Prevalence and transmission of bovine TB between cattle and badgers

Prevalence in badgers

The graph below shows the prevalence of bovine TB in badgers found by carrying out postmortems on road kill. The data plotted in this graph were calculated from the number of badgers diagnosed positive and negative. These numbers are shown in the legends of the following Road Traffic Accident maps.

• Northern counties in 2000, 2002, 2003, 2004
• Southern counties in 2000, 2002, 2003, 2004

It should be noted that statistical confidence due to small sample size is lowest for the year 2000.



It is possible that the detected prevelances in the above graph largely underestimate the true prevelances in view of the following explanation⁷.

During the Random Badger Culling Trial (RBCT)⁴ it was recognised that in interpreting patterns of TB prevalence in badgers, the standard diagnostic methods used were not 100% sensitive and so would provide an underestimate of the number of infected badgers. The sensitivity of the standard post-mortem examination protocol was compared with that of a detailed protocol. A sample of 205 post mortem examinations conducted under lesser time constraints than was possible for the majority of RBCT badgers revealed substantially more infected animals than did standard post mortem examinations of the same animals. In fact the sensitivity of the standard post mortem protocol was estimated to be 54.6% (95% CI 44.9% - 69.8%)⁸. Hence it is conceivable that true prevalence levels in badgers may be double those shown in the above graph.

Prevalence in cattle

The following graph shows the prevalence of bovine TB in cattle. This prevalence was calculated by dividing the total number of reactors taken from county animal statistics by the total number of cattle and calves taken from census tables. This data is contained in an archive which can be downloaded by clicking here.


Transmission between badgers and cattle

It is widely reported that the prevalence of TB in badgers and cattle is closely related and this is borne out in the above graphs in so far as the prevalence in the northern counties in both species is greater than in the southern counties. Foot and Mouth occurred in 2001 during which time testing⁵ was substantially reduced during 11 months. When the Foot and Mouth movement restrictions started to be lifted in the second half of 2001 this resulted in a large number of untested cattle being moved to different parts of the country during the restocking process. This resulted in increased cattle-to-cattle transmission and the peak in the prevalence of TB in badgers in the northern counties in 2002 suggests that significant transmission from cattle to badgers may have occurred on top of any badger-to-badger perturbation effect.

Regarding badger-to-cattle transmission, this is widely recognised and this has been reported in DEFRA-supported work by Zuckerman¹ (1980), Dunnet² (1986), Krebs³ (1997) and Bourne⁴ (2007). Christl Donnelly in March 2010 submitted a report¹⁰ which stated that based on a subset of data taken from the RBCT, results indicate that TB in cattle herds could be substantially reduced, possibly even eliminated, in the absence of transmission from badgers to cattle. Christl Donnelly was deputy chair of the Independent Scientific Group on Cattle TB (1998 to 2007), through which she was jointly responsible for the design and analysis of the RBCT¹¹.

Epidemiology of bovine TB

Spoligotyping is a very simple, inexpensive and effective tool for TB research. It is a PCR-Based method to detect and type TB bacteria in clinical samples. The following maps⁶ show the spoligotypes identified in reactor cattle in 2000 and 2005.


Dispersal of different spoligotypes to remote locations is thought to be due to cattle to cattle transmission. However it is a concern that wildlife sanctuaries are releasing a small number of badgers each year to new locations. Also a large number of traps went missing during the Random Badger Culling Trials and these traps may have contained badgers which were relocated to areas outside the culling zones. In fact, between December 1998 and 10 October 2003, 1827 cages were removed⁹. Culling during these trials started in 1998 and ended in October 2005.

Compared to cattle, badgers are relatively non-mobile and the clustering is an indication of how badger-to-cattle transmission is likely to be contributing to the spread of the disease in hotspot areas.

Impact of Foot and Mouth

After Foot and Mouth, bovine TB spoligotypes largely confined to the South West appeared in counties such as Cumbria as can be seen in the maps shown above. As stated above, this is likely to have been due to the large number of untested cattle being moved to different parts of the country during the restocking process and the associated increase in cattle-to-cattle transmission. Shown below are graphs which shows the impact of this restocking on bovine TB in Devon and Cumbria and what has been happening in subsequent years. Cumbria was by far the worst affected county where Reference 14 reports that 209,106 cattle were slaughtered due to Foot and Mouth. Devon was the second most affected county where 67,677 cattle were slaughtered.



In the above graphs cattle numbers were extracted from Reference 12 and the number of cattle slaughtered due to TB were taken from the total number of reactors given in Reference 13.

As can be seen in Reference 5 the test interval in much of Devon in the year 2000 was either 2 or 3 years. Also in Cumbria the test interval was 4 years throughout the county. Hence disclosure of the extent to which infection had increased due to restocking and movement of diseased cattle did not become fully apparent until 2004 in Cumbria. Although there was no relaxation of bovine TB restrictions to allow restocking as outlined in the DEFRA email shown in Reference 16, it was not until November 2004 that many cattle-based measures were introduced in England to tighten surveillance and reduce the risk of bovine TB spreading to new areas. In addition to this, as current in March 2012, most of Cumbria remains as 4-year testing parishes as can be seen in the map in Reference 17.

Unlike Devon where TB incidence has continued to rise after 2004, in Cumbria levels started to recover to what they were before Foot and Mouth. However levels do not appear to have totally recovered. It is possible that pockets of resistance may be due to infection becoming established in wildlife which were not infected prior to Foot and Mouth. In 2006 the European Commission issued a guidance report¹⁵ which states the following.

"It has now been reliably demonstrated that the persistence of an infected wildlife reservoir that enters into contact with cattle is a major obstacle to the eradication of TB. This obstacle should be addressed in tandem with the measures implemented in relation to the cattle population.
...
While future prospects for the development of suitable TB vaccines for use in wildlife are promising, considerable obstacles remain which make it difficult to foresee the use of such vaccination on its own as the most suitable tool to use to address the persistence of the variety of infected wildlife reservoirs worldwide in the near future. In the meantime, therefore, alternatives to vaccination, in order to address the role of infected wildlife in the persistence of TB should be implemented without any delay so as to allow the progress of the eradication programmes.
...
The elimination or reduction of the risk posed by an infected wildlife reservoir enables the other measures contained in the programme to yield the expected results, whereas the persistence of TB in these wildlife populations impedes the effective elimination of the disease."

References

1. Zuckerman, S., Badgers, cattle and tuberculosis, HMSO, London. (1980)
2. Dunnet, G.M., Jones, D.M. and McInerney, J.P., Badgers and bovine tuberculosis,HMSO, London. (1986)
3. Krebs, J.R., Anderson, R.M., Clutton-Brock, T., Morrison, W.I., Young, D. and Donnelly,C.A., Bovine tuberculosis in cattle and badgers, MAFF Publications, PB3423, London.(1997)
4. Final Report of the Independent Scientific Group on Cattle TB (2007)
5. Parish testing frequencies for England and Wales in 2000
6. Government Veterinary Journal
7. IAA statistics and epidemiological information
8. Comparison of a standard and a detailed postmortem protocol for detecting Mycobacterium bovis in badgers.
9. A critique of the Randomised Badger Culling Trial (RBCT)
10. Association between Levels of TB in Cattle Herds and Badgers
11. Members of the MRC Centre for Outbreak Analysis and Modelling Research Group
12. Cattle numbers by county from census data from 1998 to 2010 downloaded from the DEFRA web site
13. Animal and herd bovine TB prevalence by county from 1998 to 2010 downloaded from the DEFRA web site
14. Numbers of cattle slaughtered per month by county in Great Britain due to Foot and Mouth
15. Eradication of Bovine Tuberculosis in the EU
16. Bovine TB: Restrictions after Foot and Mouth in 2001
17. Map of TB testing intervals 2012