Date: 8 Jan 2001 18:32:06 +0000
From: Nick Honhold <nick@plewland.demon.co.uk>
The Epidemiology of Variant CJD: Some Questions
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I accept that I have no particular expertise in Creutzfeldt-Jakob disease
(CJD), or even bovine spongiform encephalopathy (BSE), apart from having
seen several cases. But as an epidemiologist looking at the figures
recently posted for CJD [see: CJD (new var.) - UK: update June 2000
20000707.1126], I have some questions that may be of interest.
I worked out 5-year rolling averages to try and take out annual
fluctuations. The rise in variant CJD is of course clear from 1995 and the
overall [number of] cases. But what is also clear is that the numbers of
sporadic cases started to rise earlier and have risen until 1999. There was
then a substantial drop in 2000. Of course, some of this rise in sporadic
cases may be due to increased vigilance, but the fact that it rises until
1999 suggests something else is happening.
The questions I have are the following: (1) Is it possible that variant CJD
is (mostly) an age-specific presentation, i.e., that this new form, which
is occurring mainly in younger people, is different from the sporadic form
mainly because of the age of the person affected rather than the prion
involved? (2) Does this suggest that variant CJD was occurring but not
recognized prior to 1995, being attributed to one of the other types? (3)
Was there a change in the diagnostic criteria between the 1999 and 2000
data that may have led to a shift between variant CJD and sporadic cases?
(4) How specific are the diagnostic tests for variant CJD in terms of
differentiating between sporadic CJD and variant CJD? Is it as clear-cut as
seems to be suggested in the limited digests that I see? (5) What a priori
reason is there that older people are not equally at risk of vCJD as young
people?
In short, is it possible that not all sporadic cases are in fact sporadic?
If so, this might seem to be worrying in terms of the possible final size
of the epidemic. But in fact it might suggest that [the epidemic] has
already peaked rather than carrying on rising since TOTAL cases have fallen
for the last 2 years.
I have one other question. In most diseases, date of onset is used for
generating epidemic curves. But, in general, variant CJD is being reported
by date of death. Would there be any change in the data if it is looked at
by date of onset? I tried this with the data quoted from Hansard in
ProMED-mail last year, and it did seem there was some difference between
the 2 curves. I'm sure someone must have looked at this.
--
Nick Honhold
<nick@plewland.demon.co.uk>
[The questions posed by Nick Honhold are interesting and relevant to our
understanding of the variant CJD outbreak. Answers and appropriate
discussion are invited. - Mod.CP]
.
A ProMED-mail post
http://www.promedmail.org
ProMED-mail, a program of the
International Society for Infectious Diseases
http://www.isid.org