First direct evidence of tuberculosis transmission between cattle and badgers

Scientists at the University of Glasgow and the Agri-Food and Biosciences Institute in Northern Ireland have established the first direct evidence that tuberculosis epidemics in badgers and cattle are related at a local scale.

Using next-generation genome sequencing technology (NGS), the team from the Institute of Biodiversity, Animal Health and Comparative Medicine were able to trace mutations in the bovine TB bacteria – Mycobacterium bovis – as it passed from animal to animal.

The study showed that bacterial isolates from 26 badgers and four cattle taken from animals in Northern Ireland over a 10-year period were extremely closely related, with bacterial types in badgers often indistinguishable from the types found in cattle from nearby farms. In fact, the two types were closer to each other than they were to farms even a few kilometres away.

TB in cattle is a serious problem for farmers that can cost them dearly as animals testing positive for TB have to be destroyed. The disease is widespread and badgers are thought to be responsible for spreading the bacteria to cattle through their urine and faeces. However, TB can be passed both ways and can also be spread by infected cattle amongst a herd so the role of the badger in its transmission is not entirely clear.

With TB said to have cost the taxpayer in England £500m in the last 10 years, the Government has proposed a cull of badgers to help curb the spread of the disease, however the plan is controversial and has not yet got underway. Scotland is declared Officially TB-free.

Rowland Kao, a Wellcome Trust Senior Research Fellow at the University of Glasgow who led the study, said: “While the study was small, it provides the first direct evidence that the epidemic in the two species are closely linked at the farm scale and highlights the potential for use of NGS as a tool for disentangling the impact of badgers and cattle on TB.”

“However, only a larger study will allow us to quantify the extent and direction of transmission between cattle and badgers and reliably inform disease control policies.”

The study identified 26 cattle and four badgers infected with the same variable number tandem repeat type of M. bovis. Single nucleoid polymorphisms (SNPs) between sequences identified differences that were consistent with bacterial lineages being persistent on or near farms for several years, despite multiple clear whole herd tests in the interim.

Badger isolates showed between zero to four SNP differences from the nearest cattle isolate, providing evidence for recent transmissions between the two hosts. However, directionality of transmission cannot be inferred at this stage.

The research is published in the journal PLoS Pathogens.