Mycobacterium bovis is a slow-growing (16 to 20 hour generation time), aerobic bacterium and the causative agent of tuberculosis in cattle (known as bovine TB). Related to M. tuberculosis—the bacterium which causes tuberculosis in humans—M. bovis can also jump the species barrier and cause tuberculosis in humans.
During the first half of the 20th century, M. bovis is estimated to have been responsible for more losses among farm animals than all other infectious diseases combined. Infection occurs if the bacterium is ingested.
M. bovis is usually transmitted to humans via infected milk, although it can also spread via aerosol droplets. Actual infections in humans are rare, mainly because pasteurisation kills any bacteria in infected milk; also, cattle are randomly tested for the disease and immediately culled if infected, but can still be used for human consumption. However, in areas of the developing world where pasteurisation is not routine, M. bovis is a relatively common cause of human tuberculosis.
Bovine TB is a chronic infectious disease which affects a broad range of mammalian hosts, including humans, cattle, deer, llamas, pigs, domestic cats, wild carnivores (fox, coyotes) and omnivores (possums, mustelids and rodents); it rarely affects equids or sheep. The disease can be transmitted in several ways; for example, badgers excrete M. bovis in exhaled air, sputum, urine, faeces and pus, so the disease can be transmitted by direct contact, contact with the excreta of an infected animal, or inhalation of aerosols, depending on the species involved.
Epidemiology and control
In New Zealand, the common brushtail possum is the main vector for the spread of M. bovis. The disease now endemic in possums across approximately 38% of New Zealand (known as ‘vector risk areas’). In these areas, nearly 70% of new herd infections can be traced back to possums or ferrets. The Biosecurity Act 1993, which established a National Pest Management Strategy, is the legislation behind control of the disease in New Zealand. The Animal Health Board (AHB) operates a nationwide programme of cattle testing and possum control, with the goal of eradicating M. bovis from wild vector species across 2.5 million hectares – or one quarter – of New Zealand’s at-risk areas, by 2026 and, eventually, eradicating the disease entirely.
The TBfree New Zealand programme is regarded as "world-leading". It has successfully reduced cattle and deer herd infection rates from more than 1700 in 1994 to fewer than 100 herds in July 2011. Much of this success can be attributed to sustained possum control reducing cross-infection and breaking the disease cycle. For example, at Hohotaka, in New Zealand's central North Island, control work from 1988 to 1994 achieved a sustained mean reduction of 87.5% in the density of TB‐infected possums. As expected, annual TB incidence in local cattle herds consequently declined by a similar amount (83.4%).
From 1979 - 1984, possum control was stopped due to lack of funding. In spite of regular and frequent TB testing of cattle herds, the number of infected herds snowballed and continued to increase until 1994. The area of New Zealand where there were TB wild animals expanded from about 10 to 40%.
That possums are such effective transmitters of TB appears to be facilitated by their behaviour once they succumb to the disease. Terminally ill TB possums will show increasingly erratic behaviour, such as venturing out during the daytime to get enough food to eat, and seeking out buildings in which to keep warm. As a consequence, they may wander on to paddocks, where they naturally attract the attention of inquisitive cattle and deer. This behaviour has been captured on video.
In the 1930s, 40% of cows in the UK were infected with M. bovis and there were 50,000 new cases of human M. bovis infection every year. According to DEFRA and the Health Protection Agency, the risk to people contracting TB from cattle in Great Britain today is very low. The HPA has said that three quarters of the 440 human cases reported to the HPA between 1994 and 2006 were aged 50 years and above and only 44 cases (10%) were known to be non-UK born.
Badgers (Meles meles) were first identified as carriers of M. bovis 30 years ago, but the report of an independent review committee in 1997 concluded badgers made an important contribution to the spread of M. bovis between herds of cattle. This was the major cause of the current battle between animal conservationists (keen to save the badger) and farmers (keen to cull badgers, to reduce livestock losses). The Randomised Badger Culling Trial  (designed, overseen and analysed by the Independent Scientific Group on Cattle TB, or ISG ) was a large field trial of widescale (proactive) culling and localised reactive culling (in comparison with areas which received no badger culling). In their final report, the ISG concluded: "First, while badgers are clearly a source of cattle TB, careful evaluation of our own and others’ data indicates that badger culling can make no meaningful contribution to cattle TB control in Britain. Indeed, some policies under consideration are likely to make matters worse rather than better. Second, weaknesses in cattle testing regimes mean that cattle themselves contribute significantly to the persistence and spread of disease in all areas where TB occurs, and in some parts of Britain are likely to be the main source of infection. Scientific findings indicate that the rising incidence of disease can be reversed, and geographical spread contained, by the rigid application of cattle-based control measures alone." On 26 July 2007, the Minister of State, Department for Environment, Food and Rural Affairs (Lord Rooker) said "My Lords, we welcome the Independent Scientific Group’s final report, which further improves the evidence base. We are carefully considering the issues that the report raises, and will continue to work with industry, government advisers and scientific experts in reaching policy decisions on these issues."
In the UK, many other mammals have been found to be infected with M. bovis, although the frequency of isolation is generally much less than cattle and badgers. In some areas of South-West England, deer, especially fallow deer due to their gregarious behaviour, have been implicated as a possible maintenance host for transmission of bovine TB, a disease which in the UK in 2005 cost £90 million in attempts to eradicate. It has been argued that in some localised areas, the risk of transmission to cattle from fallow deer is greater than it is from badgers.
In a 2010 opinion piece in Trends in Microbiology, Paul and David Torgerson argued that bovine tuberculosis is a negligible public health problem in the UK, providing milk is pasteurized. Bovine TB is very rarely spread by aerosol from cattle to humans. Therefore, the bovine tuberculosis control programme in the UK in its present form is a misallocation of resources and provides no benefit to society. Indeed, there is even very little evidence of a positive cost benefit to the livestock industry, as few studies have been undertaken on the direct costs of bovine TB to animal production. Milk pasteurisation was the single public health intervention that prevented the transmission of bovine TB to humans, and there is no justification for the present test and cull policy in the UK.
In July 2010 the 2nd issue of the discussion document 'Bovine TB, Time for a Rethink'  was published by Rethink Bovine TB, an independent research group. The paper considers current policy in England and Wales. It proposes an alternative solution that is both practical and cost effective. In the paper evidence is drawn from Defra and the work by Professors Paul and David Torgerson.
In March 2012, think tank the Bow Group published a target paper urging the Government to reconsider its plans to cull thousands of badgers to control Bovine TB, stating that the findings of Labour's major badger culling trials several years prior were that culling does not work. The paper was authored by Graham Godwin-Pearson with a foreword by Dr Brian May and contributions by leading tuberculosis scientists, including Lord Krebs.
United States of America
In the United States, M. bovis is endemic in white-tailed deer (Odocoileus virginianus) in the northeastern portion of Michigan and northern Minnesota, and sporadic import of the disease from Mexico. The white-tailed deer has been confirmed as the sole maintenance host in the Michigan outbreak of bovine tuberculosis, which remains a significant barrier to the US nationwide eradication of the disease in livestock. In 2008, 733,998 licensed deer hunters harvested approximately 489,922 white-tailed deer in attempts to control the disease spread. These hunters purchased more than 1.5 million deer harvest tags. The economic value of deer hunting to Michigan’s economy in the drive to eradicate TB is substantial. For example, in 2006, hunters spent US$507 million hunting white-tailed deer in Michigan.
The disease is found in cattle throughout the globe, but some countries have been able to reduce or limit the incidence of the disease through process of 'test and cull' of the cattle stock. Most of Europe and several Caribbean countries (including Cuba) are virtually free of M. bovis. Australia is officially free of the disease since the successful BTEC program, but residual infections might exist in feral water buffalo in isolated parts of the Northern Territory. In Canada, there are affected wild elk and white-tailed deer in and around Riding Mountain National Park in Manitoba. To improve control and eliminate bovine TB, the Canadian Food Inspection Agency (CFIA) has split Manitoba into two management areas: The Riding Mountain TB eradication area (RMEA), the area where the disease has been found and the Manitoba TB Eradication Area (MTEA), the rest of the province outside RMEA where the disease has not been found. The disease has also been found in wild buffalo in South Africa.
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