Defra’s public consultation on badger culling and bovine tuberculosis reported in July, 2006. 95 per cent of respondents and half the interested organisations opposed a cull of badgers. The government has not yet responded to the consultation. Here, the Independent Scientific Group on Cattle TB explains the effects of badger culling on bovine TB.

Bovine tuberculosis (TB) raises concerns for farmers, conservationists, and government. The disease can also infect people, and in Britain in the 1930s around 2,500 people died annually, contracting the disease from unpasteurised milk. The post-war government committed itself to eradicating the disease from cattle.

Controlling TB

Cattle testing is the mainstay of TB control. Herds are tested regularly, infected animals slaughtered and the herd quarantined until subsequent tests reveal no further cases. 

By 1970, this approach had eliminated TB across most of Britain except for the South West. Infection was discovered in badgers in 1971 and, from 1973, various badger culling strategies of this legally protected species complemented cattle testing. Eliminating badgers from one large area by gassing over several years was associated with a substantial decline in cattle TB; however, despite many years of localised culling on and around infected farms in the region, the incidence of the disease progressively increased and spread to new areas. By 2005, six per cent of cattle herds tested were affected, compared with less than one per cent in 1970.

Randomised trial

Uncertainty about the merits of badger culling continued. An independent scientific review in 1997 described evidence for badgers’ role in TB transmission as ‘compelling’, but noted that the effectiveness of badger culling could not be evaluated because the available data lacked a scientific basis. It therefore recommended that different culling strategies be formally tested under field conditions. The Randomised Badger Culling Trial (RBCT) was launched by Defra (Department for Environment Food and Rural Affairs) the following year; designed and overseen by the Independent Scientific Group.

The RBCT results surprised many. In 10 areas, each covering 100km2, all cattle TB outbreaks triggered localized badger culling – similar to control policies that had been practised for over 20 years. By 2003 these areas were experiencing higher TB incidence than 10 similar no-cull control areas: culling badgers was making the situation worse rather than better. In a third set of 10 areas, also 100km2, badgers were systematically culled from all accessible land. Here, results up to 2006 showed that, relative to the no-culling areas, cattle TB incidence fell by about 20 per cent – but was elevated on lands immediately surrounding the culled area. This ‘edge effect’ cancelled out the benefits experienced inside the culled areas.

Explaining the results

These unexpected findings can be explained by culling-induced changes in badger behaviour. Left undisturbed, badgers’ movements, and hence disease spread, are constrained by territorial borders.  Culling destroys this territorial structure. Remaining infected badgers, and those migrating in to occupy vacated land, range widely, spreading disease to one another and also encountering more cattle herds.

Substantial reductions in badger numbers could minimise these effects, but where overall densities are not greatly reduced – as in localised culling areas and immediately outside widespread culling areas – detrimental effects predominate. As this edge effect cannot be prevented by reasonable means, culling would be necessary over areas very much larger than the 100km2 used in the RBCT, to offset its detrimental impact on the overall incidence of disease. 

Any culling policy will create the problem of neighbouring farms facing an increased TB risk due to badger perturbation.

Reactions to RBCT results

The results have been published in international peer-reviewed journals, and are now widely accepted in the scientific community.  Acceptance by farming interests has been slower, however; partly because it seems ‘common sense’ to attack a known source of the disease, and the belief that earlier culling was effective, and partly because recent trials in the Republic of Ireland (under very different conditions and extreme culling), showed that culling can reduce the disease in cattle. These trials, however, did not seek evidence of detrimental effects.

The way forward

Accumulated scientific findings demonstrate the relationship between infected badgers and TB prevalence in cattle is complex. The data suggest that, if badger culling is to have net benefits for disease control, and not make matters worse, it would need to be conducted intensively, in a coordinated and sustained manner over very large areas. It is highly questionable whether this would be practicable, economically justifiable, or publicly acceptable.

In the meantime, parallel research suggests that enhanced cattle TB diagnosis and movement control could, alone, substantially reduce TB incidence. This would require an acceptance that some herds will continue to be infected by badgers. A badger vaccine remains a long term strategic objective, but available control approaches cannot eradicate badger TB. A policy focused solely on cattle, justifiable on scientific grounds, would probably meet strong opposition from some groups.

The members of the Independent Scientific Group on Cattle TB are Professor John Bourne, Professor Christl Donnelly, Sir David Cox, Professor George Gettinby, Professor John McInerney, Professor Ivan Morrison and Dr Rosie Woodroffe.