Washington: To answer why was there a sudden drop in the incidence of leprosy at the end of the Middle Ages, biologists and archeologists reconstructed the genomes of medieval strains of the pathogen responsible for the disease, which they exhumed from centuries old human graves.
Their results shed light on this obscure historical period and introduce new methods for understanding epidemics.
In Medieval Europe, leprosy was a common disease. The spectre of the leper remains firmly entrenched in our collective memory: a person wrapped in homespun cloth, announcing his presence in the streets by ringing a bell. The image is not unfounded.
In certain areas it is estimated that nearly one in 30 people were infected with the disease.
But at the turn of the 16th century, the disease abruptly receded over most of the continent. The event was both sudden and inexplicable. Perhaps the pathogen that causes leprosy had evolved into a less harmful form?
To find out, an international team of biologists and archaeologists joined forces.
They decoded the nearly complete genomes from five strains of Mycobacterium leprae, the bacterium responsible for leprosy, which they collected and reproduced by digging up the remains of humans buried in medieval graves.
Reconstructing the bacterial genomes was no easy task, as the material available-from old human bones-contained less than 0.1 percent of bacterial DNA.
The researchers developed an extremely sensitive method for separating the two kinds of DNA and for reconstituting the target genomes with an unprecedented level of precision.
"We were able to reconstruct the genome without using any contemporary strains as a basis," study co-author and EPFL scientist Pushpendra Singh, who worked closely with Johannes Kraus and team from Tubingen University in Germany, said.
The results are indisputable - the genomes of the medieval strains are almost exactly the same as that of contemporary strains, and the mode of spreading has not changed.
Many clues indicate that humans developed resistance to the disease.
The study is published in the journal Science.