Tuberculosis in man, cattle and badgers
This essay was written by Jo Colston and was first published in the 2002 Mill Hill Essays.
During the last century, the spread of tuberculosis (TB) from cattle to humans was a huge public health problem. A large proportion of dairy cows was infected with Mycobacterium bovis, the bacterium which causes TB in cattle. Many of these cows were kept near to large cities in order to provide fresh milk for the inhabitants. The cattle were often kept in closely confined, poorly ventilated cowsheds which provided ideal conditions for the spread of infection. Infection of the udders resulted in infected milk, which, when drunk, readily caused infection in man. In the 1930s, almost half of dairy cattle were infected with M. bovis, and this in turn resulted in more than 50,000 cases of M. bovis infection in humans every year, killing more than 2,500 people annually.
In response to this public health problem heat-treatment, or pasteurization, of milk was introduced. This led to a dramatic reduction in the number of cases of M. bovis infection in man so that in recent years there have been no more than 40 or 50 cases of bovine TB in humans per year. However, while the transmission of TB from cattle to humans has largely been controlled by heat-treatment of milk, cattle continue to become infected with M. bovis. This causes suffering to the cattle and may lead to the infection of other animal species. There is also a continued risk of transmission of the disease to humans.
M. bovis is a very close relative of the organism that causes human TB, Mycobacterium tuberculosis. Until recently it was thought that M. bovis was more ancient than M. tuberculosis and that the latter probably arose when man first began to farm animals. It was thought that M. bovis “jumped species”, infecting those early farmers, and it then gradually adapted to life in humans until it was recognisable as a separate organism, M. tuberculosis. This theory arose because M. bovis can infect a very wide-range of different animals, whereas M. tuberculosis is largely restricted to humans, suggesting that it is more highly adapted or evolved. However this theory now looks to be wrong. The genomes of the two organisms have been sequenced, and comparing them has revealed something quite remarkable. It appears that M. bovis is a much more recent organism than M. tuberculosis and has somehow evolved to be able to infect more species. However, although M. bovis is more successful in causing disease in a much wider range of animals, it doesn’t appear to be as successful at spreading between humans. Therefore stopping the spread from cattle to humans by pasteurisation of milk and by reducing the amount of TB in cattle generally, means that nowadays M. bovis infection in man is rare in most developed countries.
Attempts to reduce TB in cattle began in 1950, when compulsory testing of all herds for evidence of infection with TB was introduced. Today, all cattle are tested every one, two, three or four years, depending on the incidence of cattle TB in the particular area. If an animal shows evidence of infection, and this is confirmed by further testing, it is removed from the herd, valued, and killed. The farmer is compensated for the market value of the animal. However, finding TB infection in the herd can have serious consequences for the farmer because the herd loses its “Official Tuberculosis Free” status and restrictions on movement of the farmer’s cattle are applied. These can only be lifted after all of the animals in the herd have passed two consecutive tests 60 days apart. This may take many months, or, in some cases years, during which time no cattle are allowed in or out of the herd.
The strict application of this “test and slaughter” strategy led to a dramatic reduction in TB in tested cattle. In many countries where similar strategies have been used cattle TB has been virtually eradicated. In the UK, by the 1980s, less than 1 in 10,000 cattle were infected. This dramatic reduction in cases of TB in cattle was not, however, uniform across the whole country. In the South West of England particularly the disease did not fall at the same rate as in other parts of the UK. In this area the levels of TB in cattle remained virtually constant between 1965 and 1975, and stubbornly refused to follow the trend for the country as a whole. The strategy of controlling the disease by testing and diagnosis, followed by killing positive cattle and restricting the movement of animals with which they have been in contact, rests on the assumption that only cattle pass the disease on to other cattle. One possible explanation for the failure to control the disease was that this assumption was not correct, and there was an additional source of infection in that area, perhaps in the wildlife, from which the cattle were catching TB. In 1971, a dead badger, found on farmland in Gloucestershire where there had been a recent outbreak of TB in cattle, was found to be infected with M. bovis. Further testing suggested that TB infection in badgers might be quite common, and that badgers might be a particularly susceptible species to this infection. Gradually, suspicion that the badger might be an important source of infection for cattle started to grow.
Since the discovery that badgers in the wild could become infected with bovine TB, their role in the spread of the disease to cattle has been the subject of much controversy. In spite of three decades of debate, study and argument, it is still not clear whether badgers are responsible for what has now become an alarming rise in the spread of TB to cattle in certain parts of the country. It seems likely that there is some link between TB in cattle and TB in badgers since the distribution of infected badgers within the UK closely resembles the distribution of infected cattle herds. What isn’t clear, however, is whether badgers do spread the disease to cattle, and if so, to what extent they are responsible for the dramatic rise seen in the South West of England and South Wales. If badgers transmit TB infection to cattle, how does transmission occur? Do cattle transmit TB to badgers? Are other wildlife species involved?
By the mid-1970s the evidence that badgers played a significant role in infecting cattle was sufficiently strong that the UK Ministry of Agriculture, Fisheries and Food (MAFF) began to kill badgers in areas where it was judged that they posed a threat to the health of cattle. This policy was highly controversial. Firstly, because the role of badgers in transmitting TB to cattle was not conclusive, and secondly because the badger is a protected species, and in addition many people considered the method of killing, using poison gas, to be unacceptable. Although gassing of badgers was subsequently halted on humane grounds, the effects of mass culling of badgers on the incidence of TB in cattle was monitored in some detail in certain areas. It was found that the number of cases of TB in cattle fell sharply during and after the badger-removal period, but eventually started to increase again after the culling of badgers was stopped. This was further evidence that TB infection in badgers plays a significant role in the spread of the disease to cattle.
There is a parallel example of TB being present in a wildlife species which appears to act as a reservoir for transmission to cattle. In New Zealand, the brush-tailed possum is considered to be the culprit. As with the badger, attempts to control the disease in cattle by controlling the possum have resulted in some limited success. Although there is not a great deal of hard evidence available, what there is suggests that when there is a wildlife reservoir of bovine TB, such as the badger in the UK and the possum in New Zealand, attempts to control transmission to cattle by removing or killing the reservoir species are only successful if quite draconian removal strategies are used. Even if badgers are found to play an important role in the spread of the disease, for most people, complete removal of the badger from large parts of the UK would be too high a price to pay for controlling TB in cattle.
So, what are the options? One of the problems in the past has been that policy decisions have been taken without a good scientific understanding of the problem. The Country Landowners Association said in their evidence to a Select Agricultural Committee ” there has, for too long, been too little science and too much political judgement in this policy area”. Over the past four years, MAFF (or DEFRA; The Department for Environment, Food and Rural Affairs, as it is now called), have implemented an extensive and broad research programme to better understand the spread of TB in cattle, and how it might be controlled. Although TB in badgers plays a significant part in this programme, it is by no means the only issue being researched; other factors, such as the extent of cattle-to-cattle transmission, and the influence of farming practices are also being looked at.
One of the crucial issues in developing new policy for the control of TB in cattle is the relative importance of badger-to-cattle transmission, cattle-to-cattle transmission and transmission of the disease from other wild-life species. The badger culling trial, or Krebs trial (after Sir John Krebs whose report in 1997 provides much of the framework for the current DEFRA research programme), along with the associated extensive epidemiological study, has been set-up in an attempt to quantify the contribution of badgers to cattle transmission. In this trial ten “triplet areas” have been selected in parts of the country where the incidence of TB in cattle and badgers is known to be high. Each “triplet area” consists of three circles, each of approximately 100 square kilometers. In one of the three circles all badgers are removed from the area by cage trapping and shooting. In another of the three circles badgers are only culled on and around farms following outbreaks of TB in cattle. In the third area, no badgers are culled, but the land is surveyed and badger activity recorded. In all, thirty areas, representing ten triplets, have been incorporated into the trial.
As one can imagine, this trial has been the subject of much controversy and innumerable problems. There have been many reports of attempts to block badger culling by animal welfare protestors and other groups, and of killing of badgers in the control areas by people wishing to protect cattle. Most disruptive of all, the outbreak of Foot and Mouth Disease in the first half of 2001 meant that the trial had to be suspended due to the restrictions on movement in the countryside. Nevertheless, the trial has now resumed and results should start to become available in about 3 years’ time. Although the infrastructure required to carry out the trial has been extensive and costly, it represents an enormously important opportunity to establish and quantify exactly how significant a role the badger plays in increasing rates of TB in cattle, particularly in the South West of England.
In addition to placing the role of the badger in the spread of TB on a firm scientific footing, the trial also forms the backdrop to other research programmes. For example it has highlighted the need for better diagnosis of TB in cattle and badgers and research in this area is making significant advances. It is interesting that using sensitive techniques for genetically fingerprinting strains of M. bovis, the strains of the organism in the badger population in a particular area are the same as those in the cattle population, adding further weight to the idea that there is cross-infection between the two species. However there is also a need to look at TB in wildlife other than badgers in order to get a broader picture of how different species may transmit the disease. This is currently being undertaken, along with the development of better ways of differentiating between strains of M. bovis so that links between cases of TB in all species can be established with greater confidence. Most importantly, a better understanding of cattle-to-cattle spread of the disease and the ability of the existing techniques to diagnose early infection have been identified as crucial areas where better information is required.
Another recommendation of the Krebs Report was to implement a programme of research towards developing a vaccine, either for use in wildlife or in cattle. While innovative work on this is in progress, development of a vaccine is likely to take many years. In the meantime, over the next few years we will get a much better understanding of how bovine TB is spread, and we will have better tools for early diagnosis of the disease in both cattle and badgers. The question that will then need to be addressed is “How can we use this information to devise new policies to control the disease?” Obviously much will depend on what the results of the trial tell us, but even if it turns out that badgers do play a role in spreading the disease to cattle, what can we then do to try and break this link? Few people would wish to see large-scale removal of the badger from the British countryside. Alternatively if transmission from cattle-to-cattle is a major factor, how can the current control strategy be modified to reduce the spread between animals?
Whatever the outcome of the badger culling trial, it is likely that a range of measures will need to be put in place to reduce the incidence of TB in cattle. The exact focus of these measures will depend on what the trial shows regarding the relative importance of cattle-to-cattle and badger-to-cattle transmission. If cattle-to-cattle transmission is shown to be most important then the measures are likely to focus on better diagnosis, so that action can be taken earlier to remove infected animals and to restrict cattle movements more effectively. On the other hand if badger-to-cattle transmission appears to be most important, then the measures might involve some level of badger population control, and the development of husbandry methods to restrict contact between badgers and cattle on farms. In any event the current programme of research should provide a much sounder scientific basis on which such decisions can be made, along with improved technologies for implementing new control measures.