Detecting pathogens in medieval Venice

Medieval Venice was a trading empire, one of the busiest ports of the late medieval world. As a hub of commerce waves of plague visited and revisited Venice in 1348, 1462, 1485, 1506, 1575-1577, and 1630-1632 with the last two producing mortality rates around 30% of the population (Tran et al, 2011).

Three medieval plague graves in Venice. Grave B (upper right) produced one of the positive Yersinia pestis results. (Tran et al, 2011; doi:10.1371/journal.pone.0016735.g001)

As we all know, Venice has a land problem, or rather a lack of land problem. Thriving economies draw large populations and burial space becomes difficult to come by. Adding the plague on top and we have the perfect conditions for the discovery of mass plague burials.

On the island of Lazzaretto Vecchio in Venice 92 mass burials of 5-184 people have been uncovered and were used by the Marseille team for mass pathogen aDNA screening (Tran et al, 2011). Burials for the 14th, 15th, 16th, and 17th centuries were uncovered. Teeth in good condition (closed apex and no caries or trauma) were collected by the archaeologist who bagged them individually to prevent contamination. The team chose 173 specimens divided equally over the 14th-17th centuries. aDNA extracted from these teeth were screened by multiplex PCR for seven epidemic-causing pathogens: Yersinia pestis (plague), Bacillus anthracis (anthrax), Salmonella enterica Typhi (typhoid fever), smallpox, Barontella quintana (trench fever), Borrelia recurrentis (louse-borne relapsing fever), and Rickettsia prowazekii (epidemic typhus).

Of the seven pathogens screened for, only two were found in aDNA isolated from these teeth. Bartonella quintana, the causative agent of trench fever, was found in five teeth (2.9%) from the 15th and 16th centuries (Tran et al, 2011). B. quintana is a human louse transmitted disease that can explode in epidemic fashion, getting it’s name from epidemics in the trenches of World War I. However it does not always cause death so it’s presence does not necessarily indicate cause of death (Fournier et al, 2011; Foucault, Brouqui, & Raoult, 2006). Yersinia pestis, the causative agent of the plague, was found in three (1.7%) of the multiplex PCR screened specimens from the 14th and 16th centuries. One of these three was confirmed by suicide PCR and sequencing of the glpD gene, and a further positive specimen was found by PCR that the multiplex real-time PCR screening had missed (Tran et al, 2011). If I am reading their report correctly that would be overall four positive specimens by the two tests combined.

If these mass graves are primarily plague victims as Tran et al (2011) assert then efficiency of detection leaves much to be desired. It is possible that the aDNA is in bad shape. The graves pictured above are densely packed and I wonder about decomposition conditions. Does aDNA survive as well in these mass graves as it does in single graves?  A control  for aDNA integrity really needs to be established. They also remind us that aDNA detection is less sensitive than immunological methods, but even if they have detected only a small fraction of those that immunological methods would have found, that would still be a minority of skeletons sampled. The advantage of this study is that they were able to screen for seven pathogens at once.

The Marseille group is working under a hypothesis that plague was transmitted at least in part by the human louse. They have aired this hypothesis primarily in earlier publications and I may post on those papers someday. Their hypothesis explains the inclusion of two louse transmitted pathogens in their screening panel. If plague was transmitted by human lice this would change the transmission dynamics of the plague significantly and may explain at least some of the epidemiological conflicts with modern flea-transmitted plague dynamics.

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Tran TN, Signoli M, Fozzati L, Aboudharam G, Raoult D, & Drancourt M (2011). High throughput, multiplexed pathogen detection authenticates plague waves in medieval Venice, Italy. PloS one, 6 (3) PMID: 21423736

Fournier PE, Ndihokubwayo JB, Guidran J, Kelly PJ, & Raoult D (2002). Human pathogens in body and head lice. Emerging infectious diseases, 8 (12), 1515-8 PMID: 12498677

Foucault C, Brouqui P, & Raoult D (2006). Bartonella quintana characteristics and clinical management. Emerging infectious diseases, 12 (2), 217-23 PMID: 16494745