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What is bovine TB?

Bovine Tuberculosis (bTB) is a chronic disease of cattle caused by the bacterium Mycobacterium bovis (M. bovis), a relative of M. tuberculosis which causes TB in humans. Although it is primarily associated with TB in cattle, M. bovis can infect a wide range of other mammalian hosts including humans, sheep, goats, deer, camelids (alpacas & llamas), pigs, rodents, badgers, and other wildlife. Bovine TB principally infects the pulmonary system (lungs and associated organs) and it is believed that close contact/aerosol spread is important for the transmission of M. bovis from one animal to another. Indirect transmission (via contaminated feed, water or pasture) is also possible, as studies show that M. bovis can survive outside of a host for several months under certain conditions.

How big is the problem?

Bovine TB is a global disease, affecting countries including the UK, Ireland, United States, Canada, France, Spain, New Zealand and several African nations. In the UK bovine TB has increased sharply in recent decades, with the area effected by the disease also increasing dramatically. In 2017 there were 4,657 TB incidents (where herds had TB identified by testing or surveillance), resulting in the slaughter of more than 43,000 cattle. The disease can have a devastating impact on individual farmers and those affected by it, as well as costing the UK government an estimated 100 million pounds per year in testing and compensation.

What about the role of wildlife?

M. bovis can infect a wide range of wildlife species, in some cases these are ‘dead end’ hosts incapable of transmitting the disease further. However, in some cases wildlife can act as a ‘maintenance host’ or wildlife reservoir for the disease and a potential source of infection for cattle. In the UK and Ireland the European badger (Meles meles) is the principal wildlife reservoir for the disease and is implicated in disease transmission to cattle, although in some localised areas it is possible that deer are also involved. Options for managing the disease in badgers include culling, vaccination or biosecurity measures.

Research on badger culling in the UK has shown that culling badgers can reduce TB in cattle, if the culling is efficient at removing animals (>70% reduction) and is done on a large scale for a long period of time (4 years or more). Levels of TB reduction observed inside ‘proactive’ areas in the RBCT (randomised badger culling trial) were 19-32% (over a 10 year period, although ‘perturbation’ of badger populations around cull areas may reduce this benefit) suggesting that culling alone is unlikely to solve the problem of TB in cattle. It is unclear whether the current industry led culls will achieve similar, higher or lower levels of reductions in cattle TB. Vaccination of badgers with the BCG vaccine has been shown to reduce the likelihood that badgers become infected and also slows the progression and development of the disease. However, the effect of badger vaccination on cattle TB is unclear. For a great summary of the TB science evidence base read Godfray et al. 2013.

Why is TB such a difficult problem to tackle?

The control of bovine TB in the UK is complicated by multiple factors including the role of wildlife, the inaccuracy of diagnostic tests for the disease, as well as social, political and economic factors. A single focus on any one area is unlikely to resolve the problem. Ideally decisions about how to control bTB should be informed and directed by the scientific evidence base, which is substantial, although there are still many knowledge gaps. It is also important that efforts to control TB consider the potential human or economic aspects of this complex disease. For example, stricter measures on the movement of cattle may have disease control benefits, but they will also result in financial impacts on those affected. The requirement for more accurate cattle tests need to be balanced against the sheer scale and cost of the problem (9.8 million tests were conducted in 2017 in GB). Similarly, decisions about what to do regarding wildlife need to consider the cost vs the benefit, and may result in strong reactions from different aspects of the public. These factors combined with the highly political nature of the disease make bovine TB a complex and difficult disease to control.