First direct evidence links TB infection in cattle and local badger populations

The role of badgers in the transmission of bovine tuberculosis (bTB) among cattle remains controversial: the government's proposal to implement a widespread badger cull in England was recently delayed and has met with extensive criticism over its evidence base.

Previous studies have used lower resolution genetic typing of bacteria and information observed during an outbreak to identify links between cattle and badgers. Until now, however, direct evidence of transmission of the bacteria between the two hosts at the farm scale has been lacking.

In this study, researchers made use of advances in genetic technologies to sequence whole genomes of bacteria that had been isolated from 26 cows and four badgers from a group of neighbouring farms in Northern Ireland over a decade-long history of repeated bTB outbreaks. This approach enabled the team to retrospectively trace changes in the bacteria's DNA as it passed from animal to animal.

The findings reveal that the bacteria isolated from badgers and cattle were extremely closely related, and indistinguishable bacterial types were often obtained from badgers and nearby cattle farms. Moreover, the bacteria isolated from the two species were more closely related to each other than they were to farms even a few kilometres away.

"This study provides the first direct evidence of the close relationship between tuberculosis infections in cows and local badgers, at a very local scale," explains Professor Rowland Kao, a Wellcome Trust Senior Research Fellow who led the study jointly conducted by the University of Glasgow and the Agri-Food and Biosciences Institute in Northern Ireland. "However, 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."

The mathematical models used in this study show that different herd outbreaks were usually characterised by genetically distinct groups of bacteria, while bacteria from within single outbreaks were usually closely related, highlighting the potential to use next-generation sequencing to track the spread of the bacteria from herd to herd.

Bovine tuberculosis (bTB) is an important disease of both livestock and wildlife with severe impacts on animal health and subsequent economic consequences. Although the disease in cattle is caused by a different bacteria to human disease (Mycobacterium bovis rather than Mycobacterium tuberculosis), M. bovis is believed to have been a major historical contributor to human cases of TB worldwide and remains a health concern in both high- and low-income countries.

The study is published today in the journal 'PLOS Pathogens'.