What is bovine tuberculosis?
Bovine tuberculosis is effectively a distant cousin of the bacterium that causes tuberculosis in humans. Bovine tuberculosis, or Mycobacterium bovis, is an infectious disease that affects farm animals such as cattle and pigs, and wild mammals like badgers, foxes and deer.
An infected animal may go undetected for months, but ultimately the disease is fatal. Cattle herds are tested for bovine tuberculosis in an attempt to stop the spread. In 2018, 7.7 million of the 9.6 million cows in the UK were tested for tuberculosis, and nearly 45,000 were euthanised due to a tuberculosis-related incident to avoid further infections within the herd.
The Department for Environment, Food and Rural Affairs (Defra) published a policy paper back in 2014 announcing its aim for England to achieve ‘Officially Bovine Tuberculosis Free’ status by 2038. Objectives include reducing transmission between herds, but also between cattle and badgers, thought to be the next most prevalent source after cows.
How is bovine tuberculosis spread?
The disease is transmitted either by ingestion or inhalation. Mycobacterium bovis can jump the species barrier, from badger to cattle to human – though cases in people are rare in this country, as pasteurisation kills the bacteria in milk from an infected cow and meat is inspected before being sold.
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Where badgers are concerned, in most instances cattle contract the disease through indirect contact. “Badger and cow coming face-to-face rarely happens,” says Cally Ham, PhD researcher at ZSL’s Institute of Zoology and Imperial College London, and the lead author on a study on the impacts of badger culls. “It’s more likely to be indirect, perhaps through infected badger excretion on a cow field.”
The transmission can go both ways, which is why tackling the disease in both species is preferred, says Ham.
Why are badgers getting the blame?
Badgers aren’t the only TB-carrying animals to come into contact with cows. Ham says deer populations share pasture with cattle, and they can have high levels of infection. “Maybe badgers are more obviously present than deer, which might make them seem more of a target,” she suggests. “A lot more research needs to look at deer, especially in the South West.”
Badgers were identified as carriers of the disease in 1917 and the Badgers Act of 1973 allowed for farmers to be given licences to kill badgers on their own farms. But it wasn’t until 1997 that a large-scale, scientific review of badger culling was completed.
Known as the Krebs’ Report, the 1997 paper concluded that there was a lack of evidence to support or discredit the use of badger culling to control the spread of bovine tuberculosis. What followed was the Randomised Badger Culling Trials (RBCT), where 11,000 badgers were caught and killed between 1998-2005.
The Defra report that followed stated: “After careful consideration of all the RBCT and other data presented in this report, including an economic assessment, we conclude that badger culling cannot meaningfully contribute to the future control of cattle TB in Britain.”
Then, in 2013, it was announced that pilot badger culls would be implemented first in Gloucestershire and Somerset, and later in Dorset. This came after a steady rise in the number of cows infected with tuberculosis, and a request from the National Farmer’s Union to the government.
The pilots hoped to investigate whether disruption to badger populations would increase the distance that individual badgers travelled, called the ‘perturbation effect’. This effect was suggested in the Krebs’ report, where further roaming could lead to wider spread of tuberculosis, the exact opposite of the cull’s intentions.
What did the pilots show?
A report published earlier this year, led by Sara Downs from the Animal and Plant Health Agency and funded by Defra, found no evidence of the perturbation effect. Tuberculosis incidence rates in Gloucestershire and Somerset were lower than in comparison zones, and there was no difference in Dorset.
Down’s paper cautiously suggests that industry-led culling was associated with reductions in bovine tuberculosis, however ends by saying: “Culling badgers will not provide the entire solution to the cattle TB problem in Great Britain and the impact of the policy needs to be evaluated alongside other TB controls.”
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But the ZSL-Imperial study led by Ham tells a different story.
“We fitted GPS collars to badgers inside a government-led cull area and in three unculled areas, to record where they went before, during and after the cull,” explains Ham. “Individual ranging behaviour increased with every measure that we tested.
“There was a 20-fold increase in the odds of a badger trespassing into a neighbouring territory, which has implications for tuberculosis transmission between badgers and between badgers and cattle.”
It was previously known that badger roaming increases after a cull, but Ham’s research has shown they range further during the cull too.
What should be done instead?
“What’s missing [from our current understanding] is a properly funded trial of badger vaccination and how that might impact tuberculosis prevalence,” says Ham. “We know that vaccination doesn’t have the same perturbation effect as culling, but it’s not been fully investigated how it might impact cattle infection.”
Ham believes the public response to vaccination is better than for culling. But it is the farmers, as the current advocates for culls, who need to be won over. Things are changing, says Ham, who is part of a campaign to help farmers vaccinate badgers on their land.
The next step is to further develop vaccinations for cattle. There is currently no test that can determine whether a cow is vaccinated against TB or infected with the disease, and as such vaccines are prohibited in the UK. Until a test is ready for wide-spread use, badger populations are still under threat.