“Pre-Columbian Mycobacterial Genomes Reveal Seals As A Source Of New World Human Tuberculosis”

When, in 1994, definitive evidence of tuberculosis in humans was reported from pre-Columbian America, it was a startling. Conventional understanding had pegged tuberculosis as part of the new, exotic, and (to immunologically-naive populaces) deadly menagerie of pathogens brought by Europeans over to the Americas. While there were suggestions of pre-Columbian tuberculosis in the Americans, these were based on lesions on bones, which were ambiguous. Unlike previous cases, however, the Chiribaya mummy from 1000-1300 CE in Peru was shown beyond doubt to have been exposed to tuberculosis:

In the mummy’s right lung and a lymph node, the scientists found scars of disease. These were small, calcified lesions typical of tuberculosis. Extracting fragments from the tissue, molecular biologists isolated genetic material betraying the presence of Mycobacterium tuberculosis.

To find evidence of tuberculosis here some half a millennia before contact seems to suggest that tuberculosis had always been present in the Americas, and the increased incidences (to put it mildly) in post-contact native American populations were the result of contextual changes (such as high population densities, changes in diet or lifestyle, physiologies or immune systems compromised or changed by other diseases or factors, etc.) rather than the “virgin soil” phenomenon. It also led to other questions: given that tuberculosis was thought to have originated some 8000 years ago in the Middle East, as cowpox jumped to humans with the domestication of cattle, how did it get to the Americas before not just Old World people, but cattle got there?

A decade later, the mystery was solved. Leveraging advances not just in molecular sequencing technologies but also theoretical and computational analytic methods, it was found that the strain of tuberculosis found in pre-Columbian humans in the Americas was an entirely different one from the one that wreaked appalling havoc after the arrival of the Europeans. As can clearly be seen from the phylogenetic trees, the pre-Columbian tuberculosis probably was completely independent zoonosis event, jumping to humans from seals. The strain of tuberculosis that was endemic to the Americas did not confer any immunity to the new strain brought over by the Europeans, and hence the “virgin soil” pandemic syndrome that proved so devastating. The seals themselves were thought to have picked it up from hosts in Africa, and brought it over to the Americas through oceanic migration/dispersal.

While seals may seem a strange source given the agricultural practices with which we are currently familiar, resulting in a lot of skepticism in comments in the popular press, many pre-Columbian populations on coastal sites had marine-based resource exploitation economies that included strong interaction with a range of marine animals including seals. In fact, tantalizing clues to the marine origin of this (that only can be seen as clues with hindsight) were proximity of many of the early evidence of the pre-Columbian tuberculosis cases came from sites that were close to the coast or sea, as with the Chiribaya mummy.

All in all, a fascinating historical detective story, with many twists and turns, with the conclusion at the end a great example of the power of modern molecular phylogenetics to peer into the past at the micro- as well as the macro scales.

    Bos, K. I. et al. (2014). Pre-Columbian mycobacterial genomes reveal seals as a source of New World human tuberculosis. Nature, 514(7523), 494-497. doi:10.1038/nature13591.

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