NR ARVJ
AU Roberts,G.W.
TI Furrowed brow over mad cow
QU British Medical Journal 1995 Nov 25; 311(7017): 1419-20
IA http://bmj.bmjjournals.com/cgi/content/full/311/7017/1419/a
PT Article
VT
Anxieties over the possibility that material from or contact with cows infected with bovine spongiform encephalopathy might cause Creutzfeldt-Jakob disease revolve around three questions.
Could it happen?
Bovine spongiform encephalopathy in cows, scrapie in sheep and experimental animals, and Creutzfeldt-Jakob disease are all caused by the same class of pathogen and are more accurately called prion diseases.[i] Human prion disease is rare, but the public health importance of prion disease far exceeds its rarity for the following reasons. It has been transmitted experimentally to various animal species[i,ii] (bovine spongiform encephalopathy has been transmitted experimentally in primates), and accidental person to person transmission has occurred.[iii] In addition, it has been shown that prion disease can be transmitted orally by the ingestion of infected foodstuffs or infected tissues.[iv,v] The available data provide ample evidence to support the theoretical possibility that one of the naturally occurring animal forms of prion disease could be transmitted accidentally, to man by inoculation with, or ingestion of, infected material.
Has it happened?
It is accepted that cows infected with bovine spongiform encephalopathy entered the human food chain before the introduction of controls in 1988 and possibly until the introduction of legislation listing proscribed offal in November 1989. Only one attempt has been made to calculate the volume of infected material that entered the food chain.[vi] Dealer and Kent estimated that about 1.8 million cows incubating bovine spongiform encephalopathy would enter the food chain between 1983 and 1999. The number of people who have ingested this food and thus have had a theoretical exposure to infection is substantial (estimated at 34 million for a dose of 10 cubed infective units, through to 0.4 million at a dose of 10 to power of five units and 0.14 million at a dose of 10 to power nine units). The actual number of infective units required to cause disease in humans is unknown. Of course, these are calculations based on assumptions that are open to argument, and it would be interesting to see a full public discussion of the data. Whether any of this material could have provided sufficient infected material to cause prion disease in humans is unknown.
How will we know if it happens?
Simply put, doctors will see more patients dying of prion disease. The government has decided to monitor new cases of prion disease (Creutzfeldt-Jakob disease) as they arise in the community. Cases must be recognised during life and verified after death.[vii]With this surveillance system the number of cases of Creutzfeldt-Jakob disease in Britain has risen from around 30 cases a year during the 1980s (incidence 0.5 per million per year)[viii] to 54 in 1994 (incidence 0-94).[ix] Such rising figures have increased anxiety.
In reality the increase probably reflects improve ment in diagnostic methods. It has been proposed that the clinical spectrum of prion disease is much wider than that encompassed under Creutzfeldt-Jakob disease and in particular that atypical cases of prion disease (older age of onset, few motor signs, and prominent symptoms of dementia) are more likely to be overlooked in life.[x,xi] Such misdiagnoses have been documented in a large retrospective survey.[xii] It seems that our understanding and monitoring of the epidemiology of prion disease is not as robust as once thought. The interest aroused by media reports of patients with prion disease (be they teenagers or farmers), and any changes in public health practices they stimulate, need to be viewed against this recent exposure of the limitations in the epidemiological metrics traditionally ascribed to Creutzfeldt-Jakob disease. This problem is far from academic as cases of prion disease that slip through the diagnostic net probably have and certainly will represent a source of 'iatrogenic infection. Furthermore, if changing environmental factors (such as material infected with bovine spongiform encephalopathy) lead to a change in the clinical presentation of prion disease the situation may become even more difficult to interpret.
Year of onset No of cases
Number of cases of bovine spongiform encephalopathy in British cattle (data from John Wilesmith, Ministry of Agriculture, Fisheries and Food)
1986 12
1987 461
1988 3072
1989 7627
1990 14371
1991 25644
1992 36924
1993 33574
1994 22699
1995 (Jan-Jun) 7141
i. Prusiner SB, Collinge J, Powell J, Anderton B, eds. Prion diseases in humans and animals. London: Ellis Horwood, 1992.
ii. Schreuder BEC. Animal spongiform encephalopathies - an update. Part 1: scrapie and lesser known animal spongiform encephalopathies. Vet Q 1994;16 174-81.
iii. Brown P, Preece MA, Will RG. 'Friendly fire' in medicine: hormones, homografts and Creutzfeldt-Jakob disease. Lancet 1992-340 24-7.
iv. Gibbs CJ Jr Amyx HL, Bacote A, Masters CL Gajdusek DC. Oral transmission of kuru, Creutzfeldt-Jakob disease and scrapie to non-human primates. J Infect Dis 1980;142 705-8.
v. Barlow RM, Middleton DI. Dietary transmission of bovine spongiform encephalopathy to mice. Vet Rec 1990;126: 111-2 .
vi. Dealer SF, Kent JT. BSE: an update on the statistical evidence. British Food Journal 1995;47 3-19.
vii. Bell JE, Ironside JW. Neuropathology of spongiform encephalopathies in humans. Br Med Bull 1993;49 738-77.
vii. Will RG. Epidemiology of Creutzfeldt-Jakob disease Br Med Bull 1993;49 960-70 .
ix. Delasnerie-Laupretre N, Poser S, Pocchiari M, Wientjens DPWM, Will R. Creutzfeldt-Jakob disease in Europe. Lancet 1995;346:898-9
x. Prion disease: spongiform encephalopathies unveiled. Lancet 1990;336 71-2.
xi. Harrison PJ, Roberts GW. "Life, Jim, But Not as We Know It" Transmissible dementias and the prion protein. Br J Psychiatry 1991;158 457-70.
xii. Bruton CJ, Bruton RK, Gentleman SN, Roberts GW. Diagnosis and incidence of prion (Creutzfeldt-Jakob disease: A retrospective archival survey with implications for future research. Neuroreport (in press).
ZR 12
SP englisch
OR Prion-Krankheiten 7