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Targeted gassing of badgers and associated challenges

Some background

In South West England, badgers were gassed in the late 70's and early 80's in one of two ways.

Firstly, badgers were gassed reactively where there was evidence that cattle had recently become infected by tuberculous badgers. 166 such areas were targeted averaging 7 km2. See Page 69 of Reference 14. These operations were commonly referred to as "fire-brigade" operations.

Secondly, gassing was deployed proactively over wide areas, the largest of which was in the parish of Thornbury in Avon and Gloucestershire. This area covered 104 km2. (See Reference 3) Another was at Hartland in North Devon. This area covered 62 km2.

As stated in Section 19 on Page 4 of Reference 9, both of these types of operations were performed by persons employed by or under the control of the Government and licensed by the Ministry.

Some results

Gassing was only implemented in the South West and no gassing took place in the rest of England and Wales. See Page 69 of Reference 14. Also, although gassed setts were revisited for up to two years after initial gassing to prevent infection re-establishing, gassing was only applied to 166 areas over a period of 8 years. In view of this, the question could be asked as to whether a sufficient number of operations were performed to make a noticeable impact on the overall situation in the South West. However these areas averaged 7 km2 in size, appear to not include additional larger areas at Thornbury and Hartland (104km2 and 62km2 respectively), and TB in the late 70's was only persistent in the South West in small isolated pockets. In view of this, it could be argued that these operations when combined should have been substantial. The following graph shows the impact of all these operations when the TB level in 1974 is used as reference. This was done by setting the level in 1974 to 100%. Herd incidence was taken from the table on Page 141 of Reference 13.

Herd incidence due to bovine TB in England from 1971 to 1983

The following graph show the impact of the wide area clearance deployed at Thornbury. Data shown were taken from Tables 2 and 3 in Reference 11.

Herd incidence due to bovine TB in Thornbury from 1966 to 1990

It would appear from this that wide-area gassing had substantial beneficial impact on reducing TB in cattle (see Fig 92) whilst the localized targeted gassing had no beneficial impact (see Fig 91).

Unfortunately the situation is not as straight cut as this because in 1976 imported TB-prone Irish cattle started to be subjected to pre-export testing in addition to the post-import testing measures which were taken already. These imports were confined almost entirely to Scotland and north east England. See page 7 of Reference 14. This would have to a certain extent concealed any beneficial impact of the targeted gassing shown in Fig 91. The table below shows the extent to which the introduction of pre-export testing reduced confirmed TB incidences in cattle (a) outside the South West in the rest of England and Wales and (b) in the South West. This table was copied from Page 53 of Reference 14. $alt

Challenges presented by targeting gassing at infected setts

Healthy badgers on a farm are considered to be an asset as habitat is taken up by badgers which do not pose a threat of TB transmission to cattle on that farm. This implies that there would be a 2-fold benefit in removing badgers from setts detected to contain infected badgers and leaving setts containing healthy badgers. This approach is widely favoured as it would minimize the pointless and detrimental removal of healthy badgers and is in fact favoured by many in the farming community. However there are challenges to this approach of which two are as follows. Firstly there is the challenge of identifying which setts contain infected badgers and not leaving hazardous setts which contain infected badgers. Secondly there is the challenge of removing all badgers from a targeted sett without scattering any surviving and infected badgers. If not all badgers are removed, the surviving badgers are likely to recolonize with badgers in neighbouring social groups and this is likely to spread infection to these groups and nearby cattle.

Identifying infected setts

A method currently being developed which has potential2 for detecting badger setts which harbour infectious badgers is Polymerase Chain Reaction (PCR). PCR is a biochemical technology in molecular biology which amplifies DNA.1 Unfortunately infected badgers appear to excrete bacilli intermittently. Reference 3 reported that TB organisms from badgers in advanced stages of the disease were detected intermittently in faeces, urine, sputum and discharging bite wounds. In view of this, early detection of infected setts may prove to be an issue. However the importance of this is not known.

Another approach, which is sometimes referred to as the countryman's approach, is where very sick and isolated badgers are shot on welfare grounds as practiced by Bryan Hill.12 It is worth reading the accounts of Bryan's work, the background behind this work and the results which he has obtained in Reference 4. Mr Hill has adopted an extremely targeted approach whereby very sick badgers are shot. Although the accounts are anecdotal after reading the accounts in Reference 4, Mr Hill appears to have enjoyed considerable success on his own and neighbouring farms. Although doubts were expressed in Ref 15, this success suggests that the majority of incidences where infection crosses from badgers to cattle is due to badgers in the late stages of the disease. Indeed J.Gallagher and R.S. Clifton-Hadley in 2000 came to the following conclusion in Reference 5. Others succumb to florid disease and whilst those with lesser lesions may merely act to maintain infection amongst the badger population, it is probably the smaller number with advanced disease, that result in overspill from this reservoir of infection, to infect cattle and to a lesser extent other species.

Scattering of badgers caused by gassing

Regarding the scattering of badgers when setts are gassed, Reference 7 states that the efficacy of fumigation is often unpredictable and is unlikely to achieve more than an 80% reduction of the resident population. The following shows an extract taken from Reference 7. The success of fumigation also depends on the ability to deliver a lethal concentration throughout the badger sett, which is affected by the diffusion characteristics of the gas, the structure of the sett and the nature of the substrate.

Fumigation of setts involves either pumping gas into the sett or relying on diffusion. Diffusion is used when the gas is produced from tablets or powders deposited into the sett entrance. Although diffuse fumigation requires less equipment it is unlikely that the gas would distribute evenly throughout the sett. The effectiveness of fumigation depends on the ability to achieve a lethal dose throughout the sett, which will vary according to the characteristics of the gas, rate of input, topography, volume of the sett and the characteristics of the surrounding substrate. The behaviour of badgers may also influence gas dispersal, as badgers may evade the gas by moving deeper into the sett.

Badger setts can be large and complicated with many entrances and interconnected tunnels, as well as a number of blind ending tunnels and nesting chambers. In addition, some setts initially identified as large may in fact be composed of several separate setts with no interconnecting tunnels. An added complexity in estimating the size of a badger sett is that they are often found in woods or dense undergrowth, which makes it difficult to detect all entrances. There is no method for assessing the complexity of sett structure. The little information on sett structure highlights the variation with the volume of setts ranging from between 0.7m3 to 38.3m3. Of particular concern is the proportion of blind-ended tunnels, in which it is unlikely that gas will fully disperse and blind-ended tunnels may comprise a significant proportion of the sett. This issue led Defra to conclude that

'Due to the complexity of badger setts it is unlikely that a lethal concentration of any of the agents discussed below would always occur throughout the whole of a sett, which is also suggested from theoretical models and experimental data on burrow fumigation on other animals'.

A substantial amount of modelling and experimental work has been carried out in an attempt to determine levels of gas dispersal in a badger sett. The modelling work used computational fluid dynamics (CFD) to model gas dispersal under a range of conditions, such as different gas production methods, sett structure and soil substrate. This work indicated that target concentrations were rarely achieved. However, physical experiments, which involved measuring gas concentrations along a simple artificial tunnel, showed that target concentrations could be reached. This discrepancy in results meant that neither modelling work nor the data from physical experiments could be used to predict gas dispersal in more structurally complex setts. Further work suggested that it might be possible to reconcile results from models and experiments, although it was concluded that obtaining lethal concentrations throughout complex setts maybe difficult.

Effective control by fumigation relies on all individuals in a social group being killed with a single application, but this may not always be possible if setts are large, inaccessible or some individuals may be absent. Wet and windy weather conditions may also prevent fumigation. The efficacy of fumigation is often unpredictable and is unlikely to achieve more than an 80% reduction of the resident population. Repeating the application may improve the success of fumigation although this is unlikely to lead to 100% removal. Earlier gassing strategies relied on repeated applications and although a large proportion of the social group was removed, it was difficult to assess efficiency.
Reference 8 also states the following. Excavated badger setts suggest that approximately 20% of total tunnel length comprises blind-ended tunnels. There is a risk with all methods [Phosphine, Hydrogen cyanide, Carbon dioxide with and without argon, and Carbon Monoxide] that animals in blind-ended tunnels may not be exposed to lethal concentrations of gas. It was noted in Reference 9 that in the Thornbury exercise by 31 August 1976, 177 sets had been gassed, in some two thirds of the 100 sq km area, and it had been necessary to re-gas on 235 occasions. One sett had to be gassed 19 times. However Reference 10 states: ... following complete clearance of badgers from the Thornbury area, repopulation took a long time (~10 years). However, this recovery was characterised by a rapid initial partial re-population through immigration, followed by a slower increase through breeding. Although it was not stated in Reference 10 how this conclusion was reached, the contribution which immigrating badgers made to the need to regas setts is an unknown.

Discussion and conclusions

It would appear from the above graphs and reported findings that unless technological developments are made to address challenges associated with detecting infected setts and dispatching all badgers from those setts, deployment of targeted gassing relative to wide area deployment is high risk. This is because of (a) inability in current gassing technology to cope with blind ends in complex tunnel systems and (b) the greater perturbation in small areas where the boundary length is large relative to the area encompassed. Indeed the results of targeted gassing in the late 70's, as reported above, are not encouraging. It suggests that if there has been no significant improvements since the late 70's, targeting gassing at setts which are detected to be infected in a highly infected area may deliver insignificant benefit. In fact in the late 70's, badgers were removed across areas which averaged 7 km2. If operations were to be performed today in smaller areas which harbour higher disease levels, both such factors may exacerbate any failings in the methods used.

However strong evidence does exist that gassing over the wide area (104 km2) at Thornbury delivered substantial and long lasting benefit. This exercise and the results obtained are described and illustrated in Ref 16.

References

  1. Polymerase chain reaction. Wikipedia. Last modified on 13 October 2013.
  2. An inter-laboratory validation of a real time PCR assay to measure host excretion of bacterial pathogens, particularly of mycobacterium bovis. Travis, E. R., et al. (2011). PLoS One, 6(11), article e27369.
  3. Mycobacterium bovis in the European badger (Meles meles): epidemiological findings in tuberculous badgers from a naturally infected population. RS Clifton-Hadley et al. Epidemiol Infect. 1993 Aug;111(1):9-19.
  4. Bryan Hill
  5. Tuberculosis in badgers; a review of the disease and its significance for other animals
  6. Gassing of badger setts
  7. Fumigation as a badger culling technique
  8. Review of effectiveness, environmental impact, humaneness and feasibility of lethal methods for badger control
  9. Bovine Tuberculosis in Badgers. Report by the Ministry of Agriculture Fisheries and Food. November 1976.
  10. Science Advisory Council/Bovine Tuberculosis Science Advisory Body Joint Group on Defra's Bovine TB consultation
  11. The occurrence of Mycobacterium bovis infection in cattle in and around an area subject to extensive badger control. R S CLIFTON-HADLEY et al. Epidemiology Department, Central Veterinary Laboratory, Surrey. Accepted 3 October 1994.
  12. Bovine TB - A Way Forward
  13. 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)
  14. BADGERS AND BOVINE TUBERCULOSIS - REVIEW OF POLICY. MINISTRY OF AGRICULTURE, FISHERIES AND FOOD. G M Dunnet, D M Jones, J P Mclnerney. March 1986.
  15. Final Report of the Independent Scientific Group on Cattle TB
  16. The influence of gassing operations at Thornbury
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Last Modified 06 Jun 2017 13:42
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