Slow Diseases Caused by Prions

Slow Diseases Caused by Prions

The agents causing slow diseases were earlier called as slow virus, infectious protein, infectious amyloids, or crystal protein. These agents are now being named prions, and this term has gained wide acceptance, replacing all the previously used terms. The prions are unconventional agents that are not viruses.

      The diseases caused by prions are a large group of related neurodegenerative conditions, which affect both humans and animals. These diseases belong to a family of diseases known as the transmissible spongiform encephalopathies (TSEs).

There are six human TSEs caused by prions: (a) kuru, (b) Creutzfeldt–Jakob disease (CJD), (c) variant CJD (vCJD), (d) Gerstmann–Straussler–Scheinker (GSS) syndrome, (e) fatal familial insomnia (FFI), and (f) sporadic fatal insomnia.

Animal TSEs caused by prions include (a) scrapie and visna (diseases of sheep), (b) bovine spongiform encephalopathy (BSE; mad cow disease), (c) transmissible mink encephalopathy, and (d) chronic wasting disease of deer, mule, and elk.

Transmissible spongiform encephalopathies in humans caused by the prions show following characteristics:

a)        Long incubation period of several years.

b)       Course of illness lasting for months to years.

c)        A progressive debilitating neurological syndrome that is invariably fatal.

d)       Associated with pathological changes typically restricted to the central nervous system (CNS).

e)        Absence of specific immunological response in the host.

f)         The agents are resistant to conventionally used inactiva-tion methods.

History

Kuru was the first human disease known to be caused by a slow virus, or prion (1959). This condition was found among “Fore” people, a tribe living in the remote highlands of New Guinea. This virus was linked to ritualistic cannibalism prevalent among these tribal people. Hallow (1959) suggested that kuru could be a possible human form of scrapie, a slow viral disease seen in sheep. Gajdusek and his colleagues (1966) first demon-strated that kuru was transferable to chimpanzee after a long period of incubation, for which they were awarded the Nobel Prize. Since then many TSEs, linked to slow viruses, are being described both in humans and animals.