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DEFRA stats are likely to be obscuring the extent to which badgers cause cattle TB.

The Independent Scientific Group (ISG) reported at the end of the Randomised Badger Culling Trial (RBCT) that badgers were implicated in the spread of bovine TB based on confirmed new herd incidents in cattle. However they also reported that there was no evidence of culling impact on unconfirmed breakdowns. In fact this is what the ISG said.

Effects of proactive culling on total (confirmed and unconfirmed) TB incidence in cattle

Analyses of all (confirmed and unconfirmed) breakdowns revealed attenuated estimates of the impacts of proactive culling, in comparison with analyses considering confirmed breakdowns only (compare Table S5 with Table 1 in the main text). To investigate this, we examined analyses of unconfirmed breakdowns (Table S6) only and found considerable overdispersion inside trial areas, estimated effects that were all consistent with no effect of proactive culling on unconfirmed breakdowns and many estimates that were in the opposite direction to the significant effects found on confirmed breakdowns. For these reasons we conclude that there is no evidence of an impact of proactive culling on unconfirmed breakdowns and focus our attention on the analyses based on confirmed breakdowns only.1


In view of no noticable impact of the cull on unconfirmed incidents, the following list shows how DEFRA stats are likely to be obscuring how bovine TB is affecting the farming industry in Great Britain.
  1. When DEFRA switched from storing data in the VETNET database to the SAM database in 2011, confirmed cattle reactors were removed from the reported statistics. (Quantities shown in the VETNET 7th March 2011 download and the SAM 10th February 2016 download are shown here .) Depending on the proportion of cattle reactors for which postmortem tests are carried out, this statistic would be a valuable indicator of badger-TB impact.
  2. The proportion of cattle infected by TB is not reported in stats either nationally or by county. Although the stats published by DEFRA include the total number of herds, they omit the total number of cattle which exist. This total number is readily available per year from the British Cattle Movement Service (BCMS) in each county of England as shown here. This number is needed to present animal prevalence which is omitted from the DEFRA monthly stats and quarterly notice.
  3. Although DEFRA stats include separately the number of withdrawn and suspended new herd incidents, withdrawn and suspended TB restricted herds are omitted. Sample sizes associated with new herd incidents are a lot smaller than TB restricted herds. These small sizes present difficulties in certain situations where illustration of TB restricted herds would not.
  4. DEFRA stats no longer disclose confirmed and unconfirmed TB levels. Animal and Plant Health Agency (APHA) have now stopped classifying herds as confirmed or unconfirmed and classify them as withdrawn or suspended instead. Indeed incidents are classed as withdrawn (Officially bTB Free Withdrawn, OTFW) not only if TB infection in a herd is confirmed during tests on cattle after slaughter but also if the herd was deemed at elevated risk due to epidemiological reasons. Wales started classifying incidents this way in January 20112 and England started to adopt this classification in April 2016.3 Such classifications make meaning less clear.
  5. All the statistics are skewed by the inclusion of the large number of herds which contain between 1 and 10 animals. Herd prevalence is an important statistic which reveals how TB is hurting the farming industry. As such statistics without these tiny hobby herds included should be reported. Statistics need to be free of the distortion which these hobby herds inevitably introduce.


References

  1. Impacts of widespread badger culling on cattle tuberculosis: concluding analyses from a large-scale field trial. SUPPLEMENTARY MATERIAL. CA. Donnelly et al. 2007.
  2. Differences between bovine TB indicators in herds in the IAA and herds in the Comparison Area (Project OG0142): First three years, 1st May 2010 to 30th April 2013. Commissioned by the Welsh Government.
  3. A more robust approach for resolving TB breakdowns in the High Risk Area of England. Bovine TB Information Note 02/16. DEFRA. 10 March 2016.
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Last Modified 07 May 2017 08:04
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