Tuesday, January 17, 2012

Still no solid evidence for the Old World origin of syphilis

The first recorded syphilis epidemic flared up in war-torn Naples in 1494, only two years after Columbus discovered the New World.  From there syphilis spread throughout Europe.  Ever since then, controversy has raged about the origin of syphilis.  A popular belief is that Columbus's crew got infected in the New World and brought the spirochete back to Europe, where they transmitted the disease to others while serving as mercenaries during the first Italian War.  The competing pre-Columbian hypothesis asserts that syphilis was always present in the Old World yet wasn't recognized until 1494.

ResearchBlogging.orgAmong the treponemes, Treponema pallidum subspecies pallidum, the agent of syphilis, is the only one that's sexually transmitted.  Other treponemal diseases include yaws and endemic syphilis, each caused by a genetically distinct subspecies of Treponema pallidum.  The exuberant immune response to T. pallidum during the tertiary stage of treponemal disease leaves marks on the bones.  Historical cases of treponemal disease can therefore be identified by looking for these skeletal lesions.  Diagnosing treponemal infection in this way is tricky because a number of diseases cause similar lesions.  Nevertheless, bone deformities specific to treponemal diseases do exist, most famously the worm-eaten appearance of caries sicca.  Unfortunately, skeletal lesions cannot be used to reliably distinguish syphilis from the other treponemal diseases, so I will be speaking of treponemal disease instead of syphilis when discussing the skeletal evidence.

The findings of treponemal lesions in pre-1492 skeletal remains from the Old World would upend the Columbian hypothesis.  A number of claims of pre-Columbian treponemal lesions on Old World skeletons have appeared in the scientific literature, with some garnering widespread media attention.  One example is the findings from the excavation of an English friary at Hull Magistrate's Court, which was a subject of an episode of the PBS series Secrets of the Dead.  Skeletal remains from four individuals unearthed at the site had evidence of treponemal disease.  The site where the bones were found was dated to 1300-1450.

The Syphilis Enigma, part 1 of 4

The Syphilis Enigma, part 2 of 4

The Syphilis Enigma, part 3 of 4

The Syphilis Enigma, part 4 of 4

Harper and colleagues recently took another look at the published claims of pre-Columbian treponemal disease in Old World skeletal remains.  Their work came out last month in the Yearbook of Physical Anthropology, the supplement to the Journal of Physical Anthropology  They scrutinized the data in the 54 published reports with a standardized set of criteria for diagnosing treponemal disease and radiocarbon dating the bones.  There were two parts to their analysis.  First, they looked at the description of the skeletal lesions (including photographs, when available) to make sure they were the type caused solely by treponemal diseases.  Second, they looked at method used to date the bones.  In turns out that all 54 reports were flawed in some manner.  In many reports, diagnosis of treponemal disease was based on the types of skeletal lesions that could have been induced by other diseases.  In others, the bones were dated indirectly by archeological methods instead of directly by radiocarbon dating the bones.  Or worse, details of the dating method were sometimes omitted.

Among the 11 specimens for which the diagnosis of treponemal disease was deemed to be correct by the authors, two were dated by radiocarbon methods to the pre-Columbian years.  The time of death for the two individuals were 1424-1479 for a specimen unearthed in Safed, Israel and 1426-1486 for one dug up at the Church of St. Helen-on-the-wall in York, England.  At first glance, these specimens appear to undermine the Columbian hypothesis.  However, radiocarbon dating assumes that the ratio of 14C ("new" carbon, which decays at a known rate) to 12C ("old" carbon) remains constant in the environment and by extension living things, which exchange carbon with the environment by eating and breathing.  Slight deviations of the ratio may throw off the calculated date by hundreds of years.  One factor that must be accounted for is the "marine reservoir effect," which could skew the isotope ratio in humans who consume seafood.  The problem is that water near the surface mixes with deeper water, which reflects the higher proportion of old carbon from the material coming off the ocean floor.  The carbon in the air does not mix quickly enough with the carbon in the water to equalize the carbon ratios.  Therefore, marine organisms do not contain the same carbon ratio as land organisms.  Humans who consume seafood will have a slightly lower proportion of new carbon than expected, making bones from seafood consumers appear older than they really are if no correction is made.  When the correction is made for the marine reservoir effect, the new dates for the specimens become 1424-1953 for the Safed specimen and 1421-1669 for the St. Helen-on-the-Walls specimen.  An additional three Old World specimens were dated to the pre-Columbian period by radiocarbon dating, corrected for the marine reservoir effect.  However, the specimens lacked lesions specific for treponemal disease.  In sum, proper dating of the skeletal remains revealed that none of the Old World skeletons bearing treponemal marks could have originated from the pre-Columbian era.  There is still no convincing evidence for the Old World origin of syphilis.

So can we now reject the pre-Columbian hypothesis?  The authors make an interesting comment about this.

The trope that absence of evidence is not the same as evidence of absence will be invoked by some. While it is true that a working hypothesis may be falsified at any time, at what point does the absence of skeletal evidence of pre-Columbian treponemal disease in the Old World become compelling? After all, the best proof of a theory is failure to disprove it, and we find that despite intense research interest, credible evidence disproving the Columbian Hypothesis is lacking.

The "intense research interest" includes past examination of tens of thousands of skeletons in continental Europe and North Africa and 50,000 in England alone, with none yielding convincing evidence of pre-Columbian treponemal disease.  The authors do point out that skeletal remains from vast regions of sub-Saharan Africa and Asia have not been well-studied.  For this reason, it may be too soon to lay the pre-Columbian hypothesis to rest.

ACKNOWLEDGEMENTS:  A big thanks to Molly Zuckerman for alerting me to her group's work and for sending me the article.

Harper, K.N., Zuckerman, M.K., Harper, M.L., Kingston, J.D., & Armelagos, G.J. (2011). The origin and antiquity of syphilis revisited: an appraisal of Old World pre-Columbian evidence for treponemal infection American Journal of Physical Anthropology, 146 (S53), 99-133 DOI: 10.1002/ajpa.21613

von Hunnius, T.E., Roberts, C.A., Boylston, A., & Saunders, S.R. (2006). Histological identification of syphilis in pre-Columbian England American Journal of Physical Anthropology, 129 (4), 559-566 DOI: 10.1002/ajpa.20335

Related posts by other bloggers

Related posts in Spirochetes Unwound