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Image: Eurasian badger (Meles meles).
Home » Eurasian badger (Meles meles) » Articles :
Article 2: Background on Bovine TB
Bovine tuberculosis - what it is, how it is spread, and why we cannot yet vaccinate cattle.
See also: 1: Culling in Britain and 3: Do badgers infect cattle with TB?
On this page:
Introduction | What is bovine TB? | How is TB spread? | Can humans catch bovine TB? | How many cattle have bovine TB? | Why is bovine TB considered to be such a problem? | Why can't we vaccinate cattle against TB? | Related Badger Pages
Note: Some of the information on this page is now out of date. The level of TB in cattle has risen from the figure quoted below since this page was first prepared. Also, there has been considerable progress in the development of candidate vaccines for bovine TB. The information on this page will be updated soon. For more information, see the Badger Trust website.
If you are not sure what TB is, how it is spread, or why we cannot simply vaccinate cattle against the disease, this page is for you. Here I will attempt to answer some basic questions about bovine TB.
Bovine TB is a variety of tuberculosis which is known primarily as an infection of cattle. A wide range of other mammals can however catch bovine TB. The scientific name of the bacteria which causes this variety of TB is Mycobacterium bovis.
There are other strains of TB which affect other warm-blooded animals. Mycobacterium tuberculosis causes human TB, a disease which infects a third of the world population and kills 2-3 million people every year. Mycobacterium avium causes avian TB, mainly in birds but also in some mammals. A strain of the avian TB bacteria known as Mycobacterium paratuberculosis causes Johne's disease in cattle and other mammals. Some scientists believe that Crohn's disease in humans is also caused by this bacteria.
An animal which is infected with TB does not automatically infect others. It only passes the disease on to other animals when it becomes infectious . An animal becomes infectious when open lesions develop on the surfaces of organs such as the lungs; bacteria are then discharged and are released into the outside world via the animal's breath, saliva, milk, urine or faeces. Other animals can then become infected by these bacteria. For example, an infectious animal suffering from tuberculosis of the lungs may discharge large numbers of bacteria in droplets of moisture in its breath, particularly when it coughs; if another animal breathes in those droplets of moisture its own lungs will become heavily infected. That animal in turn may then become infectious and pass the disease on to others.
TB bacilli can also be transmitted indirectly. For example, if an animal passes infected urine, another animal can become infected if it later eats the grass on which the infectious animal urinated. The TB bacteria however do not form spores as some bacteria do, so they do not survive for long periods outside of a host animal.
Yes, but this is a rare occurrence. People used to become infected with the bovine form of TB by drinking unpasteurised milk which contained the TB bacteria. These days however, virtually all milk is pasteurised or ultra-heat treated; this process kills off the TB bacteria. Although TB is now on the increase in Britain (and indeed elsewhere), this is an increase of the human form of TB. Scare stories about people being at risk of contracting bovine TB from badgers are exactly that - scare stories, with no basis in fact.
Recently, it has been discovered that Mycobacterium paratuberculosis, which is believed by some scientists to cause Crohn's disease, can survive the pasteurisation of milk. This has led to worries that Mycobacterium bovis may also be able to survive the pasteurisation of milk in some cases, and that this may mean there is a risk of bovine TB being passed from cattle to humans through milk. As yet, there is no evidence that this happens.
In the 1930's, bovine TB affected about 40% of all cattle (beef and dairy). Today however, the disease is actually quite rare, although the number of cases has increased in recent years. Only 0.07% of cattle have the disease, that's 7 in every 10,000.
During the 1960's, it became apparent that levels of TB in cattle in certain parts of the south-west of England were not falling to the same low levels as elsewhere in the country. TB levels in those areas were in fact around 5 - 10 times higher, and this has continued to be the case. In recent years, the level of TB in cattle has been increasing, and outbreaks have been occurring in new areas. Thus there is concern that the disease may continue to increase and spread to other areas unless something is done to halt its progress.
Humans can be vaccinated against TB. It would therefore seem that a simple solution to the problem of TB in cattle would be to vaccinate the cattle. Unfortunately, the situation is not that simple. Currently, cattle have to be tested at intervals to check whether or not they have TB, so that infected animals can be slaughtered. If cattle are given any of the current TB vaccines, they would show up as positive for TB when they are tested. This would make testing cattle useless, as a positive result might mean that the animal has been vaccinated, or it might mean that it has got TB - it would not be possible to tell. (No TB vaccine is 100% effective.)
DEFRA has sponsored research to try to find a vaccine that will prevent TB, without interfering with the TB test. However, they estimate that this will take between 10 and 15 years, and this is the reason they have given for wanting to take other steps to fight TB in the meantime, such as carrying out the culling experiment.
It is interesting to note however that work on developing a suitable vaccine has been in progress elsewhere in the world for several years now. For example, the Texas A&M University is developing two types of vaccine, one based on Mycobacterium bovis DNA, and the other based on mutated Mycobacterium bovis DNA. Another vaccine is also undergoing trials in New Zealand.
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