Randomised Badger Culling Trial: Impact, based on more extensive data
In 2007 the Independent Scientific Group (ISG) reported to the UK government the impact on bovine tuberculosis (TB) in cattle of the Randomised Badger Culling Trial (RBCT). Badgers were culled between 1998 and 2005 across 100 km2 (nominal) zones in the West of England. The results were based on a model of New Herd Incidence (NHI). It was concluded that reactive culling generated overall detrimental effects, while proactive culling achieved very modest overall benefits at the cost of elevated incidence in surrounding areas. This work looks at more extensive RBCT data to examine if these findings hold true. Instead of presenting the results of a model, this work directly illustrates the data. The Animal and Plant Health Agency supplied this data in March 2016. Such data covers a greater number of years (1986 to 2012) and includes the prevalence of herd restrictions as well as herd incidence. Whilst the proactive culls substantially and sustainably reduced cattle TB in treated areas, such culls did not significantly increase TB in the surrounding areas. In fact NHIs between 2006 and 2012 dropped by 28%, 1% and 18% in the treated, outer 2km ring, and combined areas respectively. Based on the number of NHIs prevented since 1998, a break-even cost to complete a badger removal exercise was calculated to be £8,693 per km2. This figure may be under-estimated because it takes no account of any NHIs prevented after 2012. The more limited reactive culls had no impact on both the treated area and outer 2km ring. Proactive culling only reduced confirmed TB with no significant impact on unconfirmed TB. Conclusions in the RBCT Final Report, which were based on the results of a model of time-shifted early data, poorly reflect the greater benefits seen in this more extensive data. Badger culling is highly contentious in the UK and many press reports adversely report the effectiveness of badger culling in general and the culls which started in 2013 in particular. Unfortunately the RBCT conclusions are often cited to add credence to these press reports. In the RBCT, after the first year of substantial culling, this work found that 9 years of data were needed to clearly see the full extent by which TB dropped when plotted against calendar year. The impact of the culls, which started in late 2013, may not become clear until late 2023 after a 2-year reporting delay. This work was restricted to looking at data showing total TB breakdowns over all triplets. Further work to examine breakdowns by triplet or groups of triplets should reveal more.