An interesting story broke a few weeks ago while we were on our run up to the release of TWO PARTS BLOODY MURDER. So I filed it away, with the intent of coming back to it. Last week’s story on the discovery of the Bedlam Cemetery—containing Black Plague victims, among others—reminded me about it.
We’ve covered the Black Plague on several occasions—in 2013 when victims were discovered during the Crossrail project, and in 2014, when my own university colleague, Dr. Hendrick Poinar, sequenced the genome of the 14th century pathogen responsible for that specific wave of the plague. The common belief held for centuries is that rodents, specifically black rats, were responsible for the spread of the disease through Europe. The rats carried diseased fleas from location to location, moving through cities on foot and across continents by stowing away in caravans and on boats. But authors of a new paper in the Proceedings of the National Academy of Sciences propose that climate data from the time directly contradicts that explanation.
The optimum climate for rats includes warm summers with moderate precipitation. But looking at climate data for the time, there is no direct tie to European weather patterns. Instead, there is a direct correlation to Asian weather patterns, specifically wet springs followed by warm summers in central Asia. After each optimal Asian seasonal combination, Europe would experience a plague outbreak several weeks later. And while this is terrible weather for rats, it’s optimal for Asia’s gerbil population. These rodents could clandestinely travel the Silk Road, arriving several weeks later in Europe, bearing the plague where it then spread like wild fire. Finding a climate not to their liking, the gerbils would slowly die out and the epidemic would eventually abate. This would explain the way epidemics seemed to arrive in waves—each fresh wave was preceded by optimal Asian weather, prime gerbil breeding conditions, and a fresh arrival of disease-carrying gerbils in a vulnerable Europe.
Scientists will test this theory by examining DNA sequences from skeletons of European plague victims that died at various times. If the sequence of Y. pestis only slowly drifts over the decades and centuries, then that will support the previously held belief that there were local European reservoirs of disease from which each new epidemic sprung. But significant deviations in the DNA sequences will indicate that the disease arrived in fresh waves with each epidemic.
Maybe then, it will be time to apologize to the black rats for centuries of blame.