Bovine TB in Wales, England and Scotland

Data for the graph above were sourced from References 1, 2 and 3. The proportion of cattle slaughtered were taken from the TOTAL number of animals slaughtered under bovine TB control measures in Reference 1. The proportion was calculated by dividing the number of cattle slaughtered by the number of cattle existing.


Data for the graph above were sourced from References 1, 7 and 8. As before, 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 and the impact of consecutive cold winters.⁹ 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 it is likely that a five-year programme will be insufficient to produce an effect which can be reliably attributed to the vaccine even if the effect is significant and worthwhile. In addition to this the effectiveness of the vaccine will be limited for the following reasons.

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. The efficacy of the vaccine on wild badgers was unknown at the time of writing this note (June 2012). In the 4-year trial¹³ referred to above, Stat-Pak positivity in badgers was reduced by vaccination from 17.1% cases to 4.4% cases. This gives a 74% reduction and is the figure often quoted in the press with inadequate qualification. The 74% only applies to those badgers which initially tested negative to the pre-screening tests so this 74% does not represent the proportion of badgers which will become protected. Depending on the proportion already infected that proportion will be somewhat less. In addition to this, the Stat-Pak test has limited sensitivity. In fact, sensitivity of the Stat-Pak varies according to disease severity, such that sensitivity was found to be 34.4% in infected badgers with no visible lesions at post mortem, 66.1% in infected badgers with visible lesions at post mortem, 41.7% in infected badgers that excrete M. bovis; rising to 78.1% in so-called Super-Excretor badgers (Chambers et al., 2008).¹³
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 will probably require extension well beyond the frequently referenced timespan of 5 years in Reference 16. As such, completion is likely to require substantial commitment spanning more than one government. It follows that there is high 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 no government will be able to justify rolling out badger vaccination over a large area which has national significance before this is known.

Having said this, 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. (This is the situation in May 2013) As such if this study can persist for long enough to generate meaningful results, 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
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
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.