Sunday, September 09, 2007

Using the 'science'.

We have, and will continue to be highly critical of the politically skewed RBCT Badger Dispersal Trial. It told us little that we did not know ten years ago, except to reiterate how not to deal with a serious, zoonotic disease situation in a wildlife reservoir.

But the further one delves into the ISG 290 page final report, the more snippets emerge which, although not highlighted by the authors, do not support the unequivocal conclusions of its chairman. (that badger culling, as it was conducted in the 'trial', has no place in the control of bTb).

A comment from Jim on a posting below first drew our attention to the published results of the cull areas of the trial, but in much more detail;

I've done some "mathematical modelling" (based entirely on the data in the ISG Report), and this "suggests" (to use modellers' language) that, if you take a cull area of 800km2 (as opposed to the RBCT area of 100km2), then the net number of herd breakdowns saved (i.e. the number of breakdowns prevented in the cull area minus the number of breakdowns induced by the perturbation effect in the 2km ring outside the cull area) would be 121.9. This is to be compared with the RBCT figure of 1.4 net breakdowns saved per 100km2 (or 11.2 for eight separate areas of 100km2 each). This result is simply a (mathematical) function of the fact that the area culled is proportionately larger when compared to the perturbed area just outside the cull area.

and the really important bit, on which the ISG did not dwell;
I've also taken account of the fact that the ISG found that the beneficial effect of proactive culling was about 50% in an inner core area 4 - 5km from the trial boundary - see para. 5.15 and fig. 5.1B - though this doesn't seem to be a finding they are keen to highlight
So should the data that the centre of Bourne's cull areas managed to avoid the problem of his 'edges' - or any other part of the man - have been given more prominence? That depends from where the political steer was coming. Our commentator continues:
Surely, if the mathematical modellers are saying that something would have a beneficial effect (i.e. culling over a larger area), then it's up to the politicians to find the will and the means to achieve that (or not, as the case may be). Instead, no doubt they'll hide behind the good professor, who has assured them it isn't practicable - and in the meantime they've conveniently sacked all the people with the necessary expertise.

Jim stresses that he is not advocating much larger cull areas as the only or even the necessary way forward, preferring a more targetted approach, but he points out what can be achieved through the use of mathematical modelling.

"If you take the ISG's own figures at face value, the question for the politicians is why not devise a way to carry out a cull over larger areas, as the mathematics shows that this will have a significant beneficial effect? "For the avoidance of doubt", as they say, (a) in setting my calculator to work on the ISG data I'm not accepting that their report is anything other than fatally flawed, (b) if anyone thinks my maths is wonky, feel free to say so (though it appears to be in line with paras. 5.39 to 5.42 and fig. 5.4 of the ISG report), and (c) I realise that questions of badger ecology might influence the effects of a cull over a larger area (I'm just trying to show how a bit of number-crunching can affect one's conclusion

Another observation, this time from Dr. Nick Fenwick in his evidence to the Welsh Assembly makes the same point;

"There is a graph on page 99 of the ISG report in which the beneficial effects of what we would regard as not exactly efficient badger culling are plotted against the distance from the outside of the culling area. The closer you are to the outside of the culling area, the more significant the perturbation effect would be, because that is where the badgers are travelling in and out of the culling area. The badgers from outside the culling area are too far away to travel to the core of the culling area. So, ultimately, perturbation is not a problem in those core areas.

The relationship between the beneficial effect and the distance from the perimeter of the culling area is plotted. The average figure for the effect over five years is in the early twenties: there is around a 23 per cent or 24 per cent reduction in TB. However, in terms of the core areas, where we can ignore perturbation, the reduction in the incidence of TB is down to 50 per cent.

In addition to that, it is worth bearing in mind that, on that same page in the ISG report, there is also a plot of the effect after each year of culling. After the fourth year, the reduction in the incidence was some 32 per cent — I am estimating these figures from the graph. There was a 32 per cent beneficial effect, compared with around an 8 per cent negative effect on the periphery of the culling area. If you combine those two aspects, one might ask what the reduction in the incidence of TB in the core area was after the fourth cull, because that figure of 50 per cent is likely to be increased significantly.

So, in the fifth year of culling, you could imagine perhaps a 60 per cent or 70 per cent reduction in the incidence of TB, which would be a major breakthrough in TB control and would reverse the decline in our TB status that started in 1986."

We agree, but would suggest that annual culls are merely a way to prolong this agony. An intense targetted cull of weeks, would draw out dispersers from other social groups, thus leaving a 'clean ring' of badgers for probably several years. Do not forget that the length of time from exposure of a sentinel cow to a skin test flag up reaction from UK strains of bTb is 221 days. Just 7 months. (Not the seven years the strains take to manifest in Australia) There is absolutely no reason to suppose that the UK system of two 60 day skin tests plus a follow up six month check test for a herd, will not clear the problem, providing the source of infection is identified and removed.

And although they may not appreciate our pointing it out, the ISG final report has shown a huge reduction in cattle Tb at the centre of the cull zones, even and especially given their ineffective, irregular 8 night hit-and-run visits. And no cattle controls either, apart from regular skin tests. How odd, that they felt unable to share this.