As "mad cow" disease spreads outward from Britain, a silent epidemic of carriers in humans has begun to emerge. Jennifer Cooke reports.THE bad news came with the death of an elderly patient in Britain two years ago. While seemingly unremarkable, this was automatically the subject of an autopsy because the patient had a blood transfusion in 1999 from a donor who had died later from the human form of mad cow disease: variant Creutzfeldt-Jakob disease (vCJD).
In Britain, paranoia is rife about human-to-human vCJD transmission via blood. This disease is the newest type of prion disease in a group that includes bovine spongiform encephalopathy (BSE). This automatic look-back policy on British blood product recipients proved a grim genetic revelation, as well as the first solid indication of what experts had feared: that the BSE epidemic that has wiped out millions of cows was entering a new phase - secondary transmission through humans.
Reported in 2004, elderly patients were the second probable case of blood-borne vCJD transmitted between humans. The first was identified months earlier, in December 2003, in a 62-year-old man who died of vCJD six years after a blood transfusion. This man's blood donor had been an apparently healthy young man who died of vCJD three years after the donation.
A third blood transfusion-related case was identified in March this year. All three were in Britain, the epicentre of the declining but continuing two-decades-old BSE epidemic in which more than 184,000 cows have been diagnosed with BSE and millions more have been slaughtered as a precaution.
Although the patient died of unrelated causes, the spleen in the second case was "loaded" with abnormal prion protein, according to one expert. Prion protein is a Jekyll-and-Hyde protein. In normal brains it helps create nerve cells. In diseased brains it changes shape, misfolds and causes terminal damage.
In vCJD, unlike other types of CJD, it starts in the lymphoreticular tissue. This includes the spleen, appendix, lymph nodes and tonsils, and takes years to reach the brain where the abnormal prion creates tiny "spongy" holes.
The second case marked a turning point in the history of vCJD, which burst into the annals of disease just a decade ago (March 20, 1996) with the announcement in the House of Commons that a new disease had been identified in 10 young British people and was linked to the BSE epidemic.
Despite no symptoms of vCJD, the second patient was carrying the infectious rogue prion and was a danger to anyone who received that person's blood, organs or tissue, or whose surgery followed any procedure performed in which the same metal instruments were used. Most worrying was the genetic make-up in this case on one tiny point of a gene all humans and other mammals carry - the prion protein gene (PRNP) on chromosome 20.
To date, all 191 victims of vCJD have shared a genetic trait. They are MM at codon 129 of the PRNP gene. Forty per cent of caucasians have the MM genotype. It had been hoped this was the only susceptible population group.
But the second case, sadly, had the most common genetic type at the codon 129 position of the PRNP gene. It is known as MV - a trait shared by half of all caucasians.
This is the first identified vCJD-linked human to have the MV genotype. The last 10 per cent of the population is known as VV on codon 129, and until last month being VV was pretty good news if you wanted to avoid vCJD.
BEFORE vCJD, there was another orally transmitted prion disease. Kuru (the "shivering" disease, named after one symptom) probably originated with a single case of CJD around 1900, in the Fore tribe of the eastern highlands of Papua New Guinea. The disease nearly wiped out the Fore, who ate the brains of dead relatives in a mourning ritual.
However, cannibalism was prohibited in the 1950s and gradually the epidemic died. But with the odd case still emerging, the longest incubation period for this disease is now more than 40 years and serves as a model for what might happen with vCJD.
Recent genetic analysis of the scrupulously preserved records and blood samples from the kuru epidemic show victims with the MM genetic variation were younger when symptoms began (many were children), probably had a shorter incubation period and a shorter illness. Another study revealed most MV or VV individuals survived the epidemic or developed kuru when older. The VV genotype - reassuringly for those who might die of something else first - had the longest incubation period, according to a 2001 study published in the Journal of Infectious Diseases.
LAST month two papers provided additional foundation for the kuru findings - that the MM genotype may affect merely the shorter incubation victims of the human mad cow epidemic and that MV or VV genotypes will live longer, but not necessarily escape the same fate.
In the first, published in The Lancet Neurology, mice whose PRNP gene had been replaced with the human counterpart did not develop BSE when deliberately injected with it. However, when vCJD was injected into MM and MV mice they were "equally susceptible" to infection, although most of the MV mice did not develop symptoms before the end of the 700-day experiment - their natural life span.
The transmission rate of vCJD in the VV transgenic mice was much less but indicated a rather grim scenario: that all three genotypes on codon 129 showed subclinical or symptomless infection. What differed was the incubation periods until symptoms either appeared or the mice died. The mice were useful in showing that once BSE has passed into humans in the form of vCJD, it was altered, which also raised "concerns relevant to the possibility of secondary transmission of vCJD through blood transfusion, fractionated blood products, or contaminated surgical instruments", the authors warned.
And if the news could get worse, it did. A retrospective study published in the British Medical Journal analysed the genotypes of the three positive samples of infected tissue found among 11,109 appendixes and 1565 tonsils removed in operations in Britain between 1995 and 2000 from anonymous patients aged between 20 and 29. With enough DNA left in only two of the three positive appendixes, the researchers found that is was "perhaps surprising" to find they were both VV. This bad news is the first time that the VV population in humans - the smallest population of the three genotypes at codon 129 on the PRNP gene - has been found to be potentially susceptible to vCJD infection.
Reaffirming earlier findings, the researchers from the national CJD Surveillance Unit at the University of Edinburgh stated: "Genetic studies of kuru, another orally transmitted human prion disease, found that PRNP codon 129 MV and VV genotypes were associated with longer incubation periods than the MM genotype."
And the ramifications? With such a long incubation period projected for VV genotypes, a potential risk for horizontal transmission of vCJD infection by "blood transfusion, blood products or contaminated surgical instruments" arises, underscoring the vital need for continued surveillance of cases well into the future, they warned.
Dr Andrew Hill, a molecular genetics expert who heads a laboratory studying prion diseases at the Bio21 Institute at the University of Melbourne, was one of the first to reveal through mouse experiments reported in 2000 that there was a silent reservoir of infection in symptomless prion disease carriers. The latest mice revelations "confirm concerns we raised in our study", he says. "These studies underlie the urgent need for a rapid, sensitive test for infectious prions so we can identify potential asymptomatic carriers.
"The blood bans [banning the donation of blood from any Australian who has spent a cumulative six months or more in Britain between 1980 and 1996 or who has ever had a blood transfusion] still apply and people might question why."
While Australia remains free of BSE "you can't ignore this just because it's happening overseas", he says.
Given Australia's historic migratory and tourism ties with Britain in particular, Australian prion disease experts think it inevitable that vCJD will emerge in Australia from someone infected in Britain.
"I suppose it all really underscores the need for continuing CJD surveillance in Australia," Professor James Ironside, from Edinburgh's CJD Surveillance Centre, told the Herald. More silent carriers as well as symptomatic vCJD cases in the 25 BSE-affected countries affected to date also loom.
Any continuing pattern of change to the genotypes of vCJD victims could mark the beginning of the next phase of the human epidemic.
Prion diseases Incurable, untreatable.
Definitive diagnosis only at autopsy.
Widely accepted cause is that abnormal prion protein misfolds, replicates, causes spongy holes in the brain.
Prions stick to metal (cattle feed bins, surgical instruments).
Resistant to standard surgical sterilisation.
In humans includes Kuru and Creutzfeldt-Jakob disease which has four main types:
* Sporadic (no known cause).
* Familial (inherited).
* Iatrogenic (medically acquired).
* Variant, the only type linked to cows.
Bovine Spongiform Encephalopathy (BSE)
First reported 1986 in England.
Spread to other countries via exports of live carrier cattle and meat-and-bonemeal protein supplements (MBM).
Continuing cases in Britain believed linked to MBM contamination of inadequately cleaned metal feed bins.
Never appeared in Australasia.
First reported 1996 in England.
191 cases; Britain (161), France (18), other countries (12); none in Australasia.
Looks and behaves differently to other CJD types.
Only type to transmit between humans via blood.
Until now all cases are MM genotype; BUT one carrier case is MV; two VV genotype cases in appendixes.