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Effect of culling on TB infection in badgers

Summary

A major concern with the culling of badgers is that the culling of badgers is thought to substantially increase TB infection in badgers. This can result in increased transmission of infection from badgers to cattle and result in the culling of badgers making TB in cattle worse. This article illustrates data acquired in the Randomised badger Culling Trial (RBCT) to examine the basis for this concern. This work found that between 2000 and 2002, when Foot and Mouth occurred, the proportion of culled badgers detected with TB infection tripled. However TB levels remained relatively stable in years either side of Foot and Mouth.

The claim

The following shows an extract taken from the RBCT Final Report published in 2007 (Page 78 of Reference 1). The work on which this claim is based is described in Reference 7.

Effects of culling on M. bovis infection in badgers

4.26 Culling profoundly altered the prevalence and distribution of M. bovis infection in badgers. Statistical analyses adjusting for variables such as age, sex, triplet, and various measures relating to the probability of detecting infection, revealed that prevalence rose on successive proactive culls (Woodroffe et al., 2006b). Overall, by the fourth cull the prevalence of infection was approximately double that recorded on the initial cull (odds ratio 1.92, 95% confidence interval 1.51-2.45) after adjusting for other factors (Woodroffe et al., 2006b). Because of this rise in prevalence, the reduction in the density of badgers achieved by proactive culling was not associated with an equivalent reduction in the density of infected badgers.

The following graph has been extracted from the work in Reference 7.


The challenge

THE RBCT Final Report1 states the following on Page 41 in Section 2.31.

In addition to fieldwork capability, the trial was to make substantial demands on laboratory capacity within the Veterinary Laboratories Agency (VLA), who were to undertake the post mortem examination of badger carcasses as rapidly as possible after they became available from culling operations, along with tissue sampling, culturing and genetic typing of M. bovis infection when discovered. This in turn necessitated the provision of additional resources, not only of laboratory staff but also the appropriate facilities required by health and safety regulations.

The overgrowth of normal post mortem bacteria affects ability to culture TB significantly. RBCT datasets2,3 show if the number of days between chilling and necropsy exceeded seven. In 1999 and 2000 this applied to 98% of stored carcases. Badgers trapped between August 1999 and March 2000 had to be frozen following instructions from the Health and Safety Executive.4

A percentage of carcases (9·15% of total carcases) had to be stored (either chilled at 0-4ºC or frozen at -20ºC) for more than 7 days prior to necropsy. Ninety-eight percent of these stored carcases were in 1999 and 2000 with the remainder in 1998 and 2002. Badgers trapped between August 1999 and March 2000 had to be frozen following instructions from the Health and Safety Executive. At other times they were stored frozen when the rate at which they were culled and delivered exceeded that at which they could be examined. There is evidence that such storage may reduce the sensitivity of detecting M. bovis by culture [18].


The RBCT Final Report1 states that when carcasses were stored for greater than 7 days before necropsy, they were almost always frozen.

Guidelines5 issued by the United States Department of Agriculture in 2011 states the following. These guidelines imply a concern when the delay between death and postmortem reaches 24 hours. As such a delay which often reached 7 days in the case of badgers in the RBCT would be expected to substantially inhibit the ability to detect TB.

General Recommendations
Despite the stringent decontamination protocols used in the laboratory, tissue specimens can still be overgrown by environmental fungi and bacteria, thereby impeding the ability to recover any viable mycobacteria present in the tissues. To minimize overgrowth, it is important to collect tissues from the animal as soon as possible post mortem. Collect tissues from animals within 2 hours whenever possible. If the animal has been dead for more than 24 hours, contact the laboratory prior to submitting samples for histopathology.

The data

Detailed data describing badgers culled and their circumstances can be downloaded from Reference 2 and 3. The tables in References 2 and 3 refer to 8,863 of the 8,910 proactively culled badgers and 2,064 of the 2,069 reactively culled badgers reported in the RBCT Final Report.1 Although badger trapping did take place in January 2001 in triplets C and E no badger data for year 2001 is included in the data sets referred to in References 2 and 3. This is because each badger year went up to the end of January in the next calendar year. Badger year is shown along the x-axis of the graphs shown below so no badgers were culled in year 2001. That is why that year is omitted from the graphs.

The results

The following graph shows the proportion of badgers culled in the RBCT which were found to be infected using the data described above. 95% confidence intervals are also shown.

Culling effects 1

The following graph shows the proportion of badgers in the proactive triplets only which were found to be infected. This graph appears to be slightly different from the graph extracted from the work in Reference 7. Although I think the two graphs plot the same data, the prevalence in year 2000 appears to be slightly higher in the extracted graph. Although the difference is slight, this difference is influential when assessing whether or not the data indicates TB prevalence increased with each successive cull. In plotting the graph below I have made the assumption that in the data in References 2 and 3 each year extends from February 1 to January 31.

Table 436

The following graph shows the proportion of badgers in the reactive triplets.

Table 436

Please note that in 1999 only 3 badgers culled in 73 were detected to have TB. This would have made sample error very large and very little reliance can be placed on the TB level found in this year. To a lesser extent the same applies to the result in 2000.

Culling in the different triplets started in different years as shown in the following bar charts.



Culling in triplets E, F and G all started in the same year (2000) so should show a more consistent impact of culling when plotted against calendar year. The proportion of badgers infected each year in these triplets are shown in the following graph.

Culling effects 2

Foot and Mouth occurred in 2001. To reduce the impact of Foot and Mouth and bring any effect of badger culling into focus, the effect of culling in triplets D, I and J can be examined. Culling in these triplets started in 2002. Data from these triplets alone are shown in the graph below.

Culling effects 3

Discussion

Obviously something caused disclosed infection to triple between years 2000 and 2002. The RBCT Final Report in Reference 1 firstly attributes increased TB infection in badgers to reduced testing on cattle for 11 months due to Foot and Mouth, a resultant increase in infection in cattle, and consequential increased transmission from cattle to badgers. It secondly attributes increasing infection in successive years to the culling of badgers and the resultant perturbation of badgers causing increased spread of infection. However the tripling of detected infection between years 2000 and 2002 may have been largely due to an improved ability to detect infection in the fresher carcasses postmortemed after the lull caused by Foot and Mouth. Indeed the proportion of badgers found infected with TB remained relatively stable in years either side of Foot and Mouth.

Increased infection in badgers and the knock on effect of increased transmission to cattle is often used to add weight to the argument that the culling of badgers is ineffective and can actually make TB in cattle worse. This study has found that substantial demands were made on laboratory capacity to post mortem badger carcasses as rapidly as possible. Indeed the ability to recover any viable mycobacteria during post mortems is impeded by the overgrowth of environmental fungi and bacteria. This build-up is so quick that guidelines published by the US Department of Agriculture recommend post mortems are carried out within 24 hours. In the RBCT post mortems, the need to perform post mortems quickly necessitated the provision of additional resources, not only of laboratory staff but also of appropriate facilities which were required to meet health and safety regulations. If the increased number of TB positives in badgers was due to an improved ability to detect TB as a result of these stipulations, actual infection in badgers may not have increased. The RBCT Final Report claimed that culling profoundly altered the prevalence and distribution of M. bovis infection in badgers. Did it?

Conclusion

If infection in badgers did not increase, claiming infection levels increased is not valid. If so, this does not support the argument that the culling of badgers is ineffective and can actually make TB in cattle worse.

References

  1. Final Report of the Independent Scientific Group on Cattle TB
  2. Supporting Data Set 1 extracted from http://www.pnas.org/content/103/40/14713.full?tab=ds. Culling and cattle controls influence tuberculosis risk for badgers. R Woodroffe et al. 2006.
  3. Supporting Data Set 2 extracted from http://www.pnas.org/content/103/40/14713.full?tab=ds. Culling and cattle controls influence tuberculosis risk for badgers. R Woodroffe et al. 2006.
  4. The prevalence, distribution and severity of detectablepathological lesions in badgers naturally infected withMycobacterium bovis. HE JENKINS et al. Epidemiol Infect. Oct 2008; 136(10): 1350-1361. Published online Nov 30, 2007.
  5. Guidelines for Surveillance of Bovine Tuberculosis in Wildlife. USDA. June 2011.
  6. RBCT badger details received from APHA in Februrary 2018 in response to FoI Request ATIC1248.
  7. Culling and cattle controls influence tuberculosis risk for badgers. R Woodroffe et al. www.pnas.org. Contributed by David R. Cox, July 26, 2006.
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