Tuberculosis in cattle can be spread by badgers but cow-to-cow transmission is more significant than badger-to-cow, according to the first study to provide direct evidence of the disease passing between the two species.
Scientists undertook whole genome sequencing of different strains of bovine TB to detect how it moved between cows and badgers in Woodchester Park, Gloucestershire.
They found that transmission within each species was more frequent than from one species to another but that transmission occurred almost 10 times more frequently from badgers to cows than from cows to badgers.
Despite the expansion of a controversial badger cull to 43 zones across western England from Cornwall to Cheshire, bovine TB has continued to rise. In 2018, 44,656 cows were slaughtered in Britain as a result – the highest ever number and an increase of 50% on 2005, when just under 30,000 cows were slaughtered. The disease costs taxpayers £100m a year in compensation for prematurely slaughtered cattle.
Scientists from 12 institutions across Britain and Ireland including the Universities of Edinburgh, Glasgow and Oxford, and University CollegeDublin, compared the genomes of Mycobacterium bovis (M. bovis) found in cattle and badgers, the animals’ locations, when they were infected and whether they could have been in contact.
Their analyses, published in the journal eLife, found that the transmission of M. bovis within each species was most significant. By examining different mutations of the disease, the scientists estimated that transmission occurred between badgers at least 2.7 times more frequently than badger-to-cattle transmission, while the transmission from badgers to cattle was almost 10 times more frequent than the disease passing from cattle to badgers.
The study used data gathered at Woodchester Park, where bovine TB is well-established in local cattle and the badger population, which has been scientifically studied since 1976. It revealed that a prevalent type of M. bovis had been in badgers since scientific data was first collected from the local population, but that different types of the disease had subsequently been introduced into the area by cattle.
A few critics of the badger cull have argued there had been no firm evidence proving that badgers pass bovine TB to cattle, although the scientific and farming consensus is that a “wildlife reservoir” of infected badgers sustains the disease, which originally began in cattle.
Principal investigator Prof Rowland Kao of the University of Edinburgh said the new study provided not mere observation and inference but “direct evidence” that “changes the nature of the game”. He added: “The one very, very clear thing it says is that badgers are involved in transmitting to cattle in this case.”
Co-author Richard Delahay of the government’s Animal & Plant Health Agency said the study did not endorse or dismiss badger culling, but that their methodology was a tool to better detect how the disease is spreading and how it should be tackled in specific areas.
He said: “What the study shows is that in this area both species are involved and we’ve got transmission going from badgers to cattle and cattle to badgers and circulating within both of these populations. Under that scenario, you would imagine you would have to control the disease in both species but how you do that is a completely different question.”
An independent review of bovine TB last year concluded that the frequent trading and movement of cattle and poor biosecurity on farms was “severely hampering” efforts to tackle the disease and it was desirable to move from badger culling to vaccination.
Up to 64,000 badgers are likely to have been killed this autumn in the expanded badger cull, which first began in 2013.
A recent study of the first three cull zones, in Gloucestershire, Somerset and Dorset, found that the incidence of TB in cattle was two thirds lower in Gloucestershire after four years of culling than in similar areas without a cull, 37% lower in Somerset and with no significant change in Dorset. Critics of the cull have argued that these declines are partly due to tougher cattle controls introduced at the same time as the cull which have reduced cattle-to-cattle transmission.
