Some fatal degenerative encephalopathies of mammals form a distinct group because they are caused by unconventional but uncharacterized transmissible agents that evoke no classical iminune response in the affected host. The animal diseases are bovine spongiform encephalopathy (BSE), chronic wasting disease of elk and mule-deer, feline spongiform encephalopathy (FSE) of the domestic cat, scrapie in sheep and goats, and transmissible mink encephalopathy (TME). A number of exotic ruminant and felid species maintained in, or originating from, zoological collections in the United Kingdom have also developed fatal encephalopathies that, like FSE, are considered to have been caused by BSE agent; the affected species are cheetah, eland, gemsbok, kudu, nyala, ocelot, oryx, and puma. The human diseases are Creutzfeldt-Jakob disease (CJD), fatal familial insomnia (FFI), Gerstmann-Straussler-Scheinker syndrome (GSS), and kuru.