Microbiology and Immunology

BSE and CJD: Ethical issues and socio-economic impact WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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2001 outbreak of hoof and mouth disease in the United

Kingdom revealed that a common practice has been to feed

cattle “offal,” the ground up waste from the slaughter process.

Experience with kuru has shown that consumption of prion-

infected tissue is a means of spreading the disease.

This method of transmission is suspected of being

important, if not principle, for BSE. The exact origin of the

prions is not known. Sheep are considered a likely source.

Until the 1900s, scientists believed that the transmission

of the BSE agent to humans did not occur. However, several

studies conducted in the latter years of the 1990s has cast

doubt on this assumption. Studies using mice showed that the

brain injuries caused in BSE and CJD are identical. Moreover,

these brain alterations occurred in mice injected with either

brain tissue from BSE-diseased cattle, which was expected, or

with brain tissue from CJD-diseased animals, which was

unexpected. Thus, development of CJD could be due to human

consumption of BSE-diseased meat.

The currently held view is that prions from cattle

infected with BSE are capable of infecting humans and caus-

ing what is termed a variant CJD (vCJD) disease in humans.

There is evidence that the suspect vCJD has a different infec-

tious behavior than CJD. For instance, younger people can be

infected, and the neurological symptoms differ.

The existence of a vCJD is based mainly on epidemio-

logical evidence. If the existence of vCJD is proven, then the

species barrier for the transmission of BSE and CJD does not

exist. However, the possibility still remains that the contami-

nating agent in the meat is really a prion that causes normally

CJD, and that this prion is naturally present in cattle but has

escaped detection until now. If so, then BSE and CJD infec-

tions could indeed be confined to non-human and human

mammals, respectively.

See alsoBSE and CJD disease, advances in research; BSE and

CJD disease, ethical issues and socio-economic impact; Latent

viruses and diseases

BSE ANDCJD: ETHICAL ISSUES AND

SOCIOBSE and CJD: Ethical issues and socio-economic impact-ECONOMIC IMPACT

The outbreak of bovine spongiform encephalopathy (BSE) or

“mad cow disease” in the United Kingdom and continental

Europe continues to concern beef and dairy producers and the

general public in the United States. This concern has increased

recently because of the continued spread of the disease on the

European continent and the development of a similar disease

that has appeared in people, mostly in the U.K. The new dis-

ease known as variant Creutzfeldt-Jakob disease (vCJD)

appears to be more closely related to BSE in its pathology than

to traditional CJD. It is therefore assumed that vCJD has

crossed the species barrier from cattle to Man.

BSE and CJD are prion diseases, a group of rapidly pro-

gressive, fatal, untreatable neurodegenerative syndromes char-

acterized by the accumulation in the brain of a

protease-resistant protein that is the main (or only) macromol-

ecule of the transmissible agent. The prototypical prion dis-

ease of animals is scrapie, which has been long recognized in

sheep and goats as a common and economically important dis-

order. Following 1988, BSE has given rise to considerable

economic and political turmoil in the United Kingdom as it

developed in more than 150 000 cattle. At the peak of the epi-

demic in 1993, approximately 700 cattle were newly affected

each week. The epidemic has been linked to changes in the

rendering of sheep or cattle carcasses for use as protein sup-

plements to feed-meal, suggesting that inadequately inacti-

vated scrapie agent from sheep, cattle or both was the initial

cause. Following a legislation banning the feeding of ruminant

offal to livestock, the rate of diagnosed BSE has decreased. It

is still uncertain whether the origins of BSE lie in a mutant

form of scrapie or if it developed naturally in cattle.

A number of human prion diseases exist including CJD,

kuru, Gerstmann-Sträussler-Scheinker (GSS) syndrome and

fatal familial insomnia (FFI). These diseases are rare and, until

recently, were not considered of any great socio-economic sig-

nificance. However, in the wake of the BSE crisis in the U.K.

and the suspicion that contaminated beef may lie at the root of

it, many people now fear that an epidemic may be imminent.

The infectivity of prion diseases appears to reside in the prion

protein designated PrPSc. PrPSc is the abnormal, protease-

resistant isoform of a normal cellular membrane protein des-

ignated PrPC. Stanley B. Prusiner of the University of

California at San Francisco has long contended that changes in

conformation underlie the dramatic differences in the proper-

ties of the two isoforms; by abnormal molecular folding,

PrPSc acquires protease resistance and a “catalytic” ability to

recruit more conformational copies of itself from PrPC. PrPSc

is remarkably resistant to many procedures that inactivate

conventional infectious agents and, therefore, problems have

been encountered in decontamination procedures of, for exam-

ple, surgical instruments. Although 90% of prion disease cases

arise sporadically and a further 10% arise where the family has

some history of the disease, it is an unfortunate fact that about

eighty cases of CJD have arisen iatrogenically, that is, as a

result of exposure to medical treatment, facilities, or person-

nel. Cases of transmission by corneal transplant, transplant of

dura mater, exposure to infected neurosurgical instruments

and electroencephalogram probes, and transplantation of

human growth hormone have been confirmed.

The indestructibility of prionscreates real problems in

sterilizing surgical instruments; it is basically impossible, and

equipment has retained infectivity and caused infection in

patients even after repeated “sterilizations.” Currently in the

U.K., scientists are considering making all surgical instruments

disposable. Neurosurgical equipment is already disposed of

after each patient. Since vCJD is carried heavily by the lym-

phoreticular (blood/lymph) systems, the tonsils, appendix, and

most recently, the lymph nodes of vCJD patients have been

found to be full of prions, unlike in patients with classical CJD.

This has brought about the worrisome debate in the U.K. that

all surgical equipment, not just neurosurgical, could be con-

taminated and has brought on calls to destroy all surgical

equipment after use or to use disposable instruments only. The

U.K. has also banned the reuse of contact “fitting” lenses by

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