The evolution of Mycobacterium bovis and the badger culling controversy
A Complication:
Disease Spread
Bovine TB infections in cattle pose a major problem in great Britain. In recent years the number of cattle herds infected with bovine TB per year has risen dramatically; in 2010 10.8% of English herds were involved in a TB incident. Between 2010 and 2011 around 25,000 cattle had to be slaughtered due to infection with bovine TB, costing the UK taxpayer £91 million. An understanding of how bovine TB spreads may aid the construction of control measures against the disease. Cattle and badgers are the most common hosts of bovine TB. The existence of two common hosts prevents scientists from clearly identifying how bovine TB spreads in the UK. The disease can be transmitted from badger to cattle and vice versa, meaning that a spread of disease can be caused by a mix of hosts. Furthermore, the transport of cattle around the country further complicates attempts to understand the distribution of the disease.
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A diagram summarising the evolution of M. bovis from the Mycobacterium tuberculosis complex. Blue circles represent the various Mycobacterium species whilst green boxes indicate DNA regions lost during population bottlenecks.
Please click the numbers (in blue boxes) for a more detailed guided tour on this part of the evolution of M. bovis.
Finding a Home:
Clonal Expansion
Clonal expansions occur when a certain strain becomes common to an area. This area can be large (a continent) or small (a county of the UK). These expansions occur when a small number of individuals - of an M. bovis strain - invade an area unoccupied by any other strain. In these new areas, the members of the invading strains do not have to compete for resources with other strains. Therefore the invading strain thrives leading to an expansion in its population numbers. The invasion of a strain into an unoccupied area is known as a founder effect. The successive loss of DNA in M. bovis is thought to aid clonal expansion, allowing for the appearance of more bacteria with varying characteristics including host preference.
The UK is Home to...:
The European 1 Complex
The M. bovis lineage can be split into 3 major groups of clonal complexes: African 1, African 2 and European 1. European 1 is the clonal complex commonly found in the UK and former British colonies. The EDEu1 region has been deleted from the DNA of 99% of M. bovis strains in the UK. It is thought that a bottleneck occurred when a small number of individuals, of the European 1 complex, invaded the UK from mainland Europe. This led to reduced genetic diversity of M. bovis strains in the UK compared to strains in mainland Europe. It is possible that the export of Hereford beef cattle, since the early 19th century, has allowed bovine TB to spread to the British colonies. The success of this complex has been suggested to be due to chance, whereby certain strains with the EDEu1 deletion came across cows that happened to be susceptible to bovine TB, and then spread in their new host.
The distribution of the European1 Complex throughout
the globe. The pie charts display the proportion of the European 1 Complex in those areas with black and white representing the European 1 and other complexes respectively.
The distribution of the most common Mycobacterium bovis strains throughout the UK. Each colour represents a different strain. The strain SB0140 is not included in this figure.
Still Not the End:
The Distribution of the UK M. bovis Strains
It is common in the UK for some M. bovis strains to become localised to certain counties. For example a strain named SB0263 originated in Gloucestershire and underwent a selective clonal expansion in the nearby counties of Herefordshire and Worcestershire. These localised expansions are thought to occur when a new advantageous trait emerges, due to a deletion of DNA, in a new strain and locally increases in frequency. For example a strain named SB0140, one of the UK’s most common strains, is thought to have evolved the advantageous trait of the ability to evade control measures. However some strains have become well distributed around the UK due to trade; 85% of bovine TB in the UK thought to be caused by the SB0140 strain.
Continuing the Evolution...:
Clonal Complexes
Since emerging from the ‘animal’ lineage, M. bovis has undergone micro-evolutionary steps whereby small DNA sequences within a region known as DR have been deleted. Different strains of M. bovis are defined by their unique DNA sequence that has been deleted. Spoligotyping is the name for a method used to identify the different M. bovis strains. Strains can be grouped into what scientists term clonal complexes. Clonal complexes are comprised of M. bovis strains that share similar characteristics and are derived from a single common ancestor.
It can be seen that M. bovis emerged as the final member of the animal lineage. But that is not where our story stops, M. bovis is still evolving. Please scroll down to carry on the tale...
A reminder of the results from a founder effect
6 strains grouped into 2 clonal complexes.