Scientists Reconstruct Genome of Medieval Strains of Leprosy to Surprising End

The reason behind the sudden drop in cases of leprosy at the end of the Middle Ages has long remained a mystery, prompting a team of biologist and archaeologists to exhume bodies from ancient graves in order to reconstruct the genomes of the medieval strains of the pathogen responsible for the disease.

The results, published in the journal Science, reveal new insights into the obscure historical period as well as introducing a new method for understanding epidemics.

In Medieval Europe, leprosy was a common disease, as seen in the pervasiveness of the image of the specter of the leper – a person wrapped in homespun cloth, announcing his or her presence in the streets by ringing a bell.

The image is not unfounded, according to scientists, due to the fact that in certain areas, nearly one in 30 people carried the disease.

However, at the turn of the 16th century, the illness abruptly and inexplicably receded over nearly the entire continent.

One hypothesis was that, in a stroke of luck for those in the area, the pathogen simply evolved into a less harmful form.

To find out if this was the case, an international team of scientists joined forces, decoding nearly complete genomes from five strains of Mycobacterium leprae, the bacterium responsible for leprosy.

Doing so was not an easy task given the material, taken from old human bones, contained less than 0.1 percent of bacterial DNA. However, 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, according to a press release on the study.

“We were able to reconstruct the genome without using any contemporary strains as a basis,” study coauthor Pushpendra Singh said.

The results, the researchers reported, were indisputable: the genomes of the medieval strains are almost exactly the same as contemporary strains', and since the mode of transportation hasn’t changed, that leaves only one option.

“If the explanation of the drop in leprosy cases isn’t in the pathogen, then it must be in the host, that is, in us; so that’s where we need to look” Stewart Cole, co-director of the study and head of EFFL’s Global Health Institute, said.

Many clues indicate that humans developed resistance to the disease, probably due to the very high prevalence of the illness as well as the social isolation of diseased individuals.

“In certain conditions, victims could simply be pressured not to procreate,” Cole said. “In addition, other studies have identified genetic causes that made most Europeans more resistant than the rest of the world population, which also lends credence to this hypothesis.”

Among the researchers’ realizations was that a medieval strain of the Mycobacterium lerpae found in Sweden and the United Kingdom is nearly identical to the strain currently found in the Middle East.

“We didn’t have the data to determine the direction in which the epidemic spread,” Coles explained. “The pathogen could have been carried to Palestine during the Crusades. But the process could have operated in the opposite direction, as well.”

Besides the historical significance of the research, the team argues that the study improves scientists’ understanding of epidemics, as well as how the leprosy pathogen operates. Furthermore, sequencing methods designed as a part of the research are among the precise ever developed and may track down many other pathogens lurking in DNA.

Finally, the incredible resistance of the bacterium’s genetic material, likely due to its thick walls, according to researchers, opens up the possibility of going even further back in history in order to uncover the origins of the disease that continues to affect 200,000 individuals every year worldwide.