Scientists Solve Genetic Mystery of Medieval Leprosy: Sudden Drop Explained

In Medieval Europe, leprosy was a common disease. Yet its incidence dropped drastically at the end of the Middle Ages. So why did people stop contracting leprosy? Researchers may now have the answer.

Leprosy is a progressive disease that if left untreated can cause permanent damage to the skin, nerves, limbs and eyes. Contrary to popular belief, though, this disease doesn't cause body parts to simply fall off; instead secondary factors, including infection, can result in tissue loss. Although in the past the disease was a painful death sentence, today leprosy can be treated.

In Medieval Europe, about one in 30 people had the disease. Men and women in homespun cloth lived in groups as they were shunned by their peers. Yet at the turn of the 16^th century, this disease abruptly receded over much of the continent.

In order to find out why the disease suddenly "disappeared," researchers decoded the nearly complete genomes from five strains of Mycobacterium leprae, the bacterium that's responsible for leprosy. They collected the bacteria from people buried in medieval graves and then reproduced the bacteria in the lab.

What did the researchers find? It turns out that the medieval strains of the bacterium are almost identical to the contemporary strain--and the mode of spreading the disease has not changed. This meant that something else besides the evolution of the bacterium must have caused the sudden drop in leprosy cases.

The answer may actually lie in the evolution of humans. It's possible that people developed resistance to the disease, especially considering the fact that conditions were primed for natural selection.

"In certain conditions, victims could simply be pressured not to procreate," said Steward Cole, co-director of the study, in a news release. "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."

The findings not only have historical significance, but also have significance when it comes to understand epidemics. Sequencing methods designed as part of this research are among the most precise ever developed, and could enable scientists to track down many other pathogens in the future.

The findings are published in the journal Science.