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How likely is bovine TB to spread between species?
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Josh Loeb discusses new research that looks at the transmission of bovine TB from one species to another

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The fact that more infections are transmitted within species than between species suggests that controlling transmission among cattle is a priority in the strategy for eliminating TB

Bovine tuberculosis (bTB) in cattle and badgers passes between members of the same species at least twice as often as between the two species, a study conducted over a 15-year period has indicated.

A team of researchers led by University of Edinburgh scientists analysed genetic data from Mycobacterium bovis – the bacterium that causes bTB.

They carried out whole genome sequencing of bacteria from 116 infected badgers from an undisturbed population in Woodchester Park, Gloucestershire, and 189 infected cattle from nearby cattle farms.

The genetic data were then combined with detailed information about where the cattle and badgers lived, when they were infected and whether they could have had contact with one another.

By this means the researchers were able to estimate how often the two species spread bTB to members of their own species and between species.

They found that badgers play an important role in maintaining the disease in the geographical area considered, with cattle approximately 10 times more likely to catch bTB from badgers than vice versa. However, the researchers also found that cattle are more likely to be infected by transmission from other cattle than by transmission from badgers.

One graph in the paper, published in the journal eLife, provides a ranking for rates of transmission within and between species, with badger-to-badger transmission ranked the most likely route, followed by cow-to-cow transmission. Meanwhile, badger-to-cow transmission is ranked third, and cow-to-badger fourth.

Rowland Kao, professor of veterinary epidemiology and data science at the University of Edinburgh, said the rankings were derived using the ‘best’ statistical model.

However, he said it was important to bear in mind the caveat that estimates can be derived in different ways, so direct comparisons ‘should be done with caution’. He also said the results could help improve control strategies by making them more targeted.

‘Current approaches to bTB control only discriminate at a very coarse, regional level between areas where badgers are more likely to be involved in infecting cattle and areas where they are not,’ he explained. ‘This work identifies genetic signatures that could guide the interpretation of similar data if collected in other, less-intensively studied areas.

‘This would allow for more targeted control of tuberculosis in cattle and badgers, aiding efforts to control the disease and reduce the impact on the badger population.’

Lord John Krebs, emeritus professor of zoology at the University of Oxford and on whose recommendation the randomised badger culling trial was set up to test the effectiveness of culling badgers in controlling bTB in cattle, said: ‘In terms of policy, the results do not tell us whether killing badgers is more effective than controlling cattle-to-cattle transmission, but the fact that more infections are transmitted within species than between species suggests that controlling transmission among cattle is a priority in the strategy for eliminating TB.

‘One caveat is that Woodchester Park has a very high density of badgers, so the results may not be generalisable to all areas.’

The study was funded by the Biotechnology and Biological Sciences Research Council, Defra and the Wellcome Trust. ●

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