TME is classified as a transmissible spongiform encephalopathy (TSE). Different TSE's &e caused by similar as-yet uncharacterized agents that produce spongiform changes in the brain. Other TSE's include scrapie, which affects sheep and goats; bovine spongiform encephalopathy (BSE); feline spongiform encephalopathy; chronic wasting disease of deer and elk; and five rare diseases in humans, kuru, both classical and variant Creutzfeldt-iakob disease (CM)), Gerstmann-Straussler-Scheinker syndrome, and fatal familial insomnia.
TSE9s have also been reported in Europe in captive wild ruminants, cats, and monkeys. The occurrence of TSE's in captive wild animals is believed to have resulted from BSE-contaminated feed. These rare, progressively degenerative central nervous system diseases are characterized by a very long incubation period, a short clinical course, and a 100 percent mortality rate.
The infectious agent responsible for TME is smaller than the smallest known virus and has not been characterized to date. There are three main theories on the nature of this agent: (1) the agent is a virus with unusual characteristics, (2) the agent is a prion6an exclusively host-coded protein that is modified to a protease-resistant form after infection, or (3) the agent is a virinoda small, noncoding regulatory nucleic acid coated with a host-derived protective protein.
The TME agent is extremely resistant to heat and to normal sterilization processes. It also does not evoke any detectable immune response or inflammatory reaction in host animals. Public interest surrounding TSE's soared when the United Kingdom announced in March 1996 that BSE may be linked to a variant form of CE). The fear of this disease prompted countries around the world to step up measures to ensure that they remain free of BSE.
BSE has not been detected in the United States, and the U.S. Department of Agriculture (USDA) works proactively to keep it that way. USDA's Animal and Plant Health Inspection Service (APHIS) and Food Safety and Inspection Service take aggressive measures in prevention, education, surveillance, and response.