The two processes, responsive persecution and perturbation may operate in parallel, leading to positive feedbacks which may contribute to the persistence of bTB hotspots in certain areas independent of established cattle risk factors.
A paper published today in the journal Scientific Reports shows that badger persecution may have a role in perpetuating 'bovine TB hotspots' by repeatedly disrupting the animals' social structure.
Moreover the main risk factors for cattle infection with bovine TB are those linked to cattle, not badgers.
According to the paper, 'Herd-level bovine tuberculosis risk factors: assessing the role of low-level badger population disturbance':
"Cattle risk factors (movements, international imports, bTB history, neighbours with bTB) were more strongly associated with herd risk than area-level measures of badger social group density, habitat suitability or persecution (sett disturbance).
"Highest risks were in areas of high badger social group density and high rates of persecution, potentially representing both responsive persecution of badgers in high cattle risk areas and effects of persecution on cattle bTB risk through badger social group disruption."
Co-author Rowland Kao of the Boyd Orr Centre for Population and Ecosystem Health at the University of Glasgow said: "What we know from the Randomised Badger Culling Trial (RCBT) is that intense culling of badgers over a small area can have an overall negative impact on cattle btB.
"Here, we show that badger persecution over a very broad area does not appear to reduce the risk for cattle - further it is illegal, and may even make matters worse."
The RBCT, which took place in England, showed that while intensive culling was associated with a decrease in cattle bTB inside cull areas, it also prompted an increase in bTB in neighbouring herds. The likely reason is that disruptions to the social structure of badgers causes increased spread of bTB to cattle from infected badgers.
Farm level risk factors are the key to bTB
The Wellcome Trust-funded research was carried out by the University of Glasgow, Queen's University Belfast (QUB) and the Agri-Food and Biosciences Institute (AFBI). Dr David Wright of QUB, who led the study, said:
"Whilst the incidence of badger persecution was low, we hypothesised that those taking pre-emptive action against badgers may contribute to maintaining the disease. We were interested in investigating the interactions between cattle and disturbed and undisturbed badger populations."
The analysis was based on surveys of badger setts in Northern Ireland of which about one in 20 showed signs of interference such as digging indicative of badger baiting, entrances being blocked with soil, boulders, branches or other debris inserted directly into holes, dumping of farm debris including bricks on top of setts.
Also recorded was agricultural disturbance such as setts being ploughed over or damaged by livestock trampling, development such as the construction of roads or newly built houses and slurry being pumped into holes.
The UK Government spends more than £100m per annum in testing, slaughter and compensation. Badger culling trials in the UK and Ireland have failed to show definitive benefits in terms of bTB reduction - and persecution may be one reason why.
Farmers must be aware of the risks of disturbing badgers
According to the scientists, badger persecution was more common in areas that had a history of high cattle bTB risk, indicating that responsive persecution is taking place in areas where badgers are perceived to be a threat. The paper states:
"Our results do not exclude the possibility that bTB risk differentials among areas are maintained by continued high levels of persecution, potentially through badger population perturbation. The two processes, responsive persecution and perturbation may operate in parallel, leading to positive feedbacks which may contribute to the persistence of bTB hotspots in certain areas independent of established cattle risk factors."
And they advise farmers and others against disturbing badger setts: "Persecution did not appear to substantially reduce cattle bTB risk and may have exacerbated the problem by triggering perturbation. Therefore, it may be beneficial to inform stakeholders of the risks incurred by disturbing setts.
"These findings should also be considered when designing bTB control programmes that use sub-lethal interventions in the badger population (including proposed badger vaccination programme) and efforts should be made to minimise disturbance of badger social group structure in the implementation of such programmes."
The study, the authors conclude, also highlights "the importance of preventing transmission within the primary population through discouraging unnecessary cattle movement and increasing further the efficacy of testing programmes.
"Interventions to address these issues, including risk-based trading and bTB testing programmes are likely to be considerably less expensive and more publicly acceptable than schemes based on culling of badgers and may be more cost-effective and easier to monitor."
The paper: 'Herd-level bovine tuberculosis risk factors: assessing the role of low-level badger population disturbance' by David M Wright et al is published in Scientific Reports and is available open access.
Oliver Tickell edits The Ecologist.