The government’s decision to consult on a badger culling policy follows the publication of an interim report by the Independent Scientific Group overseeing the Randomised Badger Culling Trials.

The report suggests that the most effective way to reduce the incidence of bovine tuberculosis in hotspot areas is to have a few, much larger culling areas as opposed to many smaller areas.

Positive and negative effects of widespread badger culling on tuberculosis in cattle, published on 14 December in the journal Nature, concludes that culling badgers can reduce TB incidence in culled areas, although it increases its prevalence in adjoining areas.

Results from the “proactive” part of the RBCT returned a 19% fall in the incidence of infected cattle in the culled areas. This benefit increased to 23% after the first follow-up cull.

But herds within 2km of the culling boundary suffered a 29% increase in TB breakdowns after the initial cull, although this fell to 22% after the first follow-up cull. This rise is explained by increased activity by the remaining badgers once setts are disturbed.

However, it is this finding that has led the report authors to state that they expect “the overall reduction in cattle TB to be greatest for very large culling areas (with consequently lower perimeter:area ratios)”.

According to John Bourne, ISG chairman, for a badger cull to be effective it would need to cover an area of 300-400km2 – similar to the areas employed in the Irish “Four Areas” trial (News, 14 January, 2005).

Culling would also need to be sustained for between five and seven years for the maximum benefit to be realised. “In the Thornbury trials it took seven years of sustained gassing to achieve a positive result,” he said.

Natural boundaries, such as rivers, also need to be used if any cull is to be successful, said Prof Bourne. “To clear the disease in Cornwall you’d need to cull from the east all the way to the sea.”

But he warned that this may not be straightforward. About 20% of the landowners in Cornwall refused the ISG access to their land to cull badgers as part of the RBCT, he said.