TB trends in the badger cull areas
Recently a report was published by APHA which contains summary TB statistics for the first 10 licensed ‘badger control’ or cull areas. Click here to download this report.
The report contains measures of cattle TB incidence (the rate that new breakdowns occur) and TB prevalence (the % of herds under restrictions). These statistics are provided for the three years prior to each cull starting (this gives an idea of any trends or patterns prior to culling) and for each year during the culls.
The report covers up to the end of 2017, so provides data for the first 10 badger cull areas which started between 2013 and 2016. Cull areas licensed in 2017 and 2018 have not been underway for long enough for the data to be available, but they will be included in subsequent reports. The map to the right and table below show the details of the current badger cull areas active in England (in 2018).
|County||Year started||size (km2)|
The statistics in this report are potentially quite interesting and give an idea of the TB trends in these areas, indicating whether TB is increasing, decreasing or stable. However, it is very important to note that on their own these data cannot tell us why these changes are occurring. This is clearly outlined by the authors of the report who state that “these data alone cannot demonstrate whether the badger control policy is effective in reducing bovine TB in cattle”. Unfortunately several organisations and individuals have ignored this and used these latest figures as evidence the cull are “working”. They may very well be working, but these latest statistics can not prove this for a number of important reasons.
If you look at any of the TB statistics (either in various reports, using the TB dashboard, or using ibTB) you can see that the numbers vary up and down over time and from area to area. It is therefore possible that TB could increase or decrease due to other factors which are unrelated to culling. This is clearly demonstrated by the fact that TB trends are often not stable in cull areas in the years prior to culling.
To demonstrate that culling is causing changes in TB in cattle we need to compare the rates or numbers of breakdowns in cull areas to areas without culling (see example below). There may be a benefit from culling (ie fewer TB breakdowns than would have occurred without culling) even if the raw trends in the data (ie the green arrow below) are decreasing, stable or even increasing (as the increase may be much less than in areas without culling, as in the bottom left graph – suggesting a benefit). Likewise, even if TB is decreasing this does not mean that culling is the cause, as it may have declined anyway due to other factors (as in the top middle and top right graphs).
The changes in TB incidence in the RBCT (randomised badger culling trial) were estimated by comparing areas with culling to matched controls without culling, producing the following results below (Taken from Godfray et al. (2013) ).
Again I stress that the above figures are changes in TB incidence (rate of new breakdowns) relative to the control areas without culling.
To date the only study that has compared TB incidence rates in cull areas to the comparison areas without culling is the study by Brunton et al. (2017) which uses only the first two years of data and is also summarised here. There are many limitations and caveats to this work, but to date this is the only evidence that that badger culling is causing changes in TB in cattle. An updated report/paper using four years of data is currently being prepared by APHA. Until then the data in this recent report may be interesting, but unfortunately they cannot answer the question “are the culls working?”.