Friday, February 23, 2018

It's all in the Genes.

When people talk about zoonotic tuberculosis in terms of cattle v badger and who gives what to whom, they parade their ignorance of work done over decades on the genome of m.bovis, the bacterium which when inhaled or ingested in colony forming units (cfu) or clusters of bacteria, may go on to cause full blown disease.

 On its way through hosts, m.bovis loses part of its unique genetic code.

In this posting we bring together three papers which we've mentioned in isolation before, but not as a progression of thought.

According to a 2002 paper by Brosch et al - [link]  M.bovis appeared after a bottle neck about 15 - 20,000 years ago and developed separately from ancestral m.tuberculosis. Brosch  observed that it and other strains, had developed from a common ancestral strain which then split. Each time the genome altered, there was a deletion in the sequence.
..the results from this study unambiguously show that M. bovis has undergone numerous deletions relative to M. tuberculosis. This finding is confirmed by the preliminary analysis of the near complete genome sequence of M. bovis AF2122/97, a classical M. bovis strain isolated from cattle, which revealed no new gene clusters that were confined specifically to M. bovis.
So successful was m.bovis that Brosch postulates:
These deletions seem to have occurred in the progenitor of tubercle bacilli that — today — show natural host spectra as diverse as humans in Africa, voles on the Orkney Isles (UK), seals in Argentina, goats in Spain, and badgers in the UK.
Please note, these are ' natural hosts' maintaining the disease and its genome, not spill over.

 Following on from Brosch in 2006, our own Professor Noel Smith ran similar DNA screens - [link] and concluded that these deletions did not cross barriers as they migrated to other hosts. They 'nested' in the host they had chosen.

 And the spoligotype maps produced by Prof. Smith's team show that. They illustrate the Regional Accents [link] of the bacterium known as m.bovis.

And now we come to a simplified explanation of this work by Defra Chief Scientist, -[link] Professor Ian Boyd, who wrote in his blog in 2013 that following this clonal deletion process, the predominant cattle strain of m.bovis AN5 has now disappeared, replaced by the strain SBO 140 found predominantly in ... badgers. Professor Boyd then concludes:
"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?"
So the GB eradication process begun in the 1950s and 60s was very successful, eliminating the cattle strain of m.bovis completely.

And what we have now is badger adapted TB up spilling into - anything it touches.

So it is pretty crass to find populist vet, Dick Sibley tweeting that his work on phage testing and the undiscovered reservoir of m.bovis in our cattle (or one herd of cattle) is the result of his wanting to protect badgers from cattle tb.   The genome sequence of which, disappeared several decades ago.

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