MANY gave a huge sigh of relief when British authorities announced last week
that researchers who tested 3000 stored tonsil and appendix samples for variant
Creutzfeldt-Jakob disease (vCJD) did not find a single infected sample. The
study was designed to gauge the possible spread of vCJD, the fatal brain disease
thought to have originated in cattle.
Although the government plans to test another 15 000 stored samples, vCJD
experts say that the uncertainties involved in testing stored samples are too
great. Instead they are urging the government to embark on a programme to test
fresh tonsil and appendix samples for the presence of the rogue prion protein
thought to cause the disease.
British hospitals remove and store 45 000 appendixes every year. 鈥淚鈥檝e been
pressing for prospective studies to take off for the last two years. It鈥檚 a
political issue,鈥� says neuropathologist Adriano Aguzzi of the University of
Zurich. 鈥淚 have been raising this question at SEAC meetings every time.鈥� SEAC,
the Spongiform Encephalopathy Advisory Committee, advises the government on vCJD
and its equivalent in cattle, BSE.
Advertisement
Experts from the Department of Health acknowledged last week that the new
results did not provide much reassurance. The sample is so small that it might
not be representative of the whole population, and the use of old tissue stored
on slides makes testing less accurate, meaning positive results could have been
missed. 鈥淭his doesn鈥檛 take us any further on,鈥� admits the government鈥檚 Chief
Medical Officer, Liam Donaldson.
So far there have been 53 confirmed deaths from vCJD. Two suspected vCJD
victims are awaiting post-mortem diagnosis and 12 people are thought to be ill
with the disease. But in the absence of a larger study sample, scientists don鈥檛
know whether the death toll will be 100 or 100 000.
The problem with testing samples of fresh tissue is that doctors may face the
dilemma of whether to tell their patients that they have an incurable disease.
鈥淚t鈥檚 a question of ethics in that people are concerned about not informing the
people they鈥檙e testing if there are positive results,鈥� says Aguzzi. 鈥淏ut there鈥檚
an ethical question about not doing the research as well.鈥�
Researchers at the Wellcome Trust Centre for the Epidemiology of Infectious
Disease at Oxford University have argued that because we don鈥檛 know how soon
after infection the rogue prion appears in the appendix or tonsils, even a study
of 40 000 fresh samples might not reveal the extent of infection (The
Lancet, vol 352, p 1353). But others, like Aguzzi, say such studies would
provide more information than the existing ones.
Some tests of fresh samples are scheduled, however. Following delays due to
ethical concerns, John Collinge of Imperial College School of Medicine is
planning to test a new diagnostic method on 2000 freshly extracted tonsils
(快猫短视频, 23 January 1999, p 5).
However, Collinge admits the sample
size is not very large. Another SEAC member, Ian McConnell, a veterinary
scientist at Cambridge University, says that before large-scale testing can be
carried out, it will be necessary to increase the number of laboratories expert
in testing for vCJD. 鈥淭his is an extremely complex assay and we don鈥檛 want false
positives,鈥� he says.
Asked about the need for larger studies, Donaldson said it was necessary to
鈥減roceed with caution鈥� until better tests become available.