Tuberculosis (TB) in people is typically caused by a bacterium called Mycobacterium tuberculosis (MTB). This is, more or less specifically, a human pathogen and so you catch the disease from someone who has TB, and not from infected animals. The most common form of the disease affects your lungs, and so you pass it on by coughing, or indeed by anything that generates aerosols including talking and breathing. When you breathe in the contaminated air, the bacteria settle in your lungs. A curious feature of TB is that most (about 90%) of people who are infected never show any signs of the disease (and are not infectious). About 5% will develop symptoms within 12 months of being infected, and the remaining 5% will develop symptoms at some time during their lives, maybe 50 years later. This reactivation of an earlier infection generally reflects a decline in the effectiveness of your immune system as you grow older.
Until the middle of the twentieth century, there was another form of TB in the UK, caused by a very closely related bacterium called Mycobacterium bovis (MB). Unlike MTB, MB is a very versatile bacterium that is capable of infecting probably all mammals. The main way in which people were infected was by drinking contaminated milk. By this route, instead of the bacteria going for the lungs, they move to the lymph nodes that monitor material draining from the throat (neck lymph nodes) or digestive tract (abdominal lymph nodes). These sites of infection tend not to shed material into the environment, so people with this form of the disease are much less likely to be infectious.
This form of the disease in humans was virtually eliminated by two measures: tuberculin testing of cattle, and pasteurisation of milk. Tuberculin testing involves making an extract of killed mycobacteria and injecting a small amount into the skin of a cow. If the cow is infected, it will react – shown by a swelling at the site of injection, That cow is then killed (and the farmer is compensated). This is coupled with restrictions on the movement of cattle until the entire herd is declared free of the disease. Pasteurisation involves heating the milk to a specific temperature, which kills the mycobacteria (and incidentally some other significant pathogens), while producing minimal changes in the quality of the milk. For a while, consumers had a choice of tuberculin tested (TT) milk, pasteurised milk, or ‘ordinary’ milk. Nowadays, all milk in the shops is both TT and pasteurised.
There is also a potential risk from eating undercooked meat from an infected cow. While the tuberculin testing of cattle should ideally prevent this, to make sure none get through, vets at the abattoir inspect the carcasses to make sure they do not have any signs of TB.
So what’s the problem?
Since the existing control measures mean that the risk of human infection with MB is very low, why is there so much fuss about it? This comes mainly from the cost, to the government (and therefore to us as taxpayers) and to the farmers, costs arising from the testing of cattle and the killing of infected cattle. Although farmers are compensated for the cattle destroyed, this does not necessarily reflect the disruption to their business, let alone the loss of morale from the possible destruction of a herd that may have been built up over a period of many years. Understandably, the farmers argue strongly for the government to take radical action to tackle the sources of infection.
How do cows catch TB?
A rational strategy for controlling any infectious disease depends to a large extent on understanding the route of infection – i.e., how the disease spreads. The assumption behind the culling of badgers is that badgers are the source of infection. Is that true? It is undeniable that badgers can be infected with TB, but whether, and to what extent, they pass it on to cows is unproven. In a well-run farm, direct contact between cows and badgers is likely to be uncommon, hence direct transmission is unlikely. The main exception to this is if badgers can get access to cattle sheds, but this can be prevented.
Another possibility is that the cows eat grass that has been contaminated by badgers. e.g. by urine or faeces. However badgers use regular latrine areas, and these are likely to be avoided by cattle. So there are unsolved questions about the possible route of transmission from badgers.
Note that this is very different from the situation in New Zealand, where possums are a significant source of infection of cattle. Possums are very susceptible to TB, and they tend to die in the grazing areas. Cows being curious animals, they sniff at the possum carcass and thus become infected. New Zealand is making progress towards eliminating this by controlling the possum populations. Since the possum is not a native animal in New Zealand, there is less objection to control measures than is the case with badgers in the UK.
Will badger culling work?
If badgers really are the source of infection, then clearly something would need to be done about it. Is culling the right answer? Would it work?
To try to answer that question, a scientific study was carried out in selected areas, comparing what happened in those areas where badgers were culled with control areas where there was no culling. Although this seemed to show that in the culled areas there was some reduction in the level of TB in cattle (provided that a high enough proportion of badgers was removed), there was a rather surprising additional effect: In areas immediately adjacent to the culled areas, there was an increase in cattle TB. The likely reason for this is that the culling disrupted the normal badger population. Badgers are highly territorial, so removing badgers from one area will result in more contact between badgers in adjacent areas as they try to re-establish their territories. The conclusion is that culling could be effective, with two conditions – first, that a high proportion of badgers are removed, and secondly that the culled area should be large enough and surrounded by natural boundaries (e.g., large rivers, mountains) to prevent any mixing of badger populations in culled and non-culled areas. (The current culling programme largely fails on both counts).
What other methods are possible?
Firstly, we have to recognise that badgers are not the sole culprit. Leaving on one side the possibility that other wild mammals may be involved (remembering that MB can infect all mammals), we also have to take into account the transmission of the disease from one cow to another. Modern farming practice often involves movement of animals from one part of the country to another, with the possibility of spreading the disease. In this respect it might be significant that the increase in the number of cases of bovine TB, and the widening of their distribution, followed closely on the foot and mouth outbreak in 2001, when there was large-scale re-stocking of farms, involving moving cattle around the country.
When a case of TB is found on a farm, movement of cattle off the farm is prohibited until the herd is declared clear again. This may be inadequate in two ways – infected cattle may have been moved before the disease was detected, and a ‘farm’ may involve widely separated pieces of land. Tighter control of these areas, and recognition by the farming industry that they have an important role in disease prevention, would help.
The second possibility that is often raised is vaccination of cattle. There is a serious problem here. If you vaccinate a cow with the existing human vaccine, BCG, then that cow becomes a reactor – in other words it will now react with the skin test in the same way that an infected cow does. So you cannot distinguish between a vaccinated cow and one that has been infected – and since the vaccine is far from 100% effective, even a vaccinated cow can be infected. One important consequence would be that the UK would be prevented from exporting animals or meat to other countries which require all animals to be TB free.
There are two ways in which this could be overcome – either develop a better vaccine that would not interfere with the skin test, and/or develop a better testing method that could distinguish infected and vaccinated animals, (There is a need for a better test anyway). Unfortunately, promising research efforts on both fronts were terminated when the government withdrew its support.
Finally, instead of vaccinating cows, you can vaccinate badgers. This is not straightforward, as the BCG vaccine has to be given by injection. So you have to trap the badgers first, which is a skilled job. But it can be done, and is being done in some areas, by voluntary groups (the Derbyshire Wildlife Trust is involved in such a programme in Derbyshire). Ironically, the only safe and effective way of culling badgers involves trapping them first and then shooting them while they are in the cage. You may well wonder why, if they have been trapped already, they don’t vaccinate them rather than shooting them.