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January 06, 2004

Mad Cows

The UK Department of Health has just released end-of-year data for deaths from variant CJD (the so-called human form of BSE, or mad cow disease) in 2003.

The figures from 1995 (the first year of records) to 2003 inclusive are as follows:


As I pointed out in a recent post, alarmist predictions of thousands, or even hundreds of thousands, of deaths, have not materialised. In fact, the 'epidemic' seems to have peaked in 2000 and plateaued at around 20 deaths per year.

It is also interesting to look at the figures for so-called 'spontaneous' CJD, i.e. cases with no known cause. For the same years, these are 35, 40, 60, 63, 62, 49, 57, 73, and 58. It will be noted that these are much higher than the numbers for 'variant' CJD. If we add 'variant' to 'spontaneous' figures we get the following totals:


(NB: there is often a time-lag in reporting of 'spontaneous' cases, so the provisional end-year figures for 2003 may increase slightly.)

Apart from a 'blip' in 2002, there is a notable stability in these totals from 1998 onwards. I'm not sure if there is any real significance in this, as stochastic fluctuations in two independent series would often cancel out and produce a more stable total. But it is at least worth considering whether part of the 'increase' in vCJD from 1997 onwards was an artifact of diagnosis.

It is also worth noting that there has never been any worthwhile proof that variant CJD is caused by eating infected beef. Early in the 'epidemic' some commentators noted that a high proportion of the victims had had jobs which involved handling beef, including a girl who had been a vegetarian for many years but worked at a dogs' home where she regularly handled meat for the dogs' meals. The epidemiologists examined this connection and concluded that the proportion of victims with 'meat handling' jobs was not in fact significantly higher than expected. But of course, this does not prove that the disease is not transmittted by handling meat, e.g. by infected prions entering the bloodstream via cuts or scratches.

As the first case of BSE has just been reported in American cows, my message to American readers would be:

(a) don't panic
(b) do look carefully at hypotheses other than transmission by eating infected beef.

Posted by David B at 04:49 AM