TB testing in Wales since 2006

In 2006 the Tuberculosis (Wales) Order introduced enhanced cattle controls. This included prohibiting trade from herds with an overdue tuberculin test. From that date, herd restrictions were applied as soon as a test became overdue.³⁰

In 2008 TB Health Check Wales was the first major initiative to test every cattle herd in Wales for TB, with the aim of establishing a current understanding of the scale of the disease. The checks were carried out between October 2008 and December 2009.³⁴

These combined measures established with more accuracy the full extent of disease within the Welsh national herd by (a) catching up on many overdue tests and (b) testing every herd.

In January 2010 annual testing was introduced in Wales and this testing has been maintained.³¹

TB levels in England and Wales.

Data shown in the above graph was extracted from Reference 32.

Will this investment in increased testing deliver any recognisable benefit relative to the situation in England? The above graph shows the situation up to the end of 2014 and the latest available data downloaded from Reference 33 each month is shown in the following thumbnail. Click it to see the graphs at full size.

Bovine TB in Wales, England, Scotland and Ireland

Data in Figs 135 and 136 were sourced as follows. Cattle numbers were extracted from DEFRA's Cattle and Calves dataset. Reactors in England, Wales and Scotland were extracted from VETNET's COUNTY ANIMAL STATISTICS dataset and SAM's 2014.03.12 monthly download. Southern Ireland's data was extracted from Refs 25 and 26. Northern Ireland's data was extracted from Refs 27 and 28.




Data for the graph above were sourced from References 1, 7 and 8. The proportion of cattle slaughtered were taken from the TOTAL number of animals slaughtered under bovine TB control measures in Reference 1. The solid and dashed lines represent the proportion when the numbers of cattle existing were given by the Welsh government (Reference 7) and the Rural Payments Agency (Reference 8) respectively.



The above graph shows how smoothing over a 5-year window can take out the annual fluctuations.

Fig 62 suggests that bovine TB levels in Gwent halved between 2008 and 2010 whereas levels in Powys remained about the same. Gwent and Powys are neighbouring counties.

Possible reasons for why bovine TB prevalence dropped between 2008 and 2010 include the use of a less potent tuberculin when TB testing, changes in disclosure due to introduction of one year testing across the whole country which appear to have led to a jump in increased testing in 2009 (see graph below), and the impact of consecutive cold winters.⁹


The larger increase in testing in 2009 does not appear to coincide with an increase in disclosure in Fig62 so the impact of test interval changes on detected incidence is questionable. It certainly does not explain the doubling in TB levels extracted from VETNET which occurred between 2006 and 2008 shown in Fig 135.

The graph below shows the impact of the number of herd tests on the number of restricted herds.


Again the increase in testing does not appear to explain the large increase in TB levels between 2006 and 2008. Note the difference in the number of total herd tests recorded in VETNET and SAM.

Central England Temperatures¹⁰ are shown in the graphs below. It is clear from Fig 66 that temperatures in mid winter reduced each year from 2006/7 for 4 consecutive winters to 2010/11. Badger population dynamics are significantly affected by erratic weather patterns. Milder winters and longer springs/summers favour badger survival and increase the numbers of cubs born the next spring.¹¹ Although speculative, there may be some association between the number of cubs born in the spring and the level of badger-to-cattle transmission.

A note on vaccinating badgers in the IAA of Wales

Badger vaccination in the Intensive Action Area (IAA) will hopefully provide useful and valuable results and achieve 'herd immunity' in the badger population - a state in which a large enough proportion of the badgers becomes protected so that the disease is not sustained in the badger population. In fact when more than a third of a badger's social group is vaccinated, the risk to unvaccinated cubs is estimated in Reference 22 to reduce by 79%. However this is due to the reduced infectiousness of badgers they are in contact with and not due to any increase in their own immunity. In fact, the following should be noted.

1. It will not be possible to vaccinate badgers at a time in the badger's life when the badger is most receptive to the vaccine because badgers do not emerge from their setts until they are at least 6 weeks old.¹⁵
2. Vaccination will not protect an infected badger. (Table S5 of Reference 22 which is shown in Reference 23 shows that the prevalence of infection in badgers after taking into account test sensitivity was about half in the period 2006 to 2009.) Any infected badger may become infectious to other badgers at a later stage and it may take several years for any infected badger to die.
3. As yet (September 2015) there is no evidence that the BCG reduces the prevalence of TB in badgers.^29,22
4. Trapping and vaccinating badgers will be difficult and expensive. A mobile fridge has to be carried to keep the vaccine below a certain temperature. The traps have to be dug in and prebaited and this incurs large staff costs.¹⁷ More cages have to be set to account for badger recaptures. In fact FERA scientists found that in 2010 and 2011 about one in every five badgers were recaught on the following night¹⁴. This effort will need to be repeated in each year of the programme because it is not currently possible to permanently mark badgers when they are vaccinated in the field. Also the duration over which the vaccine is effective is not known.
5. There will be large standard errors due to the small area of the IAA which is only 288 km². Confidence intervals when analysing cattle herd incidence during the Randomised Badger Culling Trial (RBCT) were barely acceptable¹² and the total area of the proactive areas in that trial was 1000 km². This is over 3 times greater than in the IAA. In addition to this, factors may exist in a particular area which may influence results and may not be representative of average conditions which exist nationally or in any control area which is used. (The significance of this issue is illustrated in a list item below.) In the RBCT these factors were neutralised by splitting up each treatment and control setup into ten areas which covered different counties in different parts of the country. In Wales the setup is just one big area so these factors will not be neutralised out.
6. The number of cattle herds and hence the size of sample in the IAA may reduce as the trial progresses. If this happens, this will further increase the standard error referred to above. In the IAA the 6-monthly testing regime being used is compounding stress levels on cattle and farmers, and the blood test is being used more extensively. Specificity of the blood test is far less than that of the skin test so more cattle are falsely testing positive when it is used.²¹ This may be making farmers in the IAA quit the profession at a faster rate than outside the IAA. Switching policy from culling to vaccination may have also significantly contributed to this exodus. In 2011 there were 309 herds¹⁸ in the IAA. At the start of the RBCT there were a total of 1221 herds¹⁹ in the proactive areas.
7. TB levels in cattle across whole counties fluctuate greatly from one year to the next. As shown in Fig 62 above, levels have halved and doubled in Gwent and West Glamorgan over consecutive years since 2003. Not only that but levels drop in some counties when they stay the same in neighbouring counties. For instance, TB levels in Gwent halved between 2008 and 2010 whereas levels in neighbouring Powys remained the same.

In view of the issues listed above, successful completion of this trial may require extension of treatment well beyond the frequently referenced timespan of 5 years in Reference 16. As such, completion may require substantial commitment spanning more than one government. It follows that there is a risk that the current study will be abandoned by a successive government (which may take power in the 2016 elections) before effects and benefits can be properly attributed to the vaccine. Although any programme will contribute to current knowledge, when programmes are carried out piecemeal in different parts of the country in different periods of time it is much more difficult to pool the results in order to show a collective result.

Why is it important to know the benefit of the vaccine?

This is important because if the vaccine is to be used in areas of persistent TB in order to control the disease without culling, no government will be able to justify rolling out badger vaccination over a large area which has national significance before the benefit of doing so is known.

Although prospects for farmers in the IAA seeing a significant benefit from the programme may not be very favourable, the IAA probably represents an area in the UK and Ireland where most resource is currently being put into a combined programme of improved cattle measures (testing and biosecurity) and badger vaccination. The cost of delivering badger vaccination in the IAA, over five years, is estimated to be about £5,760,000.²⁴ As such if this study can persist for long enough to generate meaningful results (as noted above, 5 years may not be long enough to achieve this aim), the results should make a significant contribution to establishing whether or not bovine TB can be reduced in cattle without culling badgers.

References

1. Bovine TB County Animal Statistics - Great Britain regional - DEFRA.
2. Number of cattle in England, Wales, Scotland, Northern Ireland, Great Britain and the UK for each year from 1866 to 2010
3. Number of holdings and cattle in Scotland
4. Cattle slaughtered in the IAA
5. Bovine TB eradication programme science review in Wales Q & A - December 2011
6. Wales Bovine TB eradication programme Questions and Answers - 4th April 2012
7. Number of cattle in Wales by county 1998 to 2010
8. Number of cattle in Wales by county 2004 to 2012
9. Climate of the United Kingdom. Wikipedia. Last modified on 13 May 2012.
10. Met Office Hadley Centre Central England Temperature Data. Hadobs. Hadcet.
11. CLIMATE CHANGE: the impact on biodiversity. 2006. Earthwatch Institute.
12. A critique of the Randomised Badger Culling Trial (RBCT)
13. Field Trial to Assess the Safety of Bacille Calmette Guerin (BCG) Vaccine Administered Parenterally to Badgers. June 2006 - June 2010 study carried out in Gloucestershire by the Food and Environment Research Agency (Fera).
14. Trapping of badgers for vaccination. FOI request to FERA. 18 April 2012.
15. Final Report of the Independent Scientific Group on Cattle TB
16. Submitted report. Wales bovine tb eradication programme - decision on culling badgers in the Intensive Action Area. Evidence presented to the Minister for Environment and Sustainable Development: March 2012
17. Badger Vaccine Deployment Programme. Gloucestershire Wildlife Trust . October 2011.
18. IAA statistics and epidemiological information. 2011.
19. The effects of annual widespread badger culls on cattle tuberculosis following the cessation of culling - Supplementary Information. HE Jenkins et al. International Journal of Infectious Diseases Volume 12, Issue 5, Pages 457-465. September 2008.
20. Bacillus Calmette-Guerin vaccination reduces the severity and progression of tuberculosis in badgers. Mark A. Chambers, et al. 1 December 2010.
21. The risk of false negatives and positives when testing cattle
22. BCG Vaccination Reduces Risk of Tuberculosis Infection in Vaccinated Badgers and Unvaccinated Badger Cubs. PLOS ONE. Stephen P. Carter et al. Published 12th Dec 2012.
23. TB prevalence of badgers in vaccination study in Gloucestershire 2006-2009. PLOS ONE. Stephen P. Carter et al. Published 12th Dec 2012.
24. Bovine TB Eradication Programme IAA Badger Vaccination Project Year 1 Report. www.cymru.gov.uk. 2013.
25. TB stats to 2012 for the Irish Republic. Department of Agriculture, Food and the Marine. ERAD Division. Reply to request. Sent 09 May 2013.
26. TB Statistics - Department of Agriculture, Food & the Marine, Ireland
27. Animal Disease Statistics - Department of Agriculture and Rural Development, Northern Ireland
28. Incidence of Bovine TB in Northern Ireland to 2012
29. Impact of the BCG vaccine on the prevalence of TB in badgers
30. History of bovine TB controls. Welsh Government. Last updated 08 April 2014.
31. Eradication programme for Bovine Tuberculosis in the UK. EUROPEAN COMMISSION. HEALTH & CONSUMERS DIRECTORATE-GENERAL. Unit G5 - Veterinary Programmes. SANCO 10845. Date submitted: 14 September 2011. Approved in 2012.
32. Bovine TB Surveillance data from 1986 to 2014 in England, Wales And Scotland. Data supplied by APHA in response to FoI ATIC0719 in October 2015.
33. Statistics - national statistics. Latest statistics on the incidence of tuberculosis (TB) in cattle in Great Britain. National statistics. From: DEFRA & APHA. Updated monthly. Currently (October 2015) downloadable from https://www.gov.uk/government/statistics/incidence-of-tuberculosis-tb-in-cattle-in-great-britain.
34. Bovine Tuberculosis. July 2011. KA Daniels. Paper number: 11/041. National Assembly for Wales.