The tooth from a young woman who died in a Scandinavian leper colony in the 1300s has now provided key evidence for scientists studying the evolution of pathogens.

Leprosy, like tuberculosis and the plague, looms large in our history, even though it is no longer widespread in the Western world. Leprosy is an infection caused by the bacteria Mycobacterium leprae, and the hallmarks of the disease include skin lesions, nerve and bone damage, and loss of sensation in limbs. Although the disease is actually not that contagious, those suffering from it were once quarantined to leper colonies.

Although it may sound like a disease of the past and treatments are now available for it, more than 200,000 cases are diagnosed every year, mostly in the developing world. In the southern United States, new cases are linked to exposure from armadillos, one of the few animals that can carry leprosy.

Leprosy has been a challenge to study—it can't be grown in cell cultures, and armadillos are one of the only animal models available. In humans, it can take years for infected people to begin to develop symptoms.

One of the big questions that people are trying to study is why leprosy declined so dramatically hundreds of years ago in Europe (long before antibiotics), while it remains a serious health concern in India. One possibility is that different strains of the pathogen were evolving differing degrees of virulence.

Enter the ancient disease detectives.

Recent improvements in DNA sequencing technology enable researchers to study the genomes of ancient pathogens, even if the DNA is damaged. Sections of degraded DNA are often too short to sequence using the traditional polymerase chain reaction technique. New high-throughput technology can use much smaller fragments and produce results much faster.

The tooth found at the leper colony had such well-preserved bacteria DNA that the scientists were able to sequence the entire genome. Leprosy also creates bone lesions that researchers have now used to identify additional remains to analyze. Collecting more samples from other sites across Europe allowed scientists to study the genetic variation. Comparing these historic strains to modern samples from around the world can show how much leprosy has changed.

The results, published this week in Science by a team of researchers led by Verena J. Schuenemann and Johannes Krause from the University of Tübingen in Germany found that M. leprae actually evolves quite slowly. The current strains in infected people in India are pretty similar to the strain found in the Scandinavian skeletons.

If the pathogen isn’t evolving, the natural conclusion is that public health measures were probably responsible for the diseases’ decline in Europe in the 16th century.

That’s just one piece of the ancient pathogen puzzle. These techniques can help the trace leprosy’s evolution back much further and answer other questions like whether humans passed the disease to the armadillos in the southern US or if the animals had their own strain to begin with.

Science, June 2013. DOI: 10.1126/science.1238286 (About DOIs)