Bovine tuberculosis is prevalent in certain regions of the UK and has been for several years now. It is a disease of cattle that is caused by the organism Mycobacterium bovis. The human form of the disease is caused by Mycobacterium tuberculosis, a related species, although there have been isolated instances of human infection by M bovis. Because of this, cheese makers are not allowed to make raw-milk cheese if their herds are not certified as ‘TB-free’.

Testing of cows for M bovis is an inexact science. Most herds in the UK are tested every one to three years in order to maintain their TB-free status. Some raw milk cheese makers such as Jamie Montgomery test their cows every six months.

Recently, a new ultra-sensitive test of the cows (the ‘gamma–interferon test’) has been introduced, which is giving many false-positive readings. Any cow failing the tests will be slaughtered and subject to post-mortem and subsequent analysis of tissue which can take up to eight weeks. In most cases, amongst raw-milk cheesemakers, when cows fail the gamma interferon test, the cow subsequently is found to be TB free, however during the time it takes to get this result, the cheese maker is not allowed to make raw milk cheese.

As this test becomes more widely used, this is likely to happen more frequently in the future.

SPREAD OF THE DISEASE

It is not clear why bovine tuberculosis is a larger problem in the UK than in other countries. There may be some element of ‘don’t ask, don’t tell.’ The spread of the disease has been accelerated by movement of cattle between herds throughout the country, and was exacerbated by large amounts of restocking after the Foot and Mouth Disease epidemic. There is also the suspicion that M bovis is maintained in the ‘wildlife reservoir’, the community of animals that surrounds a farm. Badgers particularly are thought to act as a vector that can spread infection between herds as well as acting as a reservoir to re-infect a herd.

Currently there is no coherent government policy to prevent the further spread of the disease in livestock.

The normal mode of transmission of the infection is respiratory, through droplets of saliva. The human form of tuberculosis is associated with areas of overcrowding and malnutrition. The likelihood of M bovis infection passing from cattle to humans through unpasteurised milk and cheese is minimal.

In most cases, when a cow contracts the disease, the first place where lesions appear is in the throat. It is very unlikely that the bacteria will enter the milk unless the disease spreads to more than one site in the body through the blood and lymph systems. Because of this, appearance of the bacillus in the milk is highly unlikely during the first stages of infection. Testing the herds frequently minimises the chances of the disease progressing past the initial stages before it is detected.

BOVINE TUBERCULOSIS & RAW MILK CHEESE MAKING

The survivability of Mycobacterium bovis during cheese making is a debated topic. Research is currently being done within the UK to assess the extent of this, though it is likely to depend on the type of cheese and the concentration of the bacteria present in the milk. (In a herd with only one infected animal, the concentration of bacteria in the milk would be diluted to a very low level, which represents another protective factor.)

The threat of a person contracting bovine tuberculosis from raw milk cheese is extremely remote; however, the threat bovine TB poses to continued raw milk cheese making in the UK is high.

Randolph Hodgson
January 2010