Bovine TB prevalence in Scotland and prospects for this to increase

In April 2012 the Farmers Guardian reported⁵ that 60 cattle were slaughtered due to TB in Scotland following one of the worst outbreaks of bovine TB in Scotland for several years. In addition to this, Farmers Weekly reported⁶ the following

Epidemiologists from the Animal Health Veterinary Agency are currently attempting to discover the source of the outbreak, which has been described as a "mystery" because the dairy herd was fully closed and no animals has been introduced to the farm since 1988.

Although the Scottish Herald reported⁷ that scientists had ruled out wildlife being behind the outbreak [Editor: How is this established?], the number of badgers may have substantially increased in Scotland in the last decade. In fact when the number of applications for badger control licenses in Wales and Scotland are compared over the last decade, the number of applications appear to have reduced in Wales but have increased by a factor of about 10 in Scotland. In fact the number of applications in Scotland now roughly match the number of applications in Wales. All these trends can be seen in the graph below.


Data shown in the above graph was taken from References 10, 11 and 12. The numbers of applications each year in Wales were calculated by summing the number of applications made to the Countryside Council for Wales¹¹ and the Welsh government¹².

It should be noted that numbers for Scotland may not have increased by a factor as great as 10. In fact if numbers for Scotland prior to 2001 are included, the numbers over 15 years have only increased by a factor of about 4. However this is still a substantial increase particularly when numbers for Wales have reduced over the last decade.

Care should be taken when looking at this data because an increase in the number of applications may be due to reasons other than an increase in badger numbers. Indeed the distribution survey of badger numbers carried out between 2007 and 2009 in Reference 8 reports that there is only a moderate badger population in Scotland, which appears to have increased since previous studies. However, if badger numbers are increasing, as the number of badgers increase so will the frequency with which badgers come into contact with cattle. If the badger population were to become infected, this increasing badger population would be expected to increase the incidence of TB in cattle. Professor Donnelly commented in Reference 9 that roughly 50% of bovine TB incidents could be attributed to infectious badgers. Professor Donnelly was deputy chair of the Independent Scientific Group during the Random Badger Culling Trial which was conducted between 1998 and 2005 and was jointly responsible for designing the trial. Her comment relates to the initial proactive badger culls in the trial. This trial was conducted in areas where (unlike in Scotland) the incidence of TB is high. As such the proportion of incidences attributable to badgers in Scotland would be expected to be significantly lower than 50% in areas where TB is not endemic in badgers. Currently Scotland's badgers are considered to be largely TB free.

All this may have grave implications for Scotland in their attempts to maintain their bovine TB free status. The climate in Scotland is becoming more suitable for supporting a growing badger population. Between 1961 and 2003 "growing degree days" have increased more rapidly in Scotland than in the more southern regions of the UK (see Fig 7 in Reference 13). Grass and pasture provide a rich habitat for earthworms which form a large component of a badger's diet¹⁴. Badger population dynamics are significantly affected by erratic weather patterns, a fact that makes these mammals vulnerable to potential impacts of climate change. Milder winters and longer springs/summers favour badger survival and increase the numbers of cubs born the next spring.¹⁶ However the winters of 2008/09, 2009/10 and 2010/11 saw large snowfall amounts widespread and very low temperatures.¹⁵. Graphs which plot Central England Temperature clearly show how the temperatures in England plummeted during these years. Conversely, Fig 67 shown below shows how temperature, when smoothed over 10 years, has increased since the mid 1980's.


This is a good illustration of climate change and it is widely believed that these long-term temperature increases will continue.

It should also be pointed out that the prevalence of bovine TB in cattle in Scotland is very low as shown in the following graph.


In the above graph the percentage of restricted herds was calculated by dividing the number of herds under TB movement restriction (non-OTF) by the total number of herds.

The following graphs show the incidence in cattle rather than in herds. They were generated from data in References 1 and 2.







REFERENCES

1. Number of holdings and cattle in Scotland
2. TB restricted herds and cattle reactors by county in Great Britain
3. Badgers and Bovine Tuberculosis 2009 by the Mammal Society
4. Badgers and Bovine TB in Scotland published in 2000
5. Sixty cattle slaughtered after bovine TB outbreak in Scotland. Farmers Guardian. April 2012.
6. Scotland's TB free status 'not threatened by outbreak' - 4/13/2012 - Farmers Weekly
7. TB outbreak 'not caused by wildlife'. Herald Scotland. May 2012.
8. Scottish badger distribution survey. Scottish Badgers charity. 2007-2009.
9. Estimated proportion of confirmed herd breakdowns attributed to infectious badgers. Personal communication from Christl Donnelly. Jan 2012.
10. Applications to interfere with Badger setts. FOI request. Scottish Natural Heritage. April 2012.
11. Applications to interfere with Badger setts. FOI request. CCW. April 2012.
12. Applications to interfere with Badger setts. FOI request to Wales Government. May 2012.
13. A spatial analysis of trends in the UK climate since 1914 using gridded datasets
14. The European badger. Badger Trust. Revised October 2011.
15. Climate of the United Kingdom. Wikipedia. Last modified on 13 May 2012.
16. CLIMATE CHANGE: the impact on biodiversity. 2006. Earthwatch Institute.