Mycobacterium bovis

Mycobacterium bovis, often referred to as the bovine tubercle bacillus, is a member of the Mycobacterium tuberculosis complex, a group of organisms with the capacity to cause tuberculosis in humans. Most animals are potential reservoirs of infection. Cattle, in particular, are susceptible to infection and subsequent tuberculous lung disease caused by M. bovis.

 

M. bovis in humans

An accurate historical assessment of the proportion of disease caused by M. bovis in humans is problematic. Studies undertaken in England and Wales in 1931, 1937, and 1941 estimated that around 6% of deaths due to all forms of tuberculosis were due to M. bovis  infection.  The current risk posed by M. bovis to human health in the UK is considered negligible. Prior to the introduction of effective controls through milk pasteurisation and tuberculin screening of herds to identify infected animals, M. bovis infection was much more common.

Between 1994 and 2007, 79% of the 453 human cases reported to the HPA were aged 45 years and above and only 17% were known to be non-UK born. This suggests that the majority of the cases seen in the UK are attributable to reactivation of latent infection, probably acquired prior to more widespread implementation of controls, principally milk pasteurisation and meat inspection.  Further epidemiological data can be found on the Tables and Figures page.

The prevalence of tuberculosis caused by M. bovis in developing countries is largely unknown due to the complexities and prohibitive cost in differentiating between mycobacterial species. The organism is known to be widely distributed and the zoonotic importance of M. bovis is potentially a serious public health problem, particularly in areas badly affected by the HIV pandemic and where effective controls through pasteurisation and the slaughter of infected animals are not applied.

M. bovis in cattle

During the 1930s, 40% of slaughtered cattle in England and Wales had obvious tuberculosis. Rates of disease have fallen dramatically since this time with new confirmed cases occurring in only about 0.4% of UK cattle herds each year.  However, the incidence of tuberculous disease in UK cattle caused by M. bovis is increasing. The most significant increase in M. bovis cases is reported from the south west of England, where over 1% of cattle herds are now affected annually.

Transmission

Transmission of M. bovis can occur between animals, from animals to humans and vice versa and rarely, between humans. As with M. tuberculosis, transmission is most commonly by the aerosol route but also through the ingestion of milk and meat from infected animals. The link between drinking milk from diseased cows and the development of scrofula, cervical lymph node tuberculosis, was established mid-19th century when more than half of all cervical lymphadenitis cases in children were caused by M. bovis.  Infection acquired through ingesting M. bovis is more likely to result in non-pulmonary forms of disease.

 

Cycle of Mycobacterium bovis transmission between cattle and humans.

The thickness of the arrows suggests probability. Adapted from Collins and Grange (1987).

 

Clinical presentation

It is not possible to clinically differentiate between TB caused by M. bovis and that caused by M. tuberculosis. The course and extent of the disease is the same, as is the treatment in most cases. Standard anti-tuberculosis therapy is effective against TB caused by M. bovis, however, the organism is inherently resistant to the drug pyrazinamide which is therefore omitted once M. bovis has been identified and its drug susceptibility is known. In cases where there is extensive lymph involvement or damage or obstruction to other tissues, surgery is often indicated. Like M. tuberculosis, M. bovis too has the capacity to acquire drug resistance and ensuring that patients are able take a full course of treatment is similarly essential.

The badger debate

Recent studies have demonstrated M. bovis infection to be endemic in some wildlife species in the UK. Infection in the badger population, particularly, is self-maintaining and represents a significant obstacle to the eradication of M. bovis from domestic cattle herds.  Controlling transmission between cattle and badgers has traditionally involved culling of local badger populations. This issue remains highly contentious. While the argument that there is an epidemiological link between M. bovis disease in the two species is compelling, there is no consensus on what would constitute an effective and humane programme of control. Further information on initiatives to control the reservoir of M. bovis in the badger population can be found on the Department for Environment Food and Rural Affairs (DEFRA) website (see links below).

Further sources of information

Bovine TB Leaflet for Farmers

DEFRA: Animal health & welfare - TB in cattle

DEFRA: Bovine TB - Publications

Last reviewed: 16 March 2011