Those exhibiting neurological disease are condemned and tested for BSE.