A study of bacterial DNA has provided the first clear evidence that tuberculosis can spread between cattle and badgers.
Scientists mapped the genetic blueprints of bacteria isolated from 26 cows and four badgers during outbreaks of bovine TB in Northern Ireland.
The analysis traced changes in the bacteria's DNA as it passed from animal to animal. It revealed that bacteria taken from badgers and cows were closely related at the farm level – strains obtained from badgers and nearby cattle farms were often indistinguishable.
The findings, published in the Public Library of Science Pathogens, shed no light on the direction of transmission – whether from badgers to cows or cows to badgers.
Lead scientist Professor Rowland Kao, from the University of Glasgow, said: "Only with a larger study might we be able to quantify the extent and direction of transmission between cattle and badgers and reliably inform disease control policies."
Bacteria from within single outbreaks were usually closely related, the study showed. But different herd outbreaks were usually characterised by genetically distinct groups of bacteria.
Controversial Government plans for a mass badger cull in England to curb the spread of bovine TB have been postponed until next summer following widespread protests.
Opponents, including the RSPCA and Queen guitarist Brian May, argue that a cull would be ineffective and inhumane.
The Government's case is that the vaccine alternative is only 50-60% effective.
A nine-year trial showed that culling could slow the spread of the disease, but only if more than 70% of badgers in an area were eradicated.
A Defra spokesman said: "This research adds to existing scientific evidence that bTB is transmitted between cattle and badgers.
"No country in the world has successfully got to grips with the spread of this disease without tackling it in the wildlife population.
"That is why the Government remains fully committed to culling badgers in two pilot areas."