Tuberculosis an inverse zoonosis?
I have been pondering an interesting article that I have read recently (see end of post) on misconceptions surrounding the origins of the abominable consumption, or in today’s speak; tuberculosis. Now I’m not sure about other scientists out there, but I recall being taught in microbiology class that this pathogen most likely spread to humans from cattle in the distant past, possibly as a consequence of domestication. This it turns out is something of an erroneous assumption with a surprising cause.
This article from 2009 (yeah I’m a bit late, sorry), published in Microbiology, points out that a simple comparison of the chromosome of Mycobacterium tuberculosis (the pathogen responsible for tuberculosis in humans) to that of its bovine cousin, Mycobacterium bovis (the source of the tuberculosis inoculation) reveals that the bovine variety is shorter than the human one. In other words, it would appear that the chromosome of M. bovis has undergone some deletions in its evolutionary past, deletions that this clonal species is unable to repair via recombination. This would suggest that the movement of the pathogen was not from cattle to human, but possibly vice versa (most likely indirectly via other mammals)!
There are of course other possibilities, like the bovine strain undergoing the deletion after a strain had evolved to infect humans or that the infection of humans or cattle was via some other route, other mammals were most likely also vectors. A brief online mention was made by an American scientist that I sometimes communicate with about bison as a part of this process, this I will comment upon further when I am able to access the articles she mentioned.
While this contributes little to our knowledge of how humans became hosts to this particularly nasty infection, rather moving the question from one place to another, it does point out something of interest. An insidious thing that often creeps in to undermine good science. Anthropocentrism. An interesting point to this tune was that the article I have been speaking of asserts that the main reason the belief that tuberculosis spread to humans from cows has persisted, is that we see cows as ‘dirtier’ than us. I’m sure that is what was subconsciously shaping the words of my microbiology lecturer and the mental receptacles of the students who were no more immune to the self gratifying charm of an anthropocentric idea.
“Myths and misconceptions: the origin and evolution of Mycobacterium tuberculosis”
Smith, N., Hewinson, R., Kremer, K., Brosch, R. & Gordon, S.
Volume 7, Pages 537-544.