Bovine TB transmission from cow to cow most likely, study finds

Cattle-to-cattle bovine tuberculosis (bTB) transmission is the most common form of disease spread, but badger-to-cattle transmission is also detectable, a new study reveals.

Researchers from the Agri-Food and Biosciences Institute (AFBI) studied the disease transmission within and between cattle and badger populations (PDF).

They used whole genome sequencing of the bacterium that causes bovine TB – M Bovis – and analysed 162 gigabytes of genetic data.

See also: Northern Ireland cattle farmers rue the mounting cost of bovine TB

The five-year Daera (Department of Agriculture, Environment and Rural Affairs) Test, Vaccinate or Remove (TVR) research project near Banbridge, in County Down (2014-18), which included the culling of infected badgers, provided the detailed information on the local epidemiology of bTB. 

Scientists found that in the TVR zone, cattle-to-cattle transmission was “by far the most common” form of disease spread observed, while badger-to-badger transmission was undetected.

However, signals of inter-species transmission (both cattle-to-badger and badger-to-cattle) were detected as both hosts swapped closely related strains of bacteria. 

While being highly variable, cattle-to-badger transmission appeared to be considerably more common than badger-to-cattle transmission.

AFBI said: “The sum of the evidence suggests that cattle were playing the major role in the spread of disease in the TVR zone, with badgers involved, but to a lesser extent.”

But while badger-to-cattle transmission was detected less commonly, once it occurs, it can amplify within herds “to have a much greater impact” and “it remains plausible that a trickle of infection from badger-to-cattle could still have an outsized effect”.

A 2019 study of genetic data of TB in Woodchester Park in Gloucestershire found that cows were almost twice as likely to be infected with TB by cows than badgers.

But the same study concluded that badgers are 10.4 times more likely to spread TB to cattle than the other way round.

In April, Daera invited expressions of interest from farmer-led companies to fund badger culling as part of its TB eradication strategy.

The outcome of a legal challenge by Wild Justice against the lawfulness of Daera’s proposed badger cull is pending.