Impact of badgers on bovine TB in different areas of GB

Introduction

Results of the Randomised Badger Culling Trial showed that when badgers are culled, confirmed new incidents of TB in cattle herds clearly reduced whilst little (if any) reduction was found in unconfirmed new incidents. This page looks at the levels of confirmed and unconfirmed TB in different parts of Great Britain and questions if the relative number of confirmed and unconfirmed incidents can be used to identify how badgers are contributing to TB levels.

Pressure to downplay badgers as a source of bovine TB transmission

Mainstream research for tackling bovine TB in the UK is currently largely focussed on locating solutions which avoids the need to cull wildlife. This is largely due to the fact that badgers are the main source of TB in wildlife and badger culling is extremely unpopular. In addition to this, culling badgers effectively and humanely has technical challenges which need to be overcome. As such it is unfortunate that allocating tax payers money to address these challenges will be badly received by the paying public. On top of this, large unsolved challenges currently exist with vaccinating badgers. In fact vaccinating badgers on a national scale is currently unviable.

Whole Genome Sequencing

In spite of the resultant temptation to point the finger of blame at cattle only, one area which is currently receiving attention from publicly funded Biotechnology and Biological Sciences Research Council (BBSRC) grants is Whole Genome Sequencing (WGS). WGS is currently being used to examine the relative roles of cattle and badgers in transmitting TB on farms that repeatedly become infected in high incidence areas.^3,4

However data sampling is a particular concern for the utility of WGS methods.³ As such the method can only be applied to a small number of datasets. In fact it can only be used to examine the situation in distinct parts of the UK when and where sufficient data is available.

How does culling badgers reduce confirmed and unconfirmed new herd incidence?

The Independent Scientific Group (ISG), who were charged with analysing and reporting the results of the Randomised Badger Culling Trial (RBCT) to government, reported the impact (or lack of it) of culling on unconfirmed cattle incidents as follows.⁵

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

Analyses of all (confirmed and unconfirmed) incidents revealed attenuated estimates of the impacts of proactive culling, in comparison with analyses considering confirmed incidents only (compare Table S5 with Table 1 in the main text). To investigate this, we examined analyses of unconfirmed incidents (Table S6) only and found considerable overdispersion inside trial areas, estimated effects that were all consistent with no effect of proactive culling on unconfirmed incidents and many estimates that were in the opposite direction to the significant effects found on confirmed incidents. For these reasons we conclude that there is no evidence of an impact of proactive culling on unconfirmed incidents and focus our attention on the analyses based on confirmed incidents only.⁵

In fact, in the 2016 study of the more comprehensive RBCT data illustrated in Ref 7, proactive badger culling may have slightly reduced unconfirmed cattle incidence. Nevertheless, such culling was found to have had no detectable influence on the prevalence of unconfirmed cattle breakdowns. As such, when looking at prevalence, reducing the badger element only clearly reduced the confirmed component of cattle herd breakdowns and only had a small (if any) reduction on unconfirmed incidence.

How does culling cattle reduce confirmed and unconfirmed new herd incidence?

The following graph shows how suspending the culling of cattle impacted cattle incidents when testing was suspended in 2001 for 9 months due to Foot and Mouth.



In South West England the above graph shows that suspending the culling of cattle clearly shifted the number of unconfirmed incidents to a higher level. The impact on confirmed incidents is less clear.

In areas where TB was low such as in the North of England, suspended cattle culling clearly increased confirmed as well as unconfirmed incidents. This is likely to be due to the fact that in the North of England herds are tested less often than in South West England so infections have more time to reach a stage where they can be detected during post-mortem examination and tests. The relative contribution from badgers can only be validly attributed to the confirmed/unconfirmed proportion across an area where the testing interval does not change. The area subject to annual testing has steadily increased across South West England and Wales from 1996 up to January 2010 when all herds in these regions became subject to annual testing.

Incidents in the North of England where herds are typically tested once every 4 years are shown in the graph below.

What does the proportion of confirmed herd incidents signify?

There are several complicating issues which influence why post-mortem examination and tests fail to confirm infection. However over years and across an area where the testing interval does not change, the fact that removing badgers in the RBCT only clearly reduced the confirmed component suggests that where the confirmed-to-unconfirmed ratio is high, badger-to-cattle transmission is particularly important. This is provided that other complicating issues are small.

The TB situation in England, Wales and Scotland

The following graphs show confirmed and unconfirmed new herd incidence in England, Wales and Scotland. It should be noted that the 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 2011¹ and England started to adopt this classification in April 2016.²



In addition to this more blood tests are being used and in certain areas of Great Britain herds are now being tested every 6 months instead of 12. Both of these recent introductions are likely to reduce the proportion of incidents which are confirmed on account of the greater number of unconfirmed incidents which more frequent testing and the less specific blood tests will introduce.

Having said all this, it is very striking how the confirmed proportion in Fig 269 is much less in Wales than it is in England. Although the amount of testing in Wales started to ramp up more than in England from about 2008 onwards, the relatively low proportion of confirmed incidents in Wales before 2008 suggests that Wales had (and perhaps still has) a smaller badger problem than in England. A history of testing regime changes in Wales is described in Reference 6.

An alternative explanation stems from the fact that farmers in Wales are paid much higher compensation for TB reactors than farmers in England. This may be affecting disclosure rates. Compensation payments, unconfirmed proportions and number of cattle reactors disclosed for each new herd incident is shown in the graphs below. Both the unconfirmed proportions and the number of cattle reactors disclosed for each new herd incident are clearly higher in Wales than in England.

The TB situation in different counties of England and Wales.

The following graphs show confirmed and unconfirmed incidents in counties of England and Wales in descending order of confirmed to unconfirmed breakdown ratio. This order was determined by querying the APHA SAM May 2016 download and summing all herd incidences between Jan 1996 and Dec 2015. Click here to see the database query (MySQL) and the results returned. Click the thumbnail below to see the graphs at full size.



Note the small confirmed proportion in Somerset, Dorset and Pembrokeshire. Somerset and Dorset are 2 of the 3 counties where badger culling is currently taking place in England (May 2016). Pembrokeshire is the county in Wales where badgers are being vaccinated as part of the added measures in the Intensive Action Area. Although there are many contributing factors which reduce the meaning of these graphs, the above graphs suggest that these counties have not got a badger population in which badger-to-cattle TB transmission has been particularly high. If the areas selected in each county do not stand out from the situation in the rest of the county, this raises the question of whether these areas should have been singled out for addressing badger TB. For instance in Wales, if the confirmed proportion is an indicator of the strength of badger-to-cattle transmission, it may have been better to have selected an area in Mid Powys than an area in Pembrokeshire. Click on the thumbnail above to clearly see Figs 273b and 273p which shows the disease profiles in these 2 counties.

References

1. 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.
2. 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.
3. Exploring the richness of Mycobacterium bovis strain diversity to decipher the epidemiology of bovine tuberculosis ecology. J Wood et al. University of Cambridge. Start Date: 01 April 2016. End Date: 31 March 2019.
4. The genetic and spatial epidemiology of bovine tuberculosis in the UK: from molecular typing to bacterial whole genome sequencing. PhD Thesis, University of Glasgow. H Trewby. BBSRC funded. Last Modified: 13 May 2016 07:51.
5. Impacts of widespread badger culling on cattle tuberculosis: concluding analyses from a large-scale field trial. SUPPLEMENTARY MATERIAL. CA. Donnelly et al. 2007.
6. Prevalence of bovine TB in Wales, England, Scotland and Ireland.
7. Presentation and discussion of raw RBCT badger cull data