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 confirmed 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 reduced confirmed NHI in treated areas, such culls did not significantly increase NHI in the surrounding outer ring. In fact, between 1998 and 2012 these NHI slightly reduced in the outer ring . Between 2006 and 2012 they dropped by 28%, 1% and 18% in the treated, outer 2km ring, and combined areas respectively. Based on the total number of confirmed NHIs prevented between 1998 and 2012, a break-even cost to complete a badger removal exercise was calculated to be £8,693 per km2 with benefits continuing in 2012.
Proactive culling only reduced confirmed NHIs with no significant impact on unconfirmed NHIs. The more limited reactive culls had no impact on both the treated area and the outer 2km ring.
Conclusions in the RBCT Final Report, which were based on the results of a model of time-shifted early data, poorly reflect the overall 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. The RBCT conclusions are often cited to add credence to these press reports. After the first year of substantial culling in the RBCT, 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. This delay should be reflected on when accounting for the circumstances and assessing impact of the 2013 culls.
This work was restricted to looking at data showing total TB breakdowns over all zones. Further work to examine breakdowns by zone or groups of zones may reveal more.