Badgers and Bovine Tuberculosis
Credit: Ian Wade
Summary
The badger is not currently of conservation concern in Britain. The Mammal Society’s IUCN-compliant Red List of British Terrestrial Mammals classifies it as Least Concern, and the Review of the Population and Conservation Status of British Mammals (2018) ranks its current and future prospects as Good, based on long-term assessments of its population size and geographical range. However, despite their success they have been seriously persecuted in the distant and recent past and so are legally protected.
Badgers are affected by Bovine Tuberculosis (bTB) and can be infected by the same strains of Mycobacterium bovis (the causative agent) as are found in local cattle. The incidence of bTB in cattle has increased, and the epidemic has spread geographically since the 1980s and continues to do so.
There is compelling evidence of a wildlife reservoir of bTB infection. However, the quantitative significance of badgers and other wildlife species as agents of infection in cattle is not clear, and nor is the principal method of transmission of the disease between wildlife and cattle.
There is evidence that transmission amongst cattle and their movement patterns has an important role in the geographic spread of the disease. The results of the Randomised Badger Culling Trial (RBCT) identified both positive and negative effects of badger culling on the disease in cattle: although there was evidence of decreased bTB rates within proactive cull zones, an important finding was that bTB increased in the areas surrounding the cull. The trial suspended reactive culling (i.e. culls in response to disease outbreaks) after initial indications that it could be counter-productive in terms of bTB incidence. The steering group of the project concluded that badger culling over large areas was unlikely to yield a net benefit to cattle farmers and society.
Research is continuing to find a vaccine for cattle, and an injectable vaccine is available for use in badgers. Research is also underway to improve methods of diagnosing bovine tuberculosis in cattle and badgers.
The Badger in Britain
The badger Meles meles is Great Britain’s largest land carnivore and one of the most abundant. Results from three national badger sett surveys carried out in the 1980s, 1990s and most recently 2011-2013 show that the population in England and Wales has increased over that period. The current estimates are: England: 384,000 (95% Confidence Intervals 259,000-711,000); Scotland: 115,000 (85,000-198,000); Wales: 62,900 (47,000-104,000); Britain (562,000 (391,000-1,014,000)1.
Generally, landscapes with rolling hills and a mixture of habitat types is most suitable for badgers because it provides food resources and cover at suitable sites for their setts; whereas lower badger densities are found in upland areas.
It has been suggested that the increase in the badger population between the 1980s and 1990s was partly a consequence of the better survival of adults, owing to the effectiveness of a series of badger protection acts passed by Parliament during the period 1973 to 1992. However, the number of badgers also increased in an area where there was no persecution over a long period of time, suggesting the involvement of other factors.
Bovine Tuberculosis in Britain
Bovine Tuberculosis is a disease caused by the bacterium Mycobacterium bovis. It is primarily a disease of cattle and other ungulates (such as goats and deer) but can infect a wide range of wildlife hosts and can cause disease in humans.
The Ministry of Agriculture, Fisheries and Food, MAFF, made strenuous efforts during the 1950s and 1960s to eliminate the disease from cattle by regular testing of cattle and slaughtering of those that reacted positively to the test. This was largely successful insofar as the number of herds containing positive reactors dropped from 40% pa in 1932 to 0.2% pa in 1960.
However, success was not uniform and, in southwest England, the incidence of positive reactors in the regular testing programme remained higher than in the rest of Britain. Since the 1980s, the numerical incidence of TB in cattle has increased and the disease has spread geographically outwards from its original foci in southwest England https://www.ibtb.co.uk/. Defra currently identifies three TB risk areas in England, the most severely affected endemic area (called the High Risk Area), an adjacent zone of geographic spread (called the Edge Area) and beyond this the largely unaffected non-endemic area (Low Risk Area).
Bovine Tuberculosis in badgers
Research into badger ecology and TB epidemiology in Gloucestershire since the 1970s has shown that:
Badger density has increased overall during the course of the study, though it has declined to some extent recently;
TB prevalence varies over time, possibly in a cyclic manner;
TB does not spread rapidly through the badger population but tends to persist in clusters;
TB is not a major cause of death in badgers;
Badgers sometimes survive for many years while infected;
Transmission of the disease from mother to cub is potentially important;
Infection via the respiratory tract and bite wounding are probably the main routes of transmission between adult badgers;
The relationship between badger density and disease prevalence is not linear, i.e. it does not follow that high badger density corresponds to high levels of TB;
Badgers with advanced TB may behave differently to others.
The approach to reducing the risk of infection passing from badgers to cattle has involved killing badgers, badger vaccination and increased farm biosecurity. The control of TB in cattle involves regular testing and the slaughter of infected animals.
Badger Culling
A variety of badger culling strategies have been employed in the UK since the 1970s in combination with cattle test and slaughter. While these were associated with reduced incidence of TB in cattle in some circumstances, such evidence was largely anecdotal and the approach has clearly not succeeded in eliminating, or even containing, the problem.
The randomised badger culling trial (RBCT) represented an important milestone in our understanding of the effects of badger culling on TB in cattle. The trial aimed to compare the effects of two different culling strategies; Proactive culling, which aimed to reduce badger populations to very low levels over the whole of a trial area, and Reactive culling, where only badgers close to TB outbreaks in cattle were removed. Other areas acted as experimental controls as they were surveyed for badger activity, but no culling took place. The trial was started in 1997 and results were reviewed in 2003. An increase in the incidence of TB in cattle in reactive-culling areas was observed, compared with the no-culling areas. Although these results were preliminary, the reactive culling part of the trial was halted, meaning that there are few scientific data on the effectiveness, or otherwise, of reactive culls.
The final report of the Independent Scientific Group on Bovine TB, who oversaw the RBCT, was published in 2007. It reported a decrease in TB incidence among cattle in proactively culled areas, but an increase in TB among cattle in peripheral areas around the cull zone, resulting in little to no net benefit. Subsequent analysis of the data showed that the negative effect waned over time but that the reductions in cattle TB remained small. Cost-benefit analyses predicted that any net financial benefits were likely to be extremely small in comparison to the costs of culling.
The trial has been criticised on various grounds, particularly in relation to difficulties in enforcing the experimental protocols. For example, participation was not compulsory, so some landowners in proactive-culling and reactive-culling areas refused to allow badger culls to take place on their land. Conversely, it was difficult, if not impossible, to prevent clandestine (and illegal) culling in no-culling areas. Nevertheless, it has been argued that the RBCT represents what is practically possible in the ‘real-world’, and it still remains the only rigorous attempt to scientifically evaluate the impact of badger culling on the incidence of the disease in cattle in the UK.
Since 2013 the UK Government has been licensing farmer-led culls of badgers in the affected areas of England, with the licence conditions seeking to reduce the negative effects of culling identified during the RBCT. Two areas were licenced in 2013, and by 2020 a total of 54 areas have been licenced. However, the farmer-led culls differ in several important respects to the RBCT, including the use of ‘free-shooting’ of badgers in addition to cage trapping and are conducted over a longer time interval. Consequently, direct comparisons are difficult. Preliminary analysis of data from the first three farmer-led cull areas showed that after four years of culling there had been significant reductions in cattle TB but no change in the third area which had only undergone two years of culling. The small number of areas involved in this study limits what can be inferred from the results as they may not be representative of remaining cull areas and so it is not yet possible to draw any robust conclusions about the impacts of the culls.
Regardless of whether the remaining badger culls can be shown to reduce TB in cattle, questions regarding the ethics and proportionality of the approach remain. The current culling policy permits the indiscriminate destruction of a native species across wide areas of the English countryside, including land which may not even contain cattle. Recent announcements of the scale of culls to be licensed raise the prospect of very considerable reductions in badger populations in some regions, with potential consequences for the conservation status of the species. The effect of the culls was not assessed in the population estimates above.
Badger culling and hedgehog conservation
Hedgehogs are currently classed as Vulnerable in Great Britain, having undergone significant declines over several decades. There are suggestions that badgers are an important contributor to this decline and therefore that badger culls will benefit hedgehogs. It is important to remember that badgers and hedgehogs have co-evolved for millennia. Also, while badger numbers are currently increasing, this is from a very low starting point, their numbers having been depressed significantly over the last few hundred years by persecution. While badgers are undoubtedly predators of hedgehogs, and at a landscape-scale, there is some evidence for a negative association between badgers and hedgehogs, the causes of hedgehog declines are largely unrelated to badgers.
Vaccination
Although there is obvious potential for attempting to control the disease by vaccinating badgers, cattle, or both, the apparent simplicity of this approach belies some inherent difficulties. The available vaccine (BCG) is the same as that used to reduce the incidence of TB in humans around the globe. The vaccine does not fully protect all animals from infection but may be sufficient to reduce and possibly eventually eliminate bTB.
Vaccinating cattle would be relatively simple in principle. However, once a cow has been immunised, it is no longer possible to discriminate between an infected animal and an immunised one, using the tuberculin skin-test, which is primary screening tool used in the UK. However, there are other approaches to identifying infection, and Defra are about to fund field trials of a cattle vaccine, and a test to differentiate infected from vaccinated animals (a DIVA test). However, the roll out of a cattle vaccine would not eliminate infection from badgers.
Unfortunately, the UK Government has stopped funding work to develop a means of delivering a vaccine to badgers in the form of a bait that could be distributed at setts, although research continues in the Republic of Ireland. In the UK, an injectable vaccine has been licenced since 2010 for administration to trapped badgers, and is being used in several locations, largely by wildlife groups. Research shows that it is effective in reducing the severity of disease in badgers and helps to protect cubs from becoming infected when a sufficient number of adults in their social group are vaccinated (the herd immunity effect). Encouragingly, a recent review commissioned by the UK Government and led by Professor Charles Godfray has recommended that badger vaccination should play a more substantial role in the management of TB in cattle, and we wait to see how this may be implemented.
Cattle Testing
The relative roles of badger to cattle, and cattle to cattle, transmission in the persistence of TB in the UK herd are not well defined. However, several studies show that herd size and cattle movements are significant risk factors for new TB incidents. Also, there was a substantial increase in the geographical spread of TB in cattle after a period of restocking necessitated by the 2001 Foot and Mouth Disease epidemic. A statistical analysis of post-2001 cattle movement data confirmed that cattle imports from infected areas was a significant contributor to the geographical spread of TB in cattle in subsequent years.
The current test for cattle (the tuberculin skin test) has relatively low sensitivity at the individual level, which means that it may miss many truly positive animals. Hence, in Britain, it is only likely to detect 80% of infected cattle. On the other hand, it may indicate 1 in 5000 cattle as infected, when in fact they are not. A newer Interferon gamma test blood test, which detects more truly positive animals but also indicates a higher proportion as infected when they are not, is also now being used in conjunction with the skin test in certain circumstances.
Defra is currently seeking to improve the effectiveness of cattle testing by imposing stricter controls in relation to positive results and exploring the value of additional complementary tests.
Other Potential Wildlife Reservoirs
The possibility of other wildlife acting as reservoirs for bovine TB has long been recognised and many species have been sampled. Although infection has been identified in a variety of species (including red fox, deer, grey squirrel, field vole, stoat, mink, mole, brown rat) the prevalence is so low that they are unlikely to play a significant role and the disease would not persist in the absence of infected cattle or badgers. Possible exceptions may occur in some localised areas with high densities of fallow and red deer.
Husbandry
Studies of cattle farms largely in south-west England have shown widespread and frequent visits of badgers to farm buildings in some locations. Observations on contamination by badgers of stored farm feeds and other resources available to cattle suggested that these visits can pose a serious transmission risk to cattle. However, the evidence from these and other studies also indicate that not all farms are likely to be visited by badgers. An experiment to assess methods of preventing badger access to farm buildings clearly showed how this could be achieved using simple barriers and proofing measures, and can stop up to 100% of badger visits. Although this may not be the main route by which cattle become infected on every farm, implementing such measures seems like a sensible precautionary approach. Advice on these and other means of reducing contact between badgers and cattle can be found on the TB Hub. https://tbhub.co.uk/.
Conclusions
The increasing incidence of TB in cattle and badgers is clearly an important and complex problem which will not have a single simple solution.
It is clear that badgers are involved in the epidemiology of TB, but also that transmission between badgers and cattle of infection travels in both directions. Understanding more about the relative contributions of the two species to infection in cattle at a local level may help us to develop more effective targeted approaches to controlling the epidemic.
If the government is going to achieve its target of England being Officially TB Free by 2038 it seems likely that some form of TB management in wildlife would be necessary.
Badger culling as it is currently carried out in England is an unsustainable and disproportionate response to the problem, with little evidence that it will provide a substantial benefit in terms of reducing TB in cattle. The Mammal Society therefore opposes the current cull policy.
The recent Godfray report stated that “Moving from lethal to non-lethal control of the disease in badgers is highly desirable” and the Mammal Society supports this move.
There is a clear need to continue investigations into various aspects of the epidemiology and control of the disease, particularly into improving the efficacy and feasibility of vaccination and developing better tests for TB, in both badgers and cattle.
There is also a need to ensure that the costs and benefits of any proposed control measures are fully evaluated, not only in financial terms but also in terms of the impact on badgers and other wildlife, and on the farming industry. Any approaches to manage infection risks from badgers need to be proportionate, humane and sustainable.
References
[1] Mathews et al. (2018) A Review of the Population and Conservation Status of British Mammals. A report by the Mammal Society under contract to Natural England, Natural Resources Wales and Scottish Natural Heritage., Peterborough: Natural England.