Mycobacterium bovis

See: Tuberculosis

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 bacteria which causes tuberculosis in humans—M. bovis can also jump the species barrier and cause tuberculosis in humans.^[1]

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Pathogenesis

It has been estimated that, during the first half of the 20th century, M. bovis was responsible for more losses among farm animals than all other infectious diseases combined. Infection occurs if the bacterium is ingested.^{[citation needed]}

M. bovis is usually transmitted to humans via infected milk, although it can also spread via aerosol droplets. Actual infections in humans are rare, mostly due to pasteurisation killing any bacteria in infected milk; as well, cattle are randomly tested for the disease and immediately destroyed if infected. However, in areas of the developing world where pasteurisation is not routine, M. bovis is a relatively common cause of human tuberculosis.^[2]

Epidemiology

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.^[3] Since 1990, only one case of human M. bovis infection acquired from an animal source has been documented in the UK. In 2004, 5.6% of herds were affected by restrictions because of M. bovis infection.

Badgers (Meles meles) were first identified as carriers of M. bovis 30 years ago, but it was the report of an independent review committee in 1997 which concluded that badgers made an important contribution to the spread of M. bovis between herds of cattle.^[4] 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 ^[5] (designed, overseen and analysed by the Independent Scientific Group on Cattle TB, or ISG ^[6]) 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 ^[7], 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."^[8]

In the UK many other mammals have been found to be infected with M. bovis but the frequency of isolation is much less than cattle and badgers. 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 the United States, there is endemic M. bovis in White-tailed Deer in the northeastern portion of the state of Michigan and in northern Minnesota and sporadic import of the disease from Mexico. In Canada, there are affected wild elk and white-tailed deer around Ridning Mountiain National Park in Manitoba. The disease has also been found in wild buffalo in South Africa and Brush-tailed possums in New Zealand.

Mycobacterium bovis can be transmitted from human to human—there was an outbreak in Birmingham, England in 2004^[9]—and from human to cattle,^[10][11] but such occurrences are rare.

Treatment

See: Tuberculosis treatment

M. bovis is innately resistant to pyrazinamide: therefore the standard treatment is isoniazid and rifampicin for 9 months.

References