Only a few more deaths from vCJD likely in UKBMJ 2005; 330 doi: https://doi.org/10.1136/bmj.330.7484.164 (Published 20 January 2005) Cite this as: BMJ 2005;330:164
Relatively few further UK deaths from variant Creutzfeldt-Jakob disease (vCJD) associated with infections in the 1990s are likely, says research from Imperial College, London, published online last week in Journal of the Royal Society Interface (www.journals.royalsoc.ac.uk, doi:10.1098/rsif.2004.0017).
The study was designed to assess the full extent of vCJD cases after previous research had given contradictory results. Recent trends in the incidence of vCJD seemed to show that the primary epidemic was in decline after the peak of 28 cases in 2000 fell to nine in 2004 (www.cjd.ed.ac.uk/figures.htm).
Projections based on this pattern predict relatively few further cases. But a recent study, which assessed the presence of the abnormal prion protein associated with vCJD in appendix and tonsil tissues, indicated a higher infection rate (Journal of Pathology 2004;203:733-9). Three of the 12 674 samples tested showed signs of apparent vCJD, indicating that about 3800 people could ultimately be affected.
The researchers at Imperial College, London, Paul Clarke and Azra Ghani, used mathematical modelling to explore two possible explanations for this discrepancy and to assess which prediction was more likely to be correct. The first hypothesis proposed that a proportion of people infected do not develop clinical disease, to explain the low incidence. Instead, they simply have subclinical infection that is apparent in lymphoreticular tissue but does not progress.
The second suggestion—the worst case scenario—assumed that the existence of a genetic group in which no clinical cases have yet occurred but which is susceptible and could develop the disease in the future.
So far, only one genetic subgroup has been affected—the 40% of the population that is methionine homozygous (MM) at codon 129 of the PrP gene. But the researchers also calculated the number of possible cases that could occur if people from other genetic subgroups were to develop vCJD.
Using the first hypothesis, the results predicted that primary transmission would account for about 70 cases even though a much larger number of people might be infected. The results of the second hypothesis indicated a maximum fivefold increase in cases, totalling another 600 deaths.
Dr Clarke, a statistician at the department of infectious disease epidemiology, said, “The first scenario we analysed makes sense and fits the data so far. This suggests there is little chance of large numbers of vCJD infections from primary transmission occurring in the future.
“One of the most plausible explanations for the discrepancy between clinical case numbers and this estimated prevalence is the possibility that a proportion of infected individuals do not go on to develop clinical disease within their normal lifespan. We estimated that several thousand people might be infected but [would not go on] to develop clinical vCJD.”