Almost 4000 Britons aged between 10 and 30 may be harbouring the prion proteins that cause the human form of mad cow disease. The new estimate comes from direct analyses of human biopsies, and is much higher than epidemiological projections of the likely number of deaths from variant Creutzfeldt-Jakob Disease (vCJD).
The investigators discovered three infected tonsil or appendix samples from a total of 12,674 stored between 1995 and 1999. However, because so few positive samples were found, the projected total of 3808 can only be speculative. Furthermore, harbouring the prions may not necessarily lead to vCJD.
鈥淚 don鈥檛 think too much should be read into our findings, but they should be investigated further,鈥� says David Hilton, of the Derriford Hospital in Plymouth, UK, who led the study.
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He notes that only one of the three positive samples matched the usual pattern of prion accumulation seen in confirmed vCJD cases.
The other two are different. 鈥淚t could mean these are false positives,鈥� says Hilton. If they are, then the predicted incidence of the disease drops by two-thirds, from 237 per million British citizens to just 79 per million.
Fresh samples
Concern at Hilton鈥檚 preliminary findings in 2002 prompted the UK government鈥檚 Health Protection Agency to launch in 2003 a much larger screening study based on an archive of 100,000 freshly collected tonsil samples.
鈥淭he most important thing will be to have a very large number of samples in good condition,鈥� says Pat Troop, chief executive of the HPA. 鈥淲e鈥檒l test them in batches, and if there are significant findings en route, I would expect the Department of Health to publish them.鈥�
But many of the fresh samples will come from children whose food is now free of the cow prions thought to have passed BSE to humans. 鈥淭hat is a possible weakness,鈥� says Hilton.
His own study focused on preserved samples from the people most heavily exposed during childhood and early adulthood to meat from cattle with BSE.
Species barrier
Hilton says that the low number of three positives from such a heavily exposed group may indicate that cow prions seldom pass from cows to humans, because of a so-called 鈥渟pecies barrier鈥�.
鈥淔rom the observed cases, the species barrier does seem to be very high,鈥� he says. 鈥淏ut we need to do these large scale studies to find out more.鈥�
The Department of Health says that the uncertainties in Hilton鈥檚 studies justify its measures to protect patients. These include filtering potentially infective white blood cells from blood donations, and upgrading sterilisation equipment to stop prions spreading on surgical instruments.
Meanwhile, the number of recorded vCJD cases continues to fall from a peak of 28 deaths in 2000. In 2003 there were 18 deaths, with two so far in 2004. In April 2003, when the total deaths had reached 121, epidemiologists downgraded their 鈥渂est guess鈥� projection of all further deaths to just 40.
Journal reference: Journal of Pathology (DOI: 10.1002/path.1580)
