Introduction to the disease

Bovine tuberculosis (bTB) is a chronic, infectious and primarily respiratory disease caused by the slow-growing bacterium Mycobacterium bovis (M. bovis). It is mainly a disease of cattle and other bovines, but can affect a wide range of mammal species.

Part of the Mycobacterium Tuberculosis Complex (MTC), M. bovis is closely related to M. tuberculosis, the bacterium that is the primary cause of TB in humans. Bovine TB is a zoonotic disease, which means that it can be naturally transmitted from animals to humans under certain conditions. Since the pasteurisation of milk and with a long standing and compulsory bovine TB control plan this is now very rare in the UK, although human TB caused by M. bovis can be a significant problem in developing countries. Bovine TB is listed by the World Health Organisation (WHO) as one of seven neglected endemic zoonoses.

> Click here for Public Health England guidance on reducing the risk of human M. bovis infection for farmers

> Click here for Public Health England data on cases of human TB caused by M. bovis

Other members of the MTC include more rare bacteria such as M. microti (the ‘vole bacillus’), M. caprae (the ‘goat TB bacillus’) and M. pinnipedii (the ‘seal bacillus’).

The MTC itself is part of a wider family of more than 100 different bacterial species known as Mycobacteriaceae. Many of these bacteria are naturally found in soil and water and are harmless to animals and people, but others are responsible for diseases including human leprosy, Johne’s disease in ruminants and avian TB (tuberculosis of birds).

 

Genotypes and spoligotypes

Through the use of DNA typing techniques, M. bovis can be ‘broken down’ into different strains or genotypes, which tend to be geographically localised. To characterise the strain of the bacterium infecting an animal, the APHA national reference laboratory for bovine TB uses a combination of two such techniques that target different regions of the M. bovis DNA: spoligotyping and VNTR typing. A genotype of M. bovis is simply a combination of the spoligotype and VNTR type. So, for instance, a strain with spoligotype 17 and a VNTR type 7-5-5-5*-3-3.1 is reported as being of genotype ’17:a’.

The story of bovine TB – The attack of the clones.
By Professor Ian Boyd, Defra Chief Scientist.

Originally published on the Tackling Bovine TB Tumblr:
> Tackling bovine TB Tumblr

Mycobacterium bovis, the bacteria that is the cause of bovine TB (bTB), is a fascinating beast. Because it doesn’t have sex very often – perhaps not at all – each bacterium is a clone of every other bacterium. For the forensic microbiologist, this high genetic stability is both a blight and a blessing. It is a blight because it makes tracking short-term variation in M. bovis extraordinarily difficult, unless you use whole genome sequencing. But it is a blessing because it makes tracking the long-term origins of M. bovis much easier. So what does this tell us?

First, it says that the British Isles has its own clone of M. bovis (ingloriously referred to as the SB0140 clonal complex). About 85% of bTB in Britain is caused by this beast but it is almost absent from our continental neighbours. Therefore, for example, the M. bovis bacteria in France is very different to what we have in the UK. SB0140 does however have some closer relations in Australia, New Zealand, Canada and South Africa, probably dating way back to the days of the British Empire through cattle trading with these nations.

Second, when one drills down in to the details of this clonal complex within Britain one finds an interesting pattern. There are different forms of bTB in different areas. Put simply, if bTB could talk it would probably have regional accents. This implies, for example, that bTB from Somerset doesn’t mix much with bTB from Cornwall. Now, if you are in to bTB like I am this is just fascinating. It also an encouraging signal that cattle movement controls to prevent the spread of bTB are working, as otherwise we would probably a lot more mingling of the bTB strains and an eventual blurring of regional distinctions. That there are still such thick ‘accents’ between regional variations suggests some success in containing them within their regions.

The emergence of the SB0140 clonal strain in the British Isles is a bit of a conundrum. Could it be the result of selection caused by the kind of test we use to determine whether cattle have TB? To test cattle for TB, we use a strain of TB (called AN5) that was gathered from an English cow in the 1940s. While this test is still perfectly adept at picking up SB0140, we have found through our testing that the original strain, AN5, has been successfully eliminated from English cows. It is interesting that while this strain has died out, SB0140 has survived and flourished. This leads me to an intriguing hypothesis; is SB0140 specifically adapted to survive and thrive in badgers?

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Source: APHA Bovine Tuberculosis: Infection Status in Cattle in GB. Annual Surveillance Report for the period January to December 2014. See link below to access the report.

< The map shows the distribution of major spoligotypes in GB in 2014. There are also minor spoligotypes which can be seen in the APHA report.

Spoligotypes can be broken down again. For example, spoligotype 9 is made up of 9:a, 9:b, 9:c, 9:d, 9:e, 9:f, 9:g, 9:h, 9:j, 9:m, 9:n, 9:s, 9:an. These genotypes are also mapped in the APHA report (but are not shown on this map).

> Click here for APHA annual bovine TB surveillance reports, which provide a comprehensive account of the current descriptive epidemiology of bovine TB in Great Britain and in each area of England.

If your herd has or has previously had a breakdown with positive laboratory culture results for M. bovis, APHA should have details of the genotype involved.

As not part of statutory requirements, APHA do not automatically report these findings to farmers, although Veterinary Officers (VOs) are able to discuss findings on an individual basis.

It is standard for only one animal to be genotyped at the initial breakdown. Be aware that the whole process of culturing and then genotyping the bacterium can take several months. The results can be useful in certain circumstances to help identify possible sources of infection during a breakdown, although may need finer interpretation as to their meaning.

With 80% of bovine TB breakdowns occurring in the ‘home-range’ of the locally predominant strain of M. bovis, if you have had a confirmed breakdown and APHA have not discussed the genotype with you, it is likely due to it being your local strain.

> Click here for Plos Pathogens. Biek R, O’Hare A, Wright D, Mallon T, McCormick C, Orton RJ, McDowell S, Trewby H, Skuce RA, Kao RR. 2012. Whole Genome Sequencing Reveals Local Transmission Patterns of Mycobacterium bovis in Sympatric Cattle and Badger Populations