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12 March 2003
Current Assessment of the Future
Prevalence of vCJD in the UK
Dr James Irvine
Teviot Scientific Consultancy, Edinburgh and Perthshire
Cultybraggan Farm, Comrie, Perthshire
www.land-care.org.uk
(Filed 12/Mar/03)
© Teviot Scientific Consultancy
In 1997 Cousens et al (1) reporting
from the CJD Surveillance Unit (Edinburgh) estimated that the eventual
number of vCJD cases could be anywhere between 75 and 85,000.
To quote from the Phillips BSE Inquiry Report
published in October 2000 (2),
"4.17 Attempts to estimate the possible size of a vCJD epidemic
have been hampered by the many variables associated with the disease.
Many important factors in determining the probability of BSE transmission
to an individual, such as dose, route of exposure, incubation
period, genetic susceptibility and scale of the species barrier
between cattle and man, are unknown. Nevertheless, several groups
of epidemiologists and statisticians have attempted to predict
the possible number of cases, using complex mathematical models.
Unsurprisingly, these groups have arrived at varying estimates,
ranging from very few cases to many millions, as a result of the
different methodologies adopted".
To quote from the summary of the vCJD section
of the CJD Surveillance Unit Report for 2001 (3),
"The upward trend in vCJD cases continues to be statistically
significant with an increase of 21% per year for onsets and 23%
per year for deaths. The estimated current rate of onsets is 8.3
per quarter and deaths is 7.0 per quarter. There has been a decrease
in the delay from onset to diagnosis of about 5% per year. The
models predict that a total of 140 onsets (95%CI: 131 to 154)
had occurred by December 2001 and that in the following 12 months
there will be a further 32 deaths (95%CI: 19 to 47). There is
no evidence yet of a significant departure from an exponential
increase in cases, however this will be monitored closely because
the models with quadratic terms are consistent with an epidemic
reaching its peak".
The actual figures to December 2002 that have
just recently been published (4, click
to view) show that in fact there have only been a total of 129
cases of confirmed vCJD (consisting of 121 deaths and 8 diagnosed
but still alive) between January 1994 and December 2002.
Thus, rather than there being an exponential
increase in the number of expected cases of vCJD that was previously
predicted, there is evidence that a quadratic epidemiological model
may be more appropriate.
Using a quadratic model to compute the expected
number of future cases, it could be that the disease vCJD in man
may have reached its peak and be flattening off.
With such small numbers of confirmed vCJD cases
to date, it is difficult to make accurate statistical predictions.
However, the more extravagant claims of the early days seem no longer
to be substantiated.
The subject is further discussed by Andrews et
al (2003) in Research Letter in the Lancet 1 March 2003 (5).
Potentially all this is very good news - but
remember it is early days yet to be too confident about the natural
history of a disease, the preclinical phase of which cannot be detected
by non-invasive methods. The length of that preclinical phase could
be highly variable, being prolonged in some and shorter in others
according to such factors as dosage, genetic susceptibility and
the presence or absence of other aggravating or protective factors.
There is also a warning here. One should be wary
of the predictions of the claims of epidemiological modellers, especially
(through no fault of their own) as it is difficult to choose which
type of model is going to be appropriate.
Dr James Irvine
© Teviot Scientific Consultancy
References
1. Cousens, S.N., Vynnycky, E.,
Zeidler, M., Will, R.G. and Smith, P.G. (1997). Predicting the CJD
Epidemic in Humans, Nature, 385: 197-8
2. The Inquiry into BSE and variant
CJD in the United Kingdom. Volume 2: Science, Chapter 4: The link
between BSE and vCJD - Estimates of the size of the vCJD epidemic.
www.bseinquiry.gov.uk/report/volume2/chapteb4.htm
3. National CJD Surveillance Unit
10th annual Report (2001): www.cjd.ed.ac.uk/rep2001.html
4. Andrews, N. J. (2003). Incidence
of Variant Creutzfeldt-Jakob Disease Onsets and Deaths in the UK,
January 1994 – December 2002 (www.cjd.ed.ac.uk/vcjdq.htm).
5. Andrews, N. J., Farrington,
C. P., Ward, H. J. T., Cousens, S. N., Smith, P. G., Molesworth,
A. M., Knight, R. S. G., Ironside, J. W. and Will, R. G. (2003).
Deaths from variant Creutzfeldt-Jakob disease in the UK. Lancet
361: 751–52. (Download
PDF).
Reproduced with permission from Elsevier.
The Lancet homepage: http://www.sciencedirect.com/science/journal/01406736
Further Reading Recommended by Land-Care
Venters, G. A. (2001). New variant Creutzfeldt-Jakob Disease: the
epidemic that never was. BMJ, 323: 858-61. (View
article).
Irvine, W. J. (2003). Clarification of the Possibility of Transmitting
CJD in Man by Blood Products.
(Filed 17 February 2003, www.land-care.org.uk,
click
here to view).
Haywood, S. and Brown, D. R. (2003). Transmissible Spongiform Encephalopathies.
Veterinary Times, 33: 8-10.
(Filed 28 January 2003, www.land-care.org.uk,
click
here to view).
Leake, Jonathan (2002). Test everyone for CJD, says Nobel Prize
winner. Sunday Times, 1 December 2002.
(Filed 9 December 2002, www.land-care.og.uk,
click here
to view).
Linklater, Magnus (2002). They drive us Mad with False Fears about
Mad Cows. The Times, 5 December 2002.
(Filed 9 December 2002, www.land-care.org.uk,
click
here to view).
Has CJD been transmitted to 24 patients through the inappropriate
use of surgical instruments?
(Filed 30 October 2002, www.land-care.org.uk,
click
here to view)
Editorial comment. Predicted Future Incidence of vCJD in the UK
Population.
(Filed 2002, www.land-care.org.uk,
click
here to view).
Risk of vCJD From Blood Transfusion.
(Filed 2002, www.land-care.org.uk,
click here to view).
The English Disease: Hugh Pennington's Comments on the BSE Inquiry.
(Filed 2002, www.land-care.org.uk,
click
here to view).
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