The Badger Trust's basic argument is that TB is declining massively in Wales, especially in Dyfed where a badger cull could take place. They say that this decline means that any badger cull should be off the agenda. This is right - see the graph below: the numbers of TB reactors in Wales have declined year on year since 2008. They are still quite a lot higher than they were in the year before foot and mouth disease which is responsible for the big spike after 2001. And they are also a lot higher than in the early 90s.
But the problem with statistics is that on their own these overall numbers don't really tell use the whole picture. Medical statistics use all sorts of methods to standardise mortality rates or disease incidence across populations taking into account age profiles in different regions. Even then you have to be careful about interpreting differences between different regions - they may just be occurring by chance.
TB stats arent treated in the same way, but one thing we can do is to compare the overall amount of disease with the actual number of cattle tested. This relative figure is important as it tells us the actual incidence of disease and this is a better indicator of whether TB is going up or down. You can see this in this graph. There's little point comparing the number of reactors between different time periods if the number of cattle tested is quite different.
Reactors per 1000 cattle tested (RpT) is used as a standard indicator to show how TB levels have changed over time. You can see that for Dyfed and Wales in this graph. This graph is a bit like the other one. It shows that there has been a decline in RpT since 2008, but its still twice as high as pre-FMD, but more than half the extremely high rate of 2008. It also suggests - provisionally - that the decline may have ended. The 2011 figures are only for the first 6 months so should really be treated with a lot of caution, but if the trend continues for the next 6 months, then the decline may have ended.
Perhaps its no surprise not to see the Badger Trust commenting on this: their point is that the overall trend is still downward from 2009. Describing a new upward trend would spoil their story: although at least they do for England. Here though they are basing their data on 2010 figures. Its all getting a bit confusing: using a trend from 2009-2011 for one area and 2010-11 for another. Using stats to make a point? Maybe thats what statistics create...
There's an interesting story just looking at reactors from routine tests - that is tests where a reactor is found putting farms under TB restrictions, rather than tests designed to get disease out of an already infected heard (i.e. short interval tests). I dont have 2011 data for routine tests, so what the most recent trend is I dont know. But what is interesting is that these data suggest that, proportionately, the number of reactors is now getting back to similar levels to before FMD.
Just for good measure, here are the figures for new breakdowns from routine figures. Just to be clear, these are the % of tests of clear herds that result in either a reactor, or a reactor and/or IR. The first graph is for Dyfed, the second for England and Wales. Again the trend is similar: that its taken 10 years to more or less get back to where we were before foot and mouth disease. In fact for England, the trend is probably down from that before FMD.
So what do all these figures mean? On their own these statistics cant really be used to justify support for anything. They dont really suggest that additional cattle controls in Dyfed are having an effect: for a start the area whey they are being applied is just a very small part of the county, and then we'd need to look at other similar areas to see what was going on in them. Actually this would be a really useful thing to do and to make those figures publicly available. There's potentially a whole load of other interesting stats too: the number of confirmed breakdowns; the number of repeat breakdowns; the survival times of herds between breakdowns etc etc. Wouldnt it be good to show all these data too? After all, arent we all supposed to be "armchair auditors" these days? Hmm. The criticism of armchair auditing is that sometimes statistics need to be handled with care, even when they are presented in relatively simple terms. In fact, changing the way Defra present TB statistics was one of the recommendations of the ISG.
Do these statistics support the Trust's claim that they undermine a badger cull? The basis of a badger cull has not really been made in terms of the level of disease. That's to say no-one has said above a certain level of disease then we need to start thinking about a badger cull, and below a certain level then its not required either. Remember, when the Krebs review was commissioned it was thought that there was a problem with TB and the government needed to look at a badger cull, but those figures were a lot lower than now. So even if the decline continues it wont change that, although it would affect the cost-benefit analysis for a badger cull.
Perhaps this disconnect between the need for badger culling and the level of TB is the most interesting thing about these statistics. Maybe what it highlights is the difference in expertise involved in disease control: the difference between a field approach which relies on local grounded experience to get the job done, and one which relies on more a scientific quantitative account of disease. In many ways thats what the debate in TB is about.
Note on data
You can recreate these graphs using data held on Defra's TB statistics website. To calculate Rpt its: no. reactors divided by number of cattle tested multiplied by 1000. For some reason in recent years they've stopped giving the actual number of cattle tested which means you cant calculate RpT. The data Ive used comes from an extract from Vetnet that Ive been doing research on. Ive not included any data from pre-movement tests. The 2011 figures are from the latest TB statistics release.
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