Animal prion diseases

Transmissible spongiform encephalopathies or prion diseases are insidious, degenerative diseases affecting the central nervous system ( CNS ) of man and some animal species, in particular ruminants. Scrapie in sheep and goats, bovine spongiform encephalopathy ( BSE ), transmissible mink encephalopathy and chronic wasting disease in deer are the major members of animal prion diseases. Scrapie is considered to be the origin of other animal prion diseases. Animal feed contaminated with BSE prion caused prion diseases in domestic cat and some wild ruminants, as well as cats kept in zoos. Human infection of BSE was also suspected. Prion diseases occur after long incubation times of months or years, progress sub-acutely, and are always fatal. No gross lesions are observed in animal prion diseases, Histopathological changes, which are characterized by cytoplasmic vacuolations and loss of nerve cells and proliferation of astrocytes but no inflammation, are confined to CNS. The pathogenic agent of prion consists of an abnormal isoform ( PrP sup(Sc) ) of a host protein, prion protein ( PrP ). It is purified from affected animal brains as amyloid filaments named as scrapie associated fibrils. The affected animals do not produce antibodies to prion at any stage of the disease. All animal prion diseases and some human prion diseases are believed to occur by infection of prion, but some human prion diseases, such as Gerstmann-Straeussler syndrome and fatal familial insomnia, are known to be hereditary diseases. Spontaneous production and accumulation of prion occurs in the central nervous system of people with specific mutations in the PrP gene, and induces degeneration of nerve cells. Such prion is also infectious to other animals. Therefore, prion diseases can be defined as the diseases which are neurodegenerative disorders caused by accumulation of prion in CNS,irrespective of the cause of prion production. The PrP gene can exert a major effect on the length of the incubation period and on clinical disease occurrence in sheep scrapie, as in human prion diseaes. Scrapie have been diagnosed by histopathological examination. Recently, however, detection of PrP sup(SC), by immunohistochemistry, Western blot analysis, or enzyme linked immunosorbent assay, has been introduced. Since PrP sup(Sc) accumulates in lympho-reticular tissues in scrapie-infected sheep at a relatively early stage after infection. PrP sup(Sc) detection in the tissuds seems to be promising for diagnosis of scrapie at the preclinical stage.