Transmission of tuberculosis between cattle and badgers has been tracked at a local scale for the first time, using a combination of bacterial whole genome DNA sequencing and mathematical modelling. The findings highlight the potential for the use of next generation sequencing as a tool for disentangling the impact of badgers on TB outbreaks in cows at the farm level.
The role of badgers in the transmission of bovine tuberculosis (bTB) amongst cattle remains controversial, with the government's proposal to implement a widespread badger cull in England recently delayed and meeting with extensive criticism over the evidence base for this approach.
Previous studies have used lower resolution genetic typing of bacteria and information observed during an outbreak to identify links between cattle and badgers. However, until now, 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 twenty six 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, with often indistinguishable bacterial types 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 kilometers 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 Prof. 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 (AFBI) 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 of next generation sequencing for tracking 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 from 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 developed and developing countries.