Wild deer TB levels among highest globally in Exmoor, new research finds

More than one in four wild deer in parts of southwest England could be carrying bovine tuberculosis, according to new research by the Animal and Plant Health Agency (APHA), raising fresh concerns for farmers and disease control efforts.

The findings, from a large-scale study on Exmoor, suggest wildlife may be playing a greater role in the persistence of the disease as the UK pushes towards its target of eradicating bTB by 2038 — a programme that already costs around £150 million a year.

Researchers analysed 432 blood samples collected from wild deer over a 12-month period, mostly from red deer, and found that while 16% tested positive, the estimated true infection rate was 27.4% — meaning more than one in four animals could be infected.

The study places the Exmoor deer population “at the upper end of prevalence estimates reported in deer worldwide”, highlighting the potential scale of infection in a known high-risk area.

Bovine TB remains one of the most significant animal health challenges in England, particularly in the southwest where cattle and wildlife frequently interact.

Transmission between cattle and deer has long been suspected, but evidence has remained limited.

“In England, transmission of Mycobacterium bovis between cattle and certain species of free-roaming, wild deer has been implicated in some endemic bovine tuberculosis regions,” the study states.

However, the authors note there are still “data and knowledge gaps regarding the role of deer in the epidemiology of this disease”.

To address this, APHA researchers used samples taken during routine deer management culls on Exmoor, working with local deer managers to collect blood from animals shortly after they were shot.

The approach allowed scientists to gather large volumes of data without increasing culling activity, while offering a snapshot of infection across a free-ranging population.

Laboratory testing was then used to detect antibodies to Mycobacterium bovis, with results adjusted to reflect the limitations of the diagnostic test.

The findings showed little variation in infection rates between male and female deer, or across different species.

Younger animals appeared less affected, with no positive cases detected in deer under one year old, although infection was present across older age groups.

The authors cautioned that the results should be interpreted carefully, noting that opportunistic sampling may introduce bias and that the testing method used could miss some infected animals.

They stressed that infection levels alone do not fully explain transmission risk.

“Our results should be interpreted alongside other relevant information, including species-specific ecology, epidemiology and disease pathology,” they said.

The study also underlines the complexity of bTB in areas where multiple wildlife species may act as reservoirs.

Badgers are widely considered the primary wildlife host in Britain, but deer have also been identified as potential carriers, particularly where populations are dense.

Evidence from countries including Ireland and the United States shows deer can act as a reservoir for infection, with transmission occurring between wildlife, cattle and even humans in some cases.

However, the extent to which deer contribute to the spread of bTB in England remains debated, and is likely to vary depending on local conditions.

The researchers say improving understanding of infection in wildlife will be essential for shaping future policy, including surveillance and disease control strategies.

With eradication already proving complex and costly, the presence of infection across multiple species could present a significant challenge to achieving long-term control.