Sludge Watch ==> CJD - Related diseases can incubate for 50 years

[maureen.reilly at sympatico.ca]

http://www.nature.com/news/2006/060619/pf/060619-12_pf.html

Published online: 23 June 2006


CJD-related disease can incubate for 50 years
Last living cannibals aid predictions for modern prion epidemic.

Helen Pearson


It is still unclear how long the protein that causes vCJD can lie dormant in 
the brain.
© AP Photo/Professors Stanley Prusiner/Fred Cohen
People in Papua New Guinea who once feasted on their own relatives succumbed 
to prion disease as much as half a century later, say scientists who have 
laboriously tracked down the last sufferers in remote villages.

The discovery renews concerns that another prion disease, variant 
Creutzfeldt-Jakob disease (vCJD), might also be incubating silently and will 
only rear its head decades from now.

Neurologists have long been fascinated by kuru, which caused an 
unprecedented epidemic in the Fore people of Papua New Guinea that peaked in 
the 1950s and early 1960s. Families steamed and ate the entire bodies of 
their relatives in a death ritual, unwittingly ingesting the infectious 
prion proteins that cause the debilitating neurological disease. But the 
practice stopped when it was prohibited by Australian authorities in the 
mid-1950s.

Scientists' interest in kuru renewed with the realization that vCJD, 
transmitted from cows infected with bovine spongiform encephalopathy (BSE), 
might also cause an epidemic amongst those who ate infected beef in the 
1980s and 1990s.

So far, 156 vCJD deaths have been reported in Britain, the worst-affected 
country, and the number of new cases peaked in 2000, suggesting that the 
disease takes around 10 years to incubate. But there is still debate about 
the eventual size of the epidemic; no one knows whether many more are 
actually infected or for how many years they might incubate the disease 
before they show symptoms.

Bush mission

John Collinge at University College London, UK, and his colleagues went to 
Papua New Guinea to find out. Most people who had kuru have already died, 
but the team ramped up existing disease monitoring in an attempt to find the 
last vestiges of the epidemic: those people who harboured the infectious 
prions for many years and are only just developing the disease.

It's sobering that half a century on, this disease has not disappeared.

John Collinge
University College London

Working with local communities, they scoured isolated villages, which are 
typically located at mountainous altitudes above 2,000 metres, lost in 
dense, wet rainforest and often connected only by tracks. "It's arduous 
trekking," Collinge says.

>From 1996 until mid-2004, the team found what they believe to be the final 
11 remaining cases of kuru, which often manifests itself as problems with 
balance and coordination. The researchers also collected people's life 
histories to piece together when they were likely to have been infected.

Because the cannibalistic ritual had stopped by 1960, the team calculated 
the incubation time as the time between 1960 and the year that a patient 
first exhibited kuru symptoms. This suggested an incubation lasting between 
34 and 41 years, although the researchers did not know exactly when, before 
1960, a person was infected.

New numbers

They then sought to obtain a more accurate estimate, based on the knowledge 
that boys stopped taking part in the cannibalistic ritual at around the age 
of seven. For their male patients, they therefore subtracted seven years 
from the age that they first showed symptoms.

This gave a longest incubation time of 56 years, and perhaps up to seven 
years longer, although the average incubation time is thought to be 12 years 
they report in The Lancet1. "For the first time we can see the extraordinary 
incubation period in human prion disease," Collinge says. "It's sobering 
that half a century on, this disease has not disappeared."

Collinge says that vCJD could well have an average incubation time of 30 
years or longer (more than the average for kuru) because the prions are 
passing from cows to humans rather than from human to human. This species 
barrier is known to extend incubation times in animal tests. The people who 
have already succumbed to vCJD could have been those with the shortest 
incubation period, perhaps because they were particularly genetically 
susceptible, as other evidence has suggested.

Mathematical models used to predict the size of the likely vCJD epidemic 
could now take these findings into account. "Most people seem to be thinking 
that we're over the worst of it," Collinge says. "We have to be cautious 
about assuming this disease is going away."

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References

   1. Collinge J., et al. Lancet, 367. 2068 - 2074 (2006).