As noted on page 6 of last weeks’ edition dated 25 July, a Test and Vaccinate or Remove (TVR) approach to wildlife intervention work is seen as the lowest risk option for DAERA to implement.
That analysis is set out in documents to support the ongoing public consultation on potential ways to control bovine TB in wildlife. The consultation runs until Friday 25 September and explores the merits of non-selective culling of badgers in TB hotspot areas by controlled shooting, as well as implementation of TVR and a third option of just vaccinating badgers.
While the department has not identified a preferred approach, it is pretty clear from all the documentation that vaccination-only is not a viable alternative given the current high rates of disease among both cattle and wildlife in NI.
That leaves non-selective culling and TVR as the two main options, with a non-selective cull seen as higher risk for the department mainly because of the potential impact on biodiversity and ecosystems in a cull area if significant numbers of badgers are removed by shooting.
At the same time, the DAERA analysis points out that non-selective culling by either controlled shooting or after badgers are caught in cages or stopped restraints (a type of snare) are all likely to deliver benefits over and above costs.
The only other option likely to bring benefits that outweigh costs is when TVR is done when badgers are caught in stopped restraints.
Model
The statistical analysis relies on a mathematical model produced by the Animal and Plant Health Agency in England. It has been used to predict the number of TB herd breakdowns that would be avoided in a 100km2 area after seven years of wildlife intervention work (four years of non-selective culling or TVR, followed by three years of badger vaccination).
Each herd breakdown avoided is expected to deliver a saving of £27,500.
The model results show that non-selective culling, mainly by controlled shooting would deliver a net benefit of £1.165m in each area, compared to £0.91m when badgers are culled when caught in stopped restraints. TVR when badgers are caught in the same restraints is third at £0.67m.
However, while the output from a model often provides useful analysis, it is actual trial work in the field which provides the most definitive results.
In the case of TVR and the trial work done by DAERA in a 100km2 area of Banbridge between 2014 and 2018, the department’s own analysis showed it had “no obvious impact on cattle herd bovine TB levels”.
It remains the only completed trial work on a TVR approach. In the DAERA consultation document there are a couple of references to a pilot TVR study in southwest France, started in 2023, but no results are presented.
The results of the £4.5m TVR trial are reported in a research paper published in the journal Vet Record by Menzies et al in 2021.
The study was very resource-intensive, with bait used to lure badgers into cages.
In the first year, all captured badgers were tested, vaccinated and released, while in years 2 to 5, badgers that tested positive for TB were culled, with non-infected animals vaccinated, before being released.
Of the 280 badgers that were captured and released in the first year, 180 were tested for TB and 21.1% gave positive results. By the fifth year, the number testing positive had dropped to 5.6%, so the study did show a significant downward trend in disease rates in badgers.
Over the five years, there were 1,520 unique capture events (some badgers were caught multiple times), with 1,420 tested and 146 (10.3%) giving a test-positive result. Just 108 badgers were actually culled.
Estimates
Best estimates at the time suggested there were 560 badgers in the 100km2, with an average of 54% captured each year, and between 2% and 10% culled annually. However, badgers in the 100km2 area were actually more at risk of being hit by a car than being culled via TVR. In 2016, a total of 29 badgers in the area were killed by cars, compared to 11 that were removed in the study.
That potentially suggests there are a lot more badgers in the area than 560 and the Menzies paper does urge some caution around the population estimate. The paper also notes that caution is required in interpretation of the results, given that the trial was not replicated and there was no formal control area to compare against.
However, the report authors did conclude that the TVR study provided “invaluable data on the logistics and resources required” to undertake similar work in the future.
Other issues of concern relating to TVR
1. A key assumption underpinning the results of the Banbridge TVR trial was that there were 560 badgers in the 100km2 area (5.6 per km2). However, recent work by researchers at Queen’s University, using thermal imaging technology to actually count badgers, shows populations of 20 to 30 per km2 are commonplace in NI. A lot of resources would be required to capture a significant proportion of badgers in these areas.
2. In the TVR trial, the researchers estimated they caught 54% of badgers each year. In effect, 54% of badgers would be tested for TB, compared to 100% of cattle in the same area.
3. TVR relies on a Dual Path Platform (DPP) test to check whether badgers have TB, before deciding to vaccinate or cull. The test has a sensitivity of 69%, which means 31 out of every 100 infected badgers will be turned back into the wild, thereby maintaining a source of disease.
4. The TVR study was done in an area with “relatively higher” rates of TB in cattle in 2011 and 2012. Back then, TB herd incidence rate stood at 6 to 7%, compared to around 10% at present.
5. The TVR study showed that 10.3% of captured badgers had TB, yet analysis of badgers killed by cars consistently shows disease rates of at least 20%. It is also generally recognised that this post mortem test might only be detecting around half of TB infected badgers.