Disease Facts - Bovine tuberculosis

The past and present problem.

In the 1930s around 40% of dairy cattle in the UK were infected with the bacterium Mycobacterium bovis, and 0.5% of cows produced contaminated milk. Approximately 2000 human deaths a year are believed to have been caused by drinking contaminated milk or by being in close contact with infected cows. The disease was brought under control by pasteurising milk and testing animals using the tuberculin skin test. Infected cattle were removed from the herd and culled.

Bovine TB is increasing in the UK

However, the incidence of bovine TB has been rising in England and Wales recently. As many as 700 herds fail the tuberculin test each year. We don't know why this is happening, although the re-stocking of farms after the 2001 foot-and-mouth epidemic rapidly increased the spread of bovine TB in cattle, there are also unexplained clusters of cases in the south west of England.

Although there is no longer a public health risk from milk due to the control measures now in place, farmers are suffering huge financial losses and distress because of infections in their herds. The cost of the Government's control programme has also rocketed in recent times.

Both the Government and scientists agreed that a new structured research programme was needed to bring the disease under control. But this required a good understanding of what was happening in the environment.

Is the badger the culprit or a victim?

Badgers have been suspected of being a source of infection (reservoir) since the early 1970s. Various surveys have confirmed that they can be infected by M. bovis (see Table 1). Some infected badgers develop TB and produce large numbers of bacteria, which can spread to other animals. Infected badgers have been shown to transmit infection to cattle under experimental conditions, but the relevance of this finding has been questioned. Other wildlife species also are susceptible to M. bovis. However, apart from the feral ferret, infections in these species are not as common as in badgers and they do not develop serious symptoms or excrete as many bacteria.

Table 1
Incidence (%) of M. bovis in wild life species:
Mink 0.6
Deer 1.0
Fox 1.0
Mole 1.2
Rat 1.2
Ferret 3.8
Badger 4.0

No infections were found in bats, cats, grey squirrels, hares, harvest mice, hedgehogs, rabbits, shrews, stoats, voles, weasels. Source: MAFF.

Badgers were culled during the 1970s in areas where bovine TB was a problem. This led to a drop in disease levels in cattle at several sites where there had been intensive gassing of sets. In the 1980s gassing was replaced by trapping and more humane killing methods. Trapped badgers were only killed if they were shown to be infected. However, this has proved unreliable because of the poor sensitivity of the diagnostic test and despite these measures, the incidence of TB in cattle continued to increase.

Set against this discouraging scene, the Government commissioned a review of the evidence for a link between badgers and bovine TB chaired by Professor John Krebs. This was to include recommendations on future policy. The Review Group concluded that there was convincing evidence that badgers were a source of infection for cattle, but the extent to which they added to problem in cattle (ie what proportion of cases in cattle were due to badgers) could not be calculated. At the same time it was recommended that a vaccine should be developed against the infection. In a reversal of previous thinking the Group considered that the prospects of a vaccine for cows was more promising as there had been more progress in understanding the basic immunological responses in cattle.

Scientists at IAH are looking for new ways to control the disease in cattle

What does the future hold?

The contributions of the IAH to improving understanding of bovine immunology, together with its unequalled resources, physical, human and biological, make the Institute an ideal contender for carrying out the research that is so urgently required. The first steps towards developing a new research initiative to develop a vaccine to protect cattle against tuberculosis have already been taken.