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Tracing the origin of the Black Death (TRADE): Using tree rings to reconstruct historical re-introductions of plague from Asia to Europe

Tracing the origin of the Black Death (TRADE): Using tree rings to reconstruct historical re-introductions of plague from Asia to Europe

Newly emerging zoonotic infectious diseases entering human populations are often only weakly understood and thus exceptionally dangerous, because most of our public health measures depend on a certain level of familiarity with any given pathogen. Devastating pandemics can cause health disasters at continental, hemispheric and even global scales. Improving our understanding of the ecology, epidemiology and pathophysiology of zoonotic infectious diseases and how climate may also contribute to human pandemics, describes a well-timed cross-disciplinary research task.

Outbreaks of bubonic plague initiated by the flea-borne bacterium Yersinia pestis have repeatedly afflicted the Old World, at least, since the onset of the 'Justinian Plague' in 541 AD. The second and much better documented pandemic, the 'Black Death', rapidly killed around half of the European population after 1347AD, and recurrent local outbreaks persisted at least until the early-18th century. The origin of the catastrophic Black Death pandemic has been traced back to Caffa in 1346, and even further to the Kyrgyzian 'pestilence' in Issyk Kul, 1338-1339 AD. Reasons for the eventual cessation of plague in Europe, however, remain mysterious, particularly in light of a continued activity in Asia, where the infection is enzootic in its natural rodent hosts.

Recent efforts have proved the climatic importance on plague abundance in its major host species, the great gerbil (Rhombomys opimus) during the 20th century, which ultimately triggered the risk for human epidemics in Kazakhstan, for instance. Understanding plague ecology, epidemiology and pathophysiology and its climate-dependency is, however, yet almost exclusively limited to areas where Yersinia pestis currently persists in endemic rodent populations. The epidemiological dynamics of the wildlife reservoir determine the profusion, distribution and evolution of the pathogen, which in turn shape the frequency, distribution and virulence of human cases. At the same time, Central Asia represents a paleo-climatological hotspot where numerous tree-ring records provide annually resolved and absolutely dated insight on temperature and precipitation variability during the past centuries to millennium.

In this project, we compile, archive and improve existing, as well as to develop new tree-ring width, density and isotope chronologies from the Russian Altai and parts of the Tien Shan and Karakorum mountains in Kyrgyzstan and its surroundings. Based on state-of-the-art multi-proxy reconstruction techniques, this newly generated Central Asian paleo-archive will allow interannual to centennial-long climate variability to be estimated and compared with a database of nearly seven thousand historical plague outbreaks that stroke Central Europe, North Africa and the Near East between 1347 AD and the early-18th century. Socio-political and cultural influences, almost always compounding the complexity of wildlife plague ecology, likely increased the spillover of severe infection pulses into human populations and thus have to be considered as well.

TRADE will also contribute to exploring the potential of so far underrepresented documentary evidence and putative plague graves in Central Asia. Hence, it intends to furnish epidemiologists with precisely dated and exactly localized historical outbreak information, and biologists with material for possible extraction of ancient microbial DNA. Conceptual advancement related to the geographical origin and historical spread of the Black Death, as previously indicated by novel bio-molecular clocking techniques, will be of great value to archaeologists, biologists, ecologists, epidemiologists, historians and zoologists. The anticipated paleo-epidemiological insight appears particularly timely because the plague bacillus still induces several thousand human cases annually, and may even increase under projected climate change and, ominously, within the context of bioterrorism.

Schematic representation of the ecological plague cycle

Schematic representation of the ecological plague cycle with small mammals as hosts and fleas as vectors, and the wildlife reservoir expected to be located in the arid environment of Central Asia. Arrows represent connections affected by climate with a color-coding depending on the most influential climate variable on this link (i.e. precipitation, temperatures and other variables indirectly depending on them such as soil characteristics and soil moisture).