the brains of affected individuals. PrPScis a modified version of the
protein PrPCfound normally on the outer surface of neurons. Differ-
ences in its tertiary structure make it resistant to proteolytic degradation
and removal when its useful life is over. It is thought that prion PrPSc
finds its way to the neuron surface where its interaction with PrPCleads
to production of further PrPSc, a process known as ‘recruitment’. This
initiates a chain reaction, the accumulation of PrPSc, plaque formation,
and the onset of neurological symptoms.
Four human SEs are known, Gerstmann–Stra ̈ussler–Scheinker syn-
drome, Fatal familial insomnia, Creutzfeldt–Jacob disease and kuru,
though the last two may in fact be the same disease. Kuru, first described
in 1957, is restricted to the Fore people of Papua New Guinea where it
was the major cause of death among Fore women. It was shown to be
transmissable to chimpanzees by intracerebral inoculation with brain
extracts from dead patients and it was eventually decided that human
transmission was in fact foodborne, albeit of a rather special kind. The
Fore tribe have a tradition of cannibalism and it was the tribal custom
for women and children to eat the brains of the dead as a mark of
respect. Since this practice was suppressed in the late fifties incidence of
the disease has declined.
Creutzfeldt–Jacob disease is more widespread than kuru and in the
UK has an incidence of 1 per 2 000 000 inhabitants. Accidental trans-
mission by injection of contaminated pituitary extracts, corneal grafting
and implantation of contaminated electrodes has occurred but oral
transmission has not been described.
Much attention has been focused on these conditions since the
emergence of bovine spongiform encephalopathy (BSE), or mad cow
disease, in cattle in Britain in 1985/6. The illness is thought to result
from the scrapie agent crossing the species barrier and being transmitted
to cattle by scrapie-infected sheep protein fed to cattle. Its emergence at
this time has been linked with changes in the commercial rendering
processes used in the production of animal proteins such as reduced
processing temperatures and abandonment of the use of solvents to
extract tallow. Control measures introduced in the UK include prohi-
bition of the sale of bovine offal from animals more than six months old
and banning the feeding of ruminant-derived protein to ruminants and
the slaughter of all cattle over 30 months of age. In 1996 a ban on the
use of all mammalian-derived animal protein in feed for all farm
animals was introduced.
Incidence of BSE in cattle peaked in 1992 with 36,680 cases and by
2005 the annual total had declined to less than 150 cases. By and large
the epidemic seems to have been confined to the UK as reported cases in
other countries totalled around 600 at the end of 1997, mostly in
Switzerland (256) and Ireland (224). This is a little surprising since
308 Non-bacterial Agents of Foodborne Illness