Bovine tuberculosis (bTB) is an infectious disease of cattle caused by the bacterium Mycobacterium bovis (M. bovis).
The bacterium can also infect and cause disease in badgers, deer, goats, pigs, llamas and alpacas, as well as many other mammals. In cattle, TB is primarily a chronic respiratory disease but clinical signs are rare.
Transmission can occur through nose to nose contact and also through contact with saliva, urine, faeces and milk
How do we test for bTB in cattle?
The main screening test for bTB in cattle in Great Britain is the tuberculin skin test. It is carried out by a vet or approved tuberculin tester.
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Rather than look for the bacterium itself, the skin test in cattle relies on measuring the immune response of the animal to injections of tuberculin.
The test takes place on two days. On the first day two types of tuberculin (bovine and avian) are injected deep into the layers of the skin of the neck. By injecting two types of tuberculin, the test can better distinguish between animals infected with M. bovis and animals previously exposed to or infected with other types of mycobacteria found in the environment which do not cause bovine TB.
If an animal's immune system has previously been ‘sensitised' by infection with M. bovis or exposure to other mycobacteria, then the injection of tuberculin will trigger an inflammatory response and a lump develops at the injection site.
On day two, 72 hours after the cattle have received the injection, the test is read. The size of any reactions at the avian and bovine injection sites are measured and compared. Depending on the degree of reaction to the skin test and the interpretation of the test, the animal is classified as:
- Clear - negative result
- Fail - reactor or positive result
- Inconclusive reactor (IR) – the animal shows a reaction to bovine tuberculin greater than the avian, but not strong enough to be classified as a reactor. IRs must be isolated and re-tested after 60 days. Animals that have an inconclusive result at two consecutive skin tests are considered reactors
Why do we test cattle for bTB?
UK farmers are required to test their cattle for bTB to help control the spread of the disease.
The frequency of testing cattle for bTB will differ depending on where you are in the country, previous test results, and what the ‘risk' in your area is. Farms in ‘high risk areas and those nearby these areas will have to test cattle every six or every12 months. Farms in ‘low risk' areas will usually have to test cattle every four years.
Can we not vaccinate cattle against bTB?
There are field trials underway into the use of a BCG vaccine for cattle. This is the same vaccine used to protect people and badger from TB.
However, the BCG vaccination sensitises cattle to the tuberculin tests and would cause most cattle to react to the skin test as a false positive.
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This would mean the current skin test, which is an important part of the UK's bTB eradication programme, would be ineffective.
A detect infected cattle among vaccinated cattle (DIVA) skin test has been developed to use alongside the CattleBCG.
Trials into the CattleBCG vaccine and the DIVA skin test are underway, but would also need UK marketing authorisations from the Veterinary Medicine Directive, before they could be used, in the same way as any other veterinary medicines.
What about badgers?
Badgers can act as a wildlife reservoir for M. bovis, the bacterium which causes bTB in cattle. Defra's badger control policy in England includes both injectable vaccination and culling of badgers to control the spread of bovine TB. Badgers and their setts are protected under the Protection of Badgers Act 1992, however licensed culling is permitted to prevent the spread of bTB.
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There is also significant frustration over Government policy on bovine TB, with the Labour Government pledging to end the badger cull in England and divergent policies in Northern Ireland and Wales.
Prominent campaigners against badger culling include Queen guitarist Sir Brian May.
Yesterday, the NFU announced a new project was underway to explore the efficacy of badger vaccination as part of a comprehensive TB eradication strategy.
Mental health
Bovine TB has also had a significant impact on farmers' mental health.
In January, a comprehensive study of over 450 farmers by the Farming Community Network exposed the severe psychological impact of bTB, including sleepless nights, anger, depression and loss of control.
The report found many farmers believed the long-term impact of bTB outweighed dealing with shorter-term disease outbreaks.
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Farmers said health impacts were often generated most by the anticipation of statutory testing and fear of an outbreak.
Dr Jude McCann, FCN chief executive, said bTB caused ‘significant stress and uncertainty' in farming communities and was a factor in many cases presented to the charity.
Clarkson's Farm
It is not the first time Clarkson's Farm has discussed the issue.
In season two, the show showed dairy farmer Emma Ledbury, a dairy farmer whose herd was hit by bTB.
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The episode, entitled Badgering, highlighted the devastation badgers can have on dairy businesses with Ms Ledbury revealing she had lost around 50% of her cows to the disease.
Viewers saw the farmer telling Mr Clarkson that she is only staying afloat due to selling milk and milkshakes in the former Top Gear presenters Diddly Squat farm shop.
She also revealed she and her family were not currently taking a wage from the farm as finances were simply too tight.
In the series, Mr Clarkson was informed he was in a high-risk area, as he explored plans to enter the beef sector.
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