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. 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.
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 the 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 of M. Bovis can occur between animals, from animals to humans, and vice-versa and - rarely - between humans. As with M. tuberculosis, the 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 the disease.
Pasteurisation of milk, immunisation of humans and healthy diet has seen the number of cases reduce dramatically. It should be noted that cattle with bTB lesions do routinely enter the food chain.
WHO CARRIES BOVINE TB?
The following animals have been shown to carry bovine TB
- Deer = 36% positive (includes farmed, wild and park deer)
- Cat = 25% positive
- Dog = 27% positive
- Pig = 19% positive
- Alpaca = 56% positive
- Llama = 0%
- Sheep = 44% positive
- Goat = 0
- Ferret (!) = 0
- Farmed wild boar = 0 (NB: two cases this year confirmed for wild boar and TB, both on TB infected farms
Foxes, dogs, cats and rats are also known to carry bTB.
HUMANS have a 1% incidence of TB, skewed to farm workers.
An increasing trend is for higher occurrences (live cultural positives) of TB in cats and alpacas.