Policy paper

Setting the minimum and maximum numbers in badger cull areas in 2018: Advice to Natural England

Published 13 September 2018

Overview

Natural England is the competent authority for badger control licensing for the purpose of preventing the spread of bovine TB. It is a requirement of the Guidance and the licences to set a minimum number in advance of each year’s cull in an authorisation letter that is issued to each cull company once the licensing authority is satisfied that the cull company’s preparations, planning and funding are sufficient to deliver a successful cull. The purpose of setting a minimum number under the current licence is to ensure that the cull company delivers the required level of population reduction in order to achieve the expected benefits in controlling bovine TB.

This advice to Natural England sets out the approach for estimating the badger population in the cull areas in 2018 and the minimum number of badgers to be removed.

The minimum number is intended to achieve a 70% reduction of the population relative to the initial starting population. The culling objective is for no more than 30% of the starting population to remain on conclusion of the cull. The 70% target is derived from the Randomised Badger Control Trial (RBCT) where it was estimated that the culls achieved a mean of 70% control of the starting populations across the 10 areas ^{[footnote 1]}, which resulted in disease reduction benefits for the cattle herds in those areas.

Culling also needs to “not be detrimental to the survival of the population concerned” within the meaning of Article 9 of the Bern Convention on the Conservation of European Wildlife and Natural Habitats. For that purpose Natural England set a maximum number of badgers to be removed from the licensed area.

The approach to setting the minimum and maximum numbers was published by Defra in 2014 to 2017, in advice to Natural England. ^{[footnote 2] [footnote 3] [footnote 4] [footnote 5] [footnote 6]}

The estimate of population size must relate to the whole culling area, including any land within that area on which no culling is planned to take place. Any population estimate will have some degree of uncertainty which leads to an interval around the population estimate within which the true population is likely to lie. However, operating with uncertainty does not prevent an effective cull from being carried out, as shown during the RBCT culls, where no minimum numbers or targets were set.

This advice is divided into four sections:

• Section A covers the areas in Somerset and Gloucestershire where culling began in 2013
• Section B covers the areas where culling began in 2015 to 2017
• Section C covers the new areas that will begin culling in 2018
• Section D covers concluding remarks affecting all of the areas

Areas will be ordered for numbering firstly by starting year, secondly by TB risk area with High Risk Area and Edge areas coming before Low Risk Area, thirdly by alphabetical order of the county ^{[footnote 7]} and fourthly by decreasing area size.

This year, an area will be licenced in the Low Risk Area for the first time. Although they will not have minimum and maximum numbers issued as part of their licence due to the different objective of a cull in this area, it is included here for completeness.

Section A: Area 1-Gloucestershire, Area 2-Somerset

In 2018, Area 1-Gloucestershire and Area 2-Somerset will continue supplementary badger control.

Both minimum and maximum numbers of badgers to be removed are required in order to sustain the benefits of licensed badger control while avoiding local extinction. In 2017, Area 2-Somerset removed less than the minimum number, however sett surveys carried out by APHA in spring 2018 indicate that the number of active setts is actually slightly lower than that in 2014, 2015 and 2016, therefore the population appears to be kept at the level achieved during the intensive cull.

In recognition of the lower numbers of badgers removed in Area-2 in its fifth year of culling and the reduction in the number removed in Area-1 and to a lesser extent in Area 3-Dorset in its third year, the baseline to set the minimum and maximum number is reconsidered. In 2017, the minimum and maximum numbers were set by considering historic patterns of cull numbers in both the RBCT and over the last four years in the current culls. In general the number of badgers culled in second and subsequent culls was approximately 40% of the number culled in the first year. However in the current industry-led culls, the average has dropped to 36% of the year one cull with a declining trend in later years not seen in the RBCT. Therefore 36% of the year one cull total is set as the baseline and the minimum and maximum numbers are set equidistant above and below the baseline so that the difference between them is equivalent to 25% of the pre-cull population. For Area 1-Gloucestershire the minimum is therefore 125 and the maximum is 540, for Area 2-Somerset the minimum is 109 and the maximum is 578.

This approach will be kept under review as culling in contiguous areas and the larger size of the cull areas could affect the relative levels of immigration and reduce the comparability of cull returns to those in the RBCT. Therefore the amount of effort deployed by the cull companies and its spatial distribution will continue to be monitored given the uncertainty in the size of the remaining badger population.

Section B: Areas 3 to 21

As several hundred badgers have been removed from these areas in previous culls, methods based solely on an un-culled population are no longer appropriate. Instead, as in previous years, surveys of the number of active setts were used to estimate the current population.

In order to ensure that accurate assessments of sett activity were available to provide robust evidence to inform an estimate of the population and minimum numbers, all cull companies were instructed to carry out a thorough sett survey programme. APHA surveyors carried out a Quality Assurance check in sample parcels across the whole of the cull areas in year two areas.

As described in detail in the 2015 advice to Natural England, the population can be estimated by multiplying the number of active setts by the number of badgers per active sett.

As described last year, the starting population is estimated by reducing the estimate of the population at the start of year two by one-sixth, to account for 20% population growth in the intervening period, and adding the number culled in year one. The minimum and maximum numbers are then calculated as in previous years, see Table 1 in Annex A. Given the overall uncertainty associated with the methods and the range (lower to upper limits), we consider that it is still more prudent to manage the uncertainty by defining a realistic minimum number that aims to achieve the desired level of population reduction to secure the anticipated disease control benefits than to define it too high, with a risk of removing too many badgers.

In 2018, Area 15-Devon has no minimum number, this is because the population estimates indicate a population below 30% of the pre-cull population.

Section C: New areas for 2018

Over the last three years, 19 successful first year culls have been carried out, these have taken place across the High Risk and the Edge Area, all taking place in the autumn and all using similarly trained contractors putting in similar levels of effort and using a mixture of controlled shooting and cage trapping. Therefore, we now have a better picture of what success looks like.

However, we have not improved the method of setting initial minimum and maximum numbers as the methods based on the National Sett Survey in 2016 and 2017 have not been proven particularly accurate. Now that we have a larger sample size of areas that cover a significant proportion of the HRA and experience of what a successful cull looks like in the field, we can use data form the previous culls to set the initial minimum and maximum numbers. We therefore draw on the experience of previous culls and take the average number of badgers culled per km2 in previous first year culls as the anticipated cull and set the minimum and maximum numbers equidistant around that value.

Across the 19 areas the average number of badgers culled has been 3.18 ^{[footnote 8]} badgers per km2. The minimum and maximum numbers are therefore set at 2.70 and 3.66 per km2 which are equidistant about the average and maintains the 70% to 95% ratio between the minimum and maximum number.

Given the overall uncertainty associated with all methods and the range (lower to upper limits), we consider that it is still more prudent to manage the uncertainty this year by defining a realistic minimum number that can be revised in the light of new data, than to define it too high, with a risk of removing too many badgers.

The minimum and maximum numbers of badgers for the new areas in 2018 are shown in table 2 of Annex A. This approach simplifies the process of setting the minimum and maximum numbers and allows companies to plan earlier without the need for complex calculations. As with previous years this range may prove too high for some areas and too low for others, but provided sufficient effort is expended the minimum and maximum numbers can be updated for a given area in the light of the experience in the field following the methodology used in previous years.

In 2018 a cull will be licensed in the Low Risk Area for the first time. As explained in the December 2017 consultation document ^{[footnote 9]} and the Government’s subsequent response ^{[footnote 10]}, minimum and maximum numbers are of less utility in the Low Risk Area given the different aim of the cull. However the area, named Area 32-Cumbria, is included in this paper for completeness. Further details on the area and the evidence supporting a cull there is set out in Annex B.

Section D: Conclusions

As badger culling continues we have learnt that we were often dealing with more uncertainty than we anticipated, and therefore in defining minimum numbers in subsequent years we needed to avoid false levels of confidence. As with previous years, we need to consider two realistic scenarios:

1. that during the cull, there is accumulating evidence that the number of badgers in the cull area is low, and that the number of badgers removed, despite a high level of contractor effort sustained across the whole cull area, is towards the lower end of our estimates. In this scenario, if the minimum and maximum numbers were set too high, Natural England would need to consider adjusting the numbers down to bring them in line with the actual circumstances being observed in the cull, so as to manage the risk of too many badgers being removed; OR

2. that during the cull, there is accumulating evidence that the number of badgers is higher than the minimum and maximum numbers suggest, either because the cull company quickly exceeds the minimum number, or because feedback from observations suggests there is a higher level of activity observed than expected. In these circumstances, Natural England would need to consider the need to compel the cull company to continue the cull by revising the minimum and maximum numbers upwards to ensure that the optimum disease benefits can be secured.

The weather in 2018 has been more extreme than in recent years with heavy snow in much of England in late February and early March and a hot, dry summer with high temperatures from mid-July to mid-August. Both of these may have affect badger population growth and this should be borne in mind when assessing cull effectiveness this year and in comparing data from 2018 to subsequent and previous years.

Daily data collected through the course of the cull about the level of effort being applied across the cull area, and locations of badgers removed, will enable Natural England to build an assessment of progress towards the cull total. This will allow Natural England to assess whether the estimated population was a reasonable reflection of the true population.

If the evidence suggests that there are more badgers than the estimates indicated (e.g. because the number of badgers killed per unit effort is relatively high), Natural England will have the ability to revise the number upwards at an appropriate point, to ensure that the cull company is required to carry on the cull in order to achieve effective disease control.

Conversely, if the estimates are too high there will be a risk of removing too many badgers. In these circumstances, Natural England could, on the basis of careful consideration of the evidence and provided that the level of effort applied by the cull company has been sufficient, adjust the maximum number downwards at an appropriate point.

Annex A: Minimum and maximum numbers

Table 1: Minimum and maximum numbers for cull areas in their second, third and fourth years of badger control.

Area	Minimum number	Maximum number
Area 3-Dorset	109	329
Area 4-Cornwall	15	264
Area 5-Cornwall	20	330
Area 6-Devon	706	1,585
Area 7-Devon	195	504
Area 8-Dorset	617	1,826
Area 9-Gloucestershire	28	927
Area 10-Herefordshire	165	433
Area 11-Cheshire	147	434
Area 12-Devon	1,210	2,170
Area 13-Devon	426	956
Area 14-Devon	374	714
Area 15-Devon	None	170
Area 16-Dorset	1,733	3,363
Area 17-Somerset	946	1,583
Area 18-Somerset	320	571
Area 19-Wiltshire	959	1,976
Area 20-Wiltshire	333	772
Area 21-Wiltshire	831	1,471

Table 2: Size, and minimum and maximum numbers for new areas for 2018

Area	Size (km2)	Minimum number	Maximum number
Area 22-Cornwall	1,272	3,432	4,658
Area 23-Devon	594	1,602	2,175
Area 24-Devon	510	1,376	1,867
Area 25-Devon	311	840	1,140
Area 26-Devon	303	817	1,109
Area 27-Devon	210	566	768
Area 28-Devon	194	523	709
Area 29-Gloucestershire	431	1,163	1,579
Area 30-Somerset	622	1,679	2,278
Area 31-Staffordshire	1,180	3,184	4,321
Area 32-Cumbria	190	N/A	N/A

Annex B: Summary of area 32-Cumbria

Infection present in cattle

A ‘hotspot’ area (HS21) was established in Cumbria when a cluster of breakdowns of Mycobacterium bovis genotype 17:z was observed in 2016. This genotype of M. bovis had previously only been seen in Northern Ireland. Suspicion of wildlife involvement prompted the collection of found dead badgers and deer to determine if they were infected.

A report detailing the outbreak up to the end of June 2017 was published in December 2017 as part of the Low Risk Area of England (LRA) report ^{[footnote 11]} and a report covering the period to the end of December 2017 will be published today. The strain of M. bovis (17:z) associated with HS21 had not previously been identified in cattle herds in Great Britain. Current evidence suggests that it was most likely that an undetected infected bovine from Northern Ireland brought the strain of M. bovis to the area and then, either that animal or a subsequently infected animal, caused the infection in badgers within the area. From the index case in November 2014 to 7th August 2018 there had been a total of 30 outbreaks across 26 premises. The novel genotype in this area provides clear evidence that local spread of TB is occurring resulting in infection with M. bovis that was previously not known to exist in Great Britain. There has been onward spread of TB to other farms inside and outside of the HS21 area as a result of cattle movements. However, for a number of herds the transmission pathway is unclear and infection from wildlife (badgers) is a plausible risk pathway.

Infection present in badgers

From September 2016, found dead badger carcasses were collected and tested for M. bovis through post mortem and tissue collection for culture. M. bovis isolates have been obtained from three badger carcasses found dead in the area. This level of M. bovis positive cases in badgers is a cause for concern, although our sample is too small to estimate prevalence in badgers in the area. Initial results of surveillance in found dead badgers were published in February 2018 ^{[footnote 12]}. The latest figures (as of 7-8-18) are three positives out of 46 badger carcasses tested ^{[footnote 13]}. Seven deer carcasses have also been collected; one was unsuitable for sampling, the rest were negative.

Whole genome sequencing (WGS) of the badger isolates confirmed that M. bovis has been transmitted from cattle to badgers and that badgers are infected with the same Northern Ireland strain of M. bovis. The WGS obtained from these isolates are not exact matches to those obtained from the cattle on the farms on which the badgers were found. However, one of the badger isolates matches the ancestral WGS isolate from the first cattle breakdown case (approximately 4 km distant). Although we only have WGS from three badger isolates at present, they appear to exhibit greater diversity than the cattle isolates, suggesting that M. bovis is well established in the local badger population. The presence of infection in badgers indicates that badgers are a potential source of cattle infection and represent a risk for further onward transmission.

Additional cattle measures

Upon the confirmation of infection in badgers, additional cattle measures were introduced in HS21 in September 2017. These include:

• six monthly whole-herd testing of all cattle herds
• pre-movement testing of all cattle over 42 days moving out cattle herds
• movement restrictions applied to a herd with inconclusive reactors (IR) alone (OTF status suspended; OTFS) pending the 60-day IR retest
• discretionary parallel interferon-gamma testing of OTFS breakdown herds, in addition to the mandatory blood testing of all the OTF status withdrawn (OTFW) herds
• severe interpretation of skin tests for both OTFW and OTFS breakdown herds
• samples from all cattle with visible lesions of TB at post mortem to be submitted for culture and genotyping
• ad hoc surveillance of camelid (skin testing followed by serology) and goat (skin testing only) herds
• biosecurity advice has been provided to farmers in the area

Assessing the options

An existing simulation model developed by APHA ^{[footnote 14]} has been used to model the potential epidemic length in badgers in HS21. This model uses both the badger population distribution in the area as estimated with sett surveys and knowledge of general epidemiology of bovine TB in badgers. Modelling of the effect of different control policies e.g. culling only, vaccination only, and culling supplemented by vaccination was carried out by APHA. Culling was found to be the intervention most likely to result in the removal of infection from the badger population. The addition of vaccination post-culling appears to make little difference in the probability of eradication of TB or on the number of infected badgers. A scientific paper on this modelling work is being prepared for publication in a scientific journal.

The CVO and CSA have considered the issue and their advice is that badger culling is the most appropriate measure in this instance.

Defining the intervention area

Sett surveying was carried out by APHA across the area from late July to December 2017, with some additional areas surveyed in early 2018. These have been used to estimate the density of setts in HS21 and to estimate badger social group territory size.

The Wildlife disease control intervention area (WDCIA) is composed of:

• The minimum infected area based on:
  • the location of the infected badgers, associated farms and contiguous breakdown areas, plus a radius of the estimated average social group territory based on main sett distribution
  • the location of another farm with a TB breakdown very strongly suspected on epidemiological grounds to be badger related
• a buffer zone also based on estimated average badger social group territory size surrounding the minimum infected area, to take into account the possibility that infection may have already spread in the badger population. The boundary was adjusted to adhere to natural boundaries to badger movement as far as practical to minimise any possible perturbation effects. ^{[footnote 15]}

The size of the WDCIA defined in East Cumbria happens to be greater than 100km2 which is the minimum size of a cull area in the High Risk and Edge areas although this is not a requirement.

Culled badgers will be tested and the results of this, alongside the ongoing intensive surveillance of cattle, will inform future disease control measures in badgers and cattle in this area. The level of disease in cattle has remained constant despite the introduction of cattle measures and there has, as yet, been no significant geographical spread of the disease outside of the HS21 area.

References

1. Woodroffe, R., Gilks, P., Johnston, W. T., Le Fevre, A. M., Cox, D. R., Donnelly, C. A., Bourne, F. J., Cheeseman, C. L., Gettinby, G., McInerney, J. P. and Morrison, W. I. (2008), Effects of culling on badger abundance: implications for tuberculosis control. Journal of Zoology, 274: 28–37. doi:10.1111/j.1469-7998.2007.00353.x ↩

2. Setting the minimum and maximum numbers for Year 2 of the badger culls. Advice to Natural England. August 2014 ↩

3. Setting the minimum and maximum numbers for Year 3 of the badger culls. Advice to Natural England. August 2015. ↩

4. Setting the minimum and maximum numbers in Dorset for Year 1 of the badger cull. Advice to Natural England. August 2015. ↩

5. Setting the minimum and maximum numbers in licensed badger control areas. Advice to Natural England. August 2016 ↩

6. Advice to Natural England on setting the minimum and maximum numbers in licensed badger control areas in 2017. ↩

7. Where an area spans county borders, the county comprising the highest proportion of an area will be used to name the area. ↩

8. Standard deviation 1.18, range 1.81-7.21 ↩

9. Bovine TB: consultation on proposals to introduce licensed badger control to prevent the spread of bovine tuberculosis in the Low Risk Area (England) ↩

10. Summary of responses to the consultation on proposals to introduce licensed badger control to prevent the spread of bovine tuberculosis in the Low Risk Area (England) ↩

11. Regional Six-monthly Report of Descriptive Bovine TB Epidemiology for the Low Risk (Four Yearly Testing) Areas of England ↩

12. TB surveillance in wildlife in England ↩

13. Results are pending from a further three carcasses and a further five carcasses were unsuitable for sample collection ↩

14. Smith et al (2012) Comparing badger (Meles meles) management strategies for reducing tuberculosis incidence in cattle ↩

15. The boundary of the WDCIA and the locations of badger carcasses are not being released on security grounds. ↩