Monday, October 08, 2007

Culling Efficiency - or not?

Following on from our overview of the opinion piece in Veterinary Times, John Daykin, BVSc, MRCVS and Dr. Lewis Thomas, MA, VetMB, PhD, FRCPath, MRCVS continue to unpick the final report of the ISG. In this posting, they examine the efficiency of the cage trapping in the RBCT,Badger Dispersal Trial which was undertaken for 8 nights only, annually - if you were lucky.

Trapping efficiency was a point laboured by Professor, Sir John Krebs in his original methodology but tweaked by the ISG to deliver the appropriate answer from their ministerial skew. Mr. Daykin and Dr. Thomas describe the data produced by the ISG on culling efficiency as "grossly inconsistent and therefore open to question".
The negative effects of inefficient culling are central to this whole debate. In his introduction, Professor Bourne maintained that culling efficiency in proactive triplets was 70%. Professor John McInerney later inflated this to 80% in his presentation. How can these “opinions” from ISG members be substantiated in the face of previously published culling efficiency data? DEFRA statistics reported a culling efficiency of between 20% and 60% across the triplets (2). This has been recently revised to between 71% and 85% in 7 triplets, and between 35% and 46% in the three other triplets, removing between 32 and 77% of badgers overall (3). It is unclear exactly what practical significance the 30% non-consent land had on the actual badger population reduction in the proactive triplets and the influence that this had on disease dynamics. Hansard recorded in 2003 that 57% of traps had been interfered with, and 12% stolen (4). Paul Caruana of the DEFRA Wildlife Unit, which carried out the trapping in the RBCTs, expanded on these flaws in the culling methodology in his evidence to the EFRA Select Committee in 2006 (5). Mr Caruana’s criticisms, which Professor Bourne has dismissed in a most unscientific manner, focussed on trapping in winter when badger activity is minimal, and poor siting of traps which were subject to constant sabotage in the first four years of the Trials. The ISG’s own data show that 15 out of the first 30 triplet culls took place from November to January (6). This only reinforces our opinion that culling rates in the RBCTs were too low and too inconsistent to produce meaningful data.

These eight night, hit-and-run, very infrequent visits were such a disaster for the farmers concerned, that it is amazing that any reduction in bTb was seen in the cattle at all. But even this total shambles delivered a 50 percent reduction in cattle Tb in the centre of the proactive zones. The authors continue their critical analysis:
Rosie Woodroffe contended that there was a 70% reduction in badger activity in the proactive triplets by the end of the RBCTs and that this correlated directly with a similar reduction in the population density. Repeated trap-culling makes badgers trap shy and disperses them. A reduction in activity cannot be construed as proof of a high culling rate. She also reported that the number of badgers trapped reduced at successive culls, a fact not confirmed by the data published in Nature (6) which shows that in only one triplet did badger capture reduce consistently over all successive culls. This suggests that either trapping failed to take out a sufficient percentage of badgers to reduce the population effectively, or that the immigration of badgers into triplets following a cull was significant (or of course, both). Either way, the data are inconsistent, and this supports our view that culling was grossly inefficient. We suggest that Professor Woodroffe’s conclusions are unsafe...

And on the Reactive areas, the data from which the authors describe as "even less safe":
These data show that in 7 of the 10 reactive culling triplets, culling took place over an average of only 10 months before this part of the trial was halted. In only three triplets (A, B and C) did culling take place over a number of years, 13 cull years in total. In these three Reactive triplets, a total of 800 badgers were removed over 300 sq. kms. in this time frame, equivalent to only 2.67 badgers per sq. km. over 13 cull years. The poverty of these data needs no further comment..

What the data does not show of course is how quickly any 'reactive' response was, and for how long it continued.

From bitter experience, the answer which eluded the ISG, who slickly refer to all data from the RBCT Badger Dispersal Trial in 'triplet years', is 'too slowly', 'too short' and then 'not at all'.

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