29 January 2008

Was Columbus to blame?

ResearchBlogging.orgHarper, K., Ocampo, P., Steiner, B., George, R., Silverman, M., Bolotin, S., Pillay, A., Saunders, N., & Armelagos, G. (2008). On the Origin of the Treponematoses: A Phylogenetic Approach PLoS Neglected Tropical Diseases, 2 (1) DOI: 10.1371/journal.pntd.0000148

I remember way back in junior high science class hearing that Columbus was to blame for introducing syphilis to Europe. At the time, I didn't appreciate whether that was well-supported or not, and just remembered it as one of those little factoids. So I was a little surprised that this has actually been very contentious, and that there was a new paper about that subject.

This new paper by Harper and colleagues on the origin of syphilis is more interesting to read about than it is to read itself. The paper is very focused on molecular biology paper, with lots of As, Ts, Cs, and Gs. It's not very approachable. I had the good fortune to listen to some interviews with one of the co-authors, Dr. Mike Silverman, and those are far more interesting and accessible.

The new paper takes a lot of DNA sequence data from bacteria in the genus Treponema. These cause several diseases other than syphilis, most notably a condition I had never heard of before called yaws. Yaws hadn't been reported in the Americas for a long time, but when it was recently rediscovered in Guyana, Silverman and his team initially thought they might be looking at syphilis. Except that the disease wasn't showing up on the genitals, like syphilis, but rather on knees, arms, and so on.

Silverman got a call while he was waiting for his plane just before he was to go to Guyana to ask if he could collect DNA samples of yaws. He did, but found only two cases, and because the request came so late, they couldn't preserve the tissue very well. That is undoubtedly the biggest strength and weakness of the paper. The American yaws samples are absolutely crucial to the story, but the quality of the samples are poor. Still, 50% of something is better than 100% of nothing.

Once they had those DNA samples, the authors use some evolutionary theory. The more recently diverged species are, the more alike their genes should be. With the aid of a computer, you can group the samples into clusters based on their similarity. This is usually shown in a tree-like diagram.

The Guyana samples aren't included in that tree, presumably because they were quite degraded compared to the others, and they couldn't quite get enough DNA sequence information to analyze.

The bottom line is that although the bacteria are very similar, yaws is more diverse than syphilis, and appears to be older. Syphilis is extremely similar worldwide, suggesting it is very recent. And the most genetically similar is the American yaws.

This pretty strongly supports an hypothesis that indeed, syphilis was yaws that was passed on by sex between natives and Columbus's crew.

The next step in this research would be to get better preserved samples of American yaws. But because of medical missions, it's quite possible that the disease has been eradicated in the Americas. Great for health, but a shame for research.

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