Bovine tuberculosis (bTB) is a chronic bacterial disease resulting from infection by Mycobacterium bovis, a Gram positive, slow growing bacterium. It has a long incubation period and symptoms can take months to develop in cattle, sometimes remaining dormant for years until it becomes reactive due to a stressor (e.g. illness, pregnancy, or old age).
bTB can affect nearly all organ systems, but most commonly develops into a respiratory disease. It most often causes lesions to occur in the in the lymph nodes of the head and thorax, and in the lung, spleen, liver and surfaces of body cavities. These lesions are areas of localised M. bovis bacteria, which are then spread through respiratory secretions, faeces and milk. M. bovis is transmitted between cattle through aerosols when confined or in close contact, and can be ingested.
Badgers are one of several mammals that can become infected by M. bovis. Infected badgers rarely show signs of bTB, with a high proportion of infections resulting in a lengthy period of latency with few obvious lesions at post mortem examination. As such infected and infectious badgers often live with the disease asymptomatically throughout their natural lives, and shed M. bovis through their urine, faeces, sputum and discharge from bite-wounds. Duration of infection on pasture is fairly brief, with a 99% decay rate of M. bovis varying from 1 – 4 weeks according to excretion in urine, faeces or bronchial pus.
Transmission of M. bovis between cattle and badgers is thought to be through ingestion of the bacterium at badger latrines. Badgers often inhabit woodland close to pastureland (which typically holds a larger number of earthworms), and while cattle generally avoid areas of grass soiled with badger faces and urine, some cattle will graze contaminated herbage, particularly when over-grazing occurs. Furthermore, bTB infected badgers tend to range further than non-infected individuals, have larger home ranges and forage father away from the main sett, increasing the likelihood of encountering cattle.
The Department for Environment, Food and Rural Affairs (Defra) is responsible for reducing bovine TB in the UK. In 2011 it produced the Bovine TB Eradication Programme for England, which sets out a range of measures to tackle the disease, including the Government’s view that it is strongly minded to allow a cull of badgers in the worst affected areas. A planned pilot cull was postponed in 2012 for a variety of reasons, including a higher than expected number of badgers, and instead took place in two areas towards the end of 2013. The cull was controversial and failed to meet targets on badgers killed, despite estimates of badger numbers and length of the cull both changing. An independent expert panel produced a report in March 2014, which found that controlled shooting alone (or in combination with cage trapping) did not deliver the level of culling set by government. The report also comments on the humaneness of the culling pilots, stating that it was 'extremely likely that between 7.4% and 22.8% of badgers that were shot at were still alive after 5 min, and therefore at risk of experiencing marked pain'.
Defra launched a consultation which closed in September 2013 on a draft strategy to rid England of bovine TB within 25 years, informed by the Natural Science Evidence Base for Control of Bovine TB. The resulting bovine TB strategy for England was published in April 2013.
The eradication programme also includes reducing bovine TB through controls on cattle, advising farmers on herd biosecurity and husbandry measures, funding the development of TB vaccines for cattle and badgers, helping other industry sectors to deal with TB in non-bovine species and funding a bovine TB research programme.
We responded to the Environment, Food and Rural Affairs (EFRA) Committee inquiry examining the vaccination of badgers and cattle in relation to Bovine TB. We emphasised that the UK must learn from past and current outbreaks in cattle (such as foot and mouth disease and bluetongue) and prioritise the development of allowable vaccination as part of the tool box of control measures for bovine tuberculosis, without contravening EU regulations. More information is available in our full response.
EFRA have since published their response to this inquiry, citing the Society of Biology's evidence twice within the document.
On 21st January 2014, the Parliamentary and Scientific Committee held a meeting to discuss badgers and the spread of bovine TB, where the recent pilot culls and the effectiveness of culling in general were discussed. A report of the meeting is available to download.
The badger cull debate highlights the complexity of evidence based policy; this article in The Biologist discusses these interactions in more detail.