31 Dec 2011, 7:13 PM
Tuberculosis (TB) is a disease caused by many different strains of the bacterium mycobacterium. Despite it being a very ancient disease there is still a great deal to learn about these bacteria. In humans TB is usually caused by Mycobacterium tuberculosis. In cattle it is mainly caused by Bovine tuberculosis (M. bovis). However, the two are closely related. We know that transmission of M. bovis can occur between animals, from animals to humans and, more rarely, from humans to animals and between humans. The consumption of unpasteurised milk or dairy products from infected cows was an important cause of childhood tuberculosis in the UK until pasteurisation became widespread in the mid-20th century. However, nowadays human cases of Bovine tuberculosis are extremely rare, despite the alleged increase of the disease in many areas among cattle and other animals. Any warm-blooded animal can contract M. bovis – but again this is very rare.
In fact the various strains of TB are quite difficult to catch and usually require repeated or prolonged contact with an infected person or animal. Even then not all infected with the bacteria causing TB will develop the disease. The majority (about 9 out of 10) will completely eliminate or contain the bacteria and will not develop symptoms or disease. Around 10% of infected contacts may develop the disease, not necessarily immediately, but possibly at some later point in life (known as latency). This is because the bacteria that cause TB can survive in the body in an inactive state for decades and may cause TB later in life if an immune system is weakened by old age, other illness or poor living conditions. The disease is treatable, requiring a long course of antibiotics.
Mycobacterium tuberculosis (human form of TB) and Bovine tuberculosis
It is interesting to note that the human form of TB, Mycobacterium tuberculosis, a far more significant threat to human health than bovine TB, has risen by 4.2% in the UK and now affects around 15 per 100,000 people - mainly young adults 15-44 years old. Around 60% contracted the disease from abroad (where the disease may be rife) before they came to the UK. Most of these cases are confined to the larger cities. Such cases may not be diagnosed for two or more years – yet there appears to be very little concern regarding whether or not these cases are spreading the disease to others throughout this time. This is surprising, bearing in mind the existing policy regarding bovine TB and cattle dictates that we have to identify and kill every animal that has been exposed to the bacteria – ‘just in case’. Yet bovine TB is not a significant public health issue, so we ask – why is there so much fuss and so many resources being put into trying to eradicate bovine TB?
Is bovine TB really a public health risk?
The Health Protection Agency's report, 'Tuberculosis in the UK' (Ref. 1), on tuberculosis surveillance in the UK 2010 reveals that bovine TB accounted for JUST .5% of the total TB cases in 2010, despite it being alleged that bovine TB is on the increase and now endemic in many areas.
It would seem our current fear of the disease is based on levels that existed before pasteurisation of milk was introduced. Studies (Ref. 2) undertaken in England and Wales back in 1931, 1937, and 1941 estimated that around 6% of deaths due to all forms of tuberculosis were due to M. bovis infection. Considering that most milk samples in those days contained the bacteria, this is a remarkably small percentage, and clearly shows that, in most people, the immune system will fight off the disease. Nowadays, as virtually all milk is pasteurised, the risk from bovine TB is now negligible.
In the UK between 1994 and 2007, 79% of the 453 human cases of bovine TB reported to the Health Protection Agency (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 can be attributed to reactivation of latent infection, probably acquired prior to the more widespread implementation of controls, principally milk pasteurisation and meat inspection.
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, but this is mainly in areas badly affected by the HIV pandemic and where effective controls through pasteurisation and the slaughter of infected animals are not applied. This is not currently the case in the UK.
In fact in the UK, despite the alleged increase of bovine TB and alleged spread to other species (it can affect any warm blood mammal) human cases of the disease have not materialised, even among those working in close proximity to cattle. This ties in with the facts given in ‘Public Health and Bovine TB; What’s All the Fuss About’ (Ref 3). In this paper the authors presented evidence that even when bovine TB was common in UK residents (the early part of the last century), it was almost entirely transmitted from cattle to humans in milk, and hence pasteurization dealt with the problem.
Interestingly the meat from reactor cattle is, in most cases, still sold for human consumption and there is even talk in the government’s latest Farming Regulation Taskforce report published recently, about using milk from reactors in the human food chain. Para 10.25 says ' We agree with industry that on scientific and safety grounds, the requirements to destroy milk from TB reactors is not evidence based. However, to allow milk from TB reactors to enter the food chain, we must be confident that we can identify TB reactors and know that milk has been pasteurised. We recommend that Defra, the FSA and dairy trade associations should open discussions on how pasteurised milk from TB-reactor cows, could safely enter the food chain.'
What about other diseases?
Surely there are many other diseases that are of greater risk to the UK population (currently totalling round 65 million). For example, in 2010, there were 6,136 new cases of HIV, contributing to a cumulative total of 114,766 cases reported by the end of December 2010. As of December 2010, there have been 26,791 diagnoses of AIDS in the UK, and 19,912 people diagnosed with HIV have died. There were less than a handful of bovine TB cases.
What about other fatalities?
Also we could look at the fatality figures for various high-risk professions, including agriculture – all have higher death rates than from bovine TB (Ref. 4)
In agriculture in 2010/11 there were 24 deaths (a rate of 8 deaths per 100,000 workers). In construction there were 50 fatal injuries, with a rate of 2.4 deaths per 100 000 workers. This compares to an average rate of 2.8 for the previous five years. In manufacturing there were 27 fatal injuries, with a rate of 1.1 deaths per 100 000 workers (the same rate as the average for the previous five years). In the services sector there were 47 fatal injuries, with a rate of 0.2 deaths per 100 000 workers. (the same rate as the average for the previous five years). In the waste and recycling industry (categorised using SIC divisions 38 and 39) there were nine fatal injuries, with a rate of 8.7 deaths per 100 000 workers. This compares to an average rate of 7.0 for the previous five years.
How consistent is our approach to Bovine TB compared to other zoonoses?
A leaflet from the HSE (Health & Safety Executive), ‘Common Zoonoses in Agriculture’ (Ref. 5)
reveals how inconsistent our approach is to bovine TB, compared to all the other infections which are also transmissible to man. Some are widespread and just as unpleasant if contracted. These include:
Leptospirosis, Orf, Ringworm. E coli O157, cryptosporidiosis, chlamydia, Streptococcus suis (S suis), Salmonella, Psittacosis (Ornithosis), Q fever and Listeriosis.
Defra doesn't test or monitor for these other diseases so accurate levels are not available but, again we ask, why all the fuss about bovine TB?
The BCG vaccine and skin test
The BCG (Bacillus Calmette- Guèrin) vaccine (single injection into the skin) can be given to humans to help protect against TB. This is a modified strain of M. bovis. The bacteria in the vaccine are alive, but have been modified so that, in most people, it should not cause disease. It does not prevent TB in all cases but helps reduce severity. It was introduced into the UK in 1953 when TB was more widespread. It was part of a universal programme of testing and vaccinating but this was discontinued some years ago as rates of the disease fell. Usually a skin test is performed to determine if the immune system already recognises TB. A small amount of tuberculin (a solution of purified proteins from M. tuberculosis) is injected into the skin. The injection site is inspected two to three days later. If it is positive then there is a raised red reaction at the needle site. If this is the case the BCG vaccine is not given, as the person has already come into contact with TB (or other similar bacteria). If the reaction is severe other tests are undertaken in case the person actually has the disease. The test is similar to that given to cattle and there are similar concerns regarding its absolute reliability. Humans that react to the test are not culled as cattle are and most will continue to lead a normal life among us with no ill effects.
So what can be done?
Surely we need to justify why we spend so much public money on bovine TB when the risk to human health is so low now most milk is pasteurised and meat is cooked - heat destroys any mycobacteria?
We could opt for a control, rather than eradication policy for bovine TB and give responsibility for disease back to the farmers, perhaps even discontinuing the export of live animals in order to get a derogation from the EU. Farmers would then choose to use the BCG vaccine for their cattle, or to continue with the test and cull/treat (at their own expense), depending on what is best for their farm. This would avoid the compensation claims we are currently seeing. We know that the BCG vaccine is not perfect, but then neither are the existing skin test or blood tests, which merely identify if an animal has been exposed to the bacteria that causes TB and do not necessarily indicate infection or disease.
3. An opinion piece in Trends in Microbiology by Professor Paul and Professor David Torgerson
www.healthpromotionagency.org.uk/Resources/parentschild/immunisation/pdfs/TB%20Factsheet.pdf (Tuberculosis Fact Sheet for healthcare workers (Health Promotion Agency):)