Examination of over 12,000 appendix and tonsil tissue samples from patients in England and Scotland revealed that vCJD prions were present in people with a genotype previously thought to be resistant to the disease.
All the 161 confirmed cases of vCJD in the UK have been in people with a homozygous methionine (MM) polymorphism at codon 129 in the gene encoding the prion protein. An estimated 40 per cent of the population have this genotype.
But vCJD prions have now been found in tissue samples from two individuals homozygous for valine (VV) at this codon. This follows a previous report of an asymptomatic case of vCJD infection in a heterozygous (MV) individual.
The findings suggest that there might be no genetic subgroup immune from vCJD infection.
There is no clinical evidence to date of vCJD in people with either the VV or MV genotype. The researchers say this means they are either asymptomatic carriers of the infection, or could go on to develop the condition after a longer latency period.
'They might have a longer incubation period than the MM subgroup and be more likely to develop vCJD at a later time,' lead researcher Professor James Ironside, from the National CJD Surveillance Unit at the University of Edinburgh, said.
'This would explain why we haven't seen any cases in the VV genetic subgroup so far.'
He said that cases of vCJD could still be emerging up to 50 years from now: 'The unfolding of vCJD could still be a long time in the future.'
Even if these asymptomatic carriers do not go on to develop clinical signs of vCJD, they could be a potent reservoir of secondary infection and pass vCJD to susceptible individuals through blood transfusion and surgical instruments.
The research follows a study gauging prevalence of vCJD in the UK population, which identified vCJD prions in three out of 12,674 tissue samples. Although this number appears small, it was higher than that expected from the number of confirmed cases of vCJD.
BMJ 2006; 332: 1,186-8; Live links at GPonline.com.