Leprosy is one of those diseases that, when I first read about it, haunted my imagination. I had sad pictures of haggard, dirty people, wearing sackcloth, covered in sores or maybe with toes, noses, or hands missing, shuffling alone, wearing a bell to make that no one would go near...the leper. To be a leper seemed like one of the worst things that could happen to you in the Middle Ages.

Now, we have ways to take care of this. Combination antibiotics, taken for a year, can cure leprosy, though there are still leper colonies in places like India. And though leprosy is horrid, it's not as horrid as our Medieval picture, people's hands and noses didn't ACTUALLY fall off, and it's not actually very contagious.

But at the time, and in some places in the world now, lepers were a fact of life, and a terrifying one. At one point in history, its estimated that 1 out of 30 people in Europe was a leper. But then, in about the 1500s...leprosy started to disappear.

Why? There hadn't been any advances in medicine or anything. No one knew how leprosy spread. There was still leprosy in places like the Middle East and India, but in Europe, it became extremely rare. What changed?


Schuenemann et al. "Genome-wide comparison of medieval and modern

Mycobacterium leprae" Science, 2013.

What can cause a disease to suddenly all but disappear? Well, it could be us, or it could be "them". The disease itself could have changed in some way, becoming less contagious, less virulent, causing less infection in a ways that may it easier for us to fight off. Or it could be us, maybe our immune systems adapted to become more efficient at throwing off leprosy. It could even be another disease, say, plague killing off many people infected with leprosy and thus slowing the spread dramatically.

So which is it? It's harder to figure out than you might suppose. Leprosy is a very "slimmed down" bacteria (the two species are Mycobacterium leprae and Mycobacterium lepromatosis), it has gotten rid of a lot of the genes that would allow it to grow independently, meaning that it requires a host to proliferate. This also means that you can't culture leprosy in the lab, it needs a human body to grow in. So you can't just culture up the bacteria, sequence out the DNA, and take a look. You have to get samples from people who have had leprosy.

The authors of this study took DNA bacterial remnants in 5 medieval leprosy cases (from the UK, Sweden, and Denmark), and sequenced up the DNA, being careful to separate it from the human DNA still present in the skeletons. They then compared it to modern leprosy cases (now treatable, but they still happen), to see if and how the DNA had changed.

(Figure from the paper showing the distribution of the Medieval leprosy samples)

The result? Leprosy is one very stable disease. The genome of the bacteria hadn't really changed at all over the past 1,000 years. It also showed that leprosy as it exists in the Americas probably traveled over from Europe, and that something similar had taken place in the Middle East, as the bacterial genomes between the American leprosy, Middle Eastern leprosy, and European leprosy were so similar. It's a safe bet that leprosy traveled from Europe to the Americas (as there are no records or tales of leprosy in the Americas prior to European arrival), but for the Middle East, well, the leprosy could have come from the Middle East, or have been spread to it from Europe.

Leprosy is hardy, it's barely changed at all. So what caused the decline in leprosy cases in Europe? Well, if its not leprosy, it's got to be either us, or possibly another disease crowding it out. Perhaps we have evolved resistance to leprosy, making us throw off the bacteria easier when we become infected. Perhaps it was the result of another disease invasion, like the Plague, which could have killed off all the people with weaker immune systems (and more likely to get leprosy, and possibly a lot of the lepers as well), leaving the populace with a much lower prevalence, and stronger immune systems. It could be a combination of both.

This sequencing of leprosy also uncovered something else interesting: leprosy is tough stuff! The scientists were able to get a sequence off 14th century leprosy DNA with very little trouble. This could because the leprosy bacteria has very thick cell walls. This could be very useful, allowing scientists to look at diseases like leprosy much further back in human history, with hopes of finding enough to really make something out of it. This could help us find out where these diseases come from, and how they may have changes the shape of our species.


Genome-wide comparison of medieval and modern Mycobacterium leprae, in Science Express, 13-Jun-2013. DOI: 10.1126/science.1238286

Verena J. Schuenemann1*, Pushpendra Singh2*, Thomas A. Mendum3*, Ben Krause-Kyora4*, Günter Jäger5*, Kirsten I. Bos1, Alexander Herbig5, Christos Economou6, Andrej Benjak2, Philippe Busso2, Almut Nebel4, Jesper L. Boldsen7, Anna Kjellström8, Huihai Wu3, Graham R. Stewart3, G. Michael Taylor3, Peter Bauer9, Oona Y.-C. Lee10, Houdini H.T. Wu10, David E. Minnikin10, Gurdyal S. Besra10, Katie Tucker11, Simon Roffey11, Samba O. Sow12, Stewart T. Cole2†, Kay Nieselt5†, and Johannes Krause1†