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Dormant but deadly
Animal experiments have revealed a silent carrier role for a deadly prion disease in mice which may point to dire consequences for humans.
Jennifer Cooke reports.
Dr Andrew Hill realised the implications immediately. A flurry of calls from the animal house to the lab where he worked in London early last year warned of eight mice with signs of prion disease. It had been assumed for years that this could not happen.
But not only did these results show new evidence for a "sub-clinical" or undetectable form of disease in mice; they lent credence to theories that mad cow disease could be carried silently by apparently healthy cows and that humans incubating the mad cow-related new variant Creutzfeldt-Jakob disease (vCJD) may infect surgical and dental patients through contaminated instruments.
All prion disease experiments are lengthy and these important findings, published last week in the Proceedings of the National Academy of Sciences, capped years of lab work.
More than four years ago, Hill and colleagues at the Medical Research Council Prion Unit in London, headed by Professor John Collinge, began an experiment in which 20 normal mice were injected with the mulched-up brains of hamsters who had died from one strain of the prion disease called scrapie.
There are more than 20 known strains of scrapie, which is endemic in British sheep flocks. Scrapie is believed to have given rise to the epidemic of mad cow disease or bovine spongiform encephalopathy (BSE) that began around 1985 and has since spread to Europe.
More than 20 years ago researchers had noted that mice lived a full life with no apparent disease despite being injected with hamster scrapie. The assumption had been that this strain could not be passed on to mice.
But Hill and his colleagues took that old experiment a step further and injected the liquefied brain matter of these apparently healthy mice, into another bunch of healthy mice and hamsters - just to confirm that there was no disease present.
It was supposed to be a simple control experiment but, to the shock of everyone involved, the mice and hamsters started staggering around their cages five months afterwards, indicating that prion disease had taken hold and spongy holes had begun destroying their brains. What's more, the mice had produced their own lethal prions, which were different from the hamster prions injected into the original 20 mice.
The control experiment had evolved into a major finding itself. It proved that those original 20 mice had been silent carriers of the disease and were able to pass it on.
A new-generation test in which proteins are examined to identify the molecular signature of different prion diseases also confirmed the presence of high levels of prions in the brains of those mice.
If the same were found to be true of cattle, then many cows in Britain, and other European countries where it has surfaced may be silent carriers of the related BSE.
Diseased cattle may then freely enter the human food chain and escalate what is fast becoming a mini-epidemic of the human equivalent, vCJD.
Test findings published in 1998 showed that vCJD had an identical molecular signature to BSE - as close to absolute proof as is likely, experts say, that BSE caused the disease in humans.
Species including poultry, pigs and sheep were also exposed to BSE in Britain from the cannibalistic feeding practices of the 1980s when dead sheep and cows were ground down into cakes of meat and bonemeal protein supplements. Speculation now surrounds whether those species might also be capable of harbouring symptom-free prion disease.
The most worrying implication of all is that transmission could occur from silent animal carriers to humans and then to other humans.
No test is sensitive enough yet to detect rogue prions in a person who has no symptoms of vCJD (or any other type of CJD).
If the mice experiments prove in future to be applicable to cows, and thus humans, as BSE has already shown, then anyone who has been exposed to prions from mad cows has the potential to be a silent carrier. Include in this group British migrants (fed on baby food or at British schools, in particular), returned expatriates, and possibly even tourists who indulged in the English passion for pies, hamburgers, sausages or beef products that included brain and spinal cord material - the most infectious parts.
And if that were the case, the potential would in turn loom large for transmission between humans via CJD-contaminated surgical instruments and tissue donations - routes via which another type of CJD has already killed neurosurgical patients and recipients of corneas and pituitary-derived human growth hormone.
The infectious agent that causes CJD cannot be removed by hospital-standard sterilisation or cleaning techniques, although infectiousness can be reduced.
So far, no transmission has occurred through organ donations. And blood and plasma derivatives are not believed to be implicated in the transmission of any type of CJD. However, the fear with vCJD, which can be detected in tonsil and appendix tissue, is that it might behave differently (having sprung from animals) and pose more of a threat.
Tighter restrictions on blood transfusions and wider use of disposable surgical instruments are among the anti-vCJD measures introduced over the past two years in Britain alone, to reduce the risk of spread between humans.
Earlier this year the United States, Canada, Austria and New Zealand also banned blood donations from anyone who lived in Britain for longer than six months between 1980 and 1996. Canada last week applied the same ban to those who had lived in France.
Australia has followed suit on the grounds of the small, theoretical risk of CJD transmission via blood and fears of diminishing an already small pool of vital donors.
Only this year, nine neurosurgery patients at the Royal Melbourne Hospital were potentially exposed to CJD when instruments used on a patient later found to be suffering from CJD were reused. CJD was not initially suspected in the patient so the normal procedure of quarantining the instruments for later destruction, if CJD was confirmed, was not followed.
Just last month, to head off a similar occurrence with vCJD in Britain, the Department of Health announced that surgical instruments used in tonsil, appendix and brain operations are to be destroyed after one use in a move likely to cost millions of pounds.
At the pathology department of the University of Melbourne researcher Andy Hill, has been dogged by calls from the British media over the past week concerned at what his work might mean.
"The bottom line," he says of the implications of the mice experiment that sparked it all, "is that healthy cattle may harbour infectivity and never show any signs of BSE. It is entirely possible that, in the same way, humans might be harbouring the disease at this sub-clinical level. As we don't know how many people may be incubating the disease - which may be longer than the normal lifespan - then there are possible implications for medical and surgical procedures."
In the five years since its discovery in two British teenagers, vCJD has been reported in 79 people in Britain, two in France and one in Ireland. Alarmingly, 36 of those were reported in only the past year.
Despite the introduction of a series of controls dating from 1989, including a ban on the most infective cattle offals entering food, a bar on cattle over 30 months old being used for food, and an end to feeding cow and sheep remains to other meat-producing livestock, they were not policed effectively and the BSE epidemic was not contained quickly.
A devastating report on the British Government's inadequate role in guarding public health is expected from The BSE Inquiry, a two-year-old probe into the handling of the BSE epidemic up to 1996. It is due within weeks - about the time of the next meeting of the Spongiform EncephalopathyAdvisory Committee (SEAC) at which scientists may yet decide to re-examine controls against vCJD.
The British Government, which denied for a decade until 1996 that British beef was anything but perfectly safe to eat, has spent the past week confidently asserting that existing measures are more than adequate to guard against the possibility of silent BSE cases.
Hill's old boss, Professor John Collinge, also an SEAC member, is not in total agreement.
"My own view is that probably the 30-month rule and the offal ban would still protect us from [BSE] but I am not sure," he said.
"This data shakes me up a bit."
Jennifer Cooke is the author of Cannibals, Cows & the CJD Catastrophe, which won last year's Eureka Science Book Prize.
This is an interesting article well worth posting in view of the strangeness of this forum of late.
Regards from OZ where petrol is up, up and away!!!!
If anyone is coming over the big pond for the Olympics, welcome - the exchange rate is very good right now & in your favour...
-- Pieter (email@example.com), September 09, 2000
Thank you Pieter--
Chilling and personally worrisome.
My Mother went home to England at least 4 times during 1972-1982 and died from a progressive diesease similar to vCJD but over a longer time span. Probably no connection but.........?
-- Pam (Sheep@thistles.the pasture), September 10, 2000.