Leprosy is an ancient disease. References to leprosy and the social stigma attached to it go back to 600 BC from India and in the Old Testament. However, like the plague, it was not until relatively late (1873) that the term leprosy became attached to a particular microbe, Mycobacterium leprae. Although some medieval descriptions suggest M. leprae, it can be very difficult to sort true leprosy from other skin conditions in medieval texts. This is where paleomicrobiology and biological anthropology come in.
Bioarchaeologists have been able to identify skeletal changes suggestive of M. leprae for some time. These changes only occur in the most severe cases making it unclear how common true leprosy was in ancient and medieval times. It takes many years, if not decades, for leprosy to cause skeletal changes. As with many chronic infectious diseases, it is likely that more people died with the disease than from it. Another way to assess the level and origins of ancient and medieval leprosy is by looking at ancient DNA.
Like the plague, leprosy is considered a monomorphic pathogen that has very little genetic diversity. Four single nucleotide polymorphism (SNP)* types have been characterized for M. leprae. These SNP types are defined by three sites in each type. SNP typing can be technologically challenging with degraded ancient DNA (aDNA) because the typing is based on a single base position at each site. These four SNP types define regional types of M. leprae with type 2 found in Asia and type 3 found in Europe and the Mediterranean today.
A Swedish group led by Christos Economou investigated the M. leprae aDNA of ten skeletons, eight with osteological signs of leprosy, from Sigtuna, Sweden, dating from the 10th to 14th centuries. DNA analysis revealed that nine of the ten were positive for M. leprae aDNA, including all eight with osteological signs. Leprosy often does not have a high bacterial load so they took nine samples from each skeleton. For each skeleton ruled positive for leprosy DNA, there are two to nine positive samples. However, only three of these nine produced readable results at all three SNP sites: two SNP type 2 (G) and one SNP type 3 (I). This is the first discovery of SNP type 2 in Europe. SNP subtype 2G had previously only been found in Nepal near Uzbekistan. The SNP subtype 3I found in one skeleton is consistent with other samples found in Europe of similar date, according to Economou et al (2013).
Although this is the first discovery of SNP type 2 in Europe, it is not terribly surprising. Sweden borders Asia and was tied to Eastern and Central Asia through trade routes from the early medieval period. Economou et al (2013) note that Sigtuna was an economic and administrative center in 10th-14th Sweden producing artifacts from the Middle East. Missing data from Poland and Russia is a significant gap in the map above.
We are left to hypothesize how an Asian strain of M. leprae got to Sweden. Ultimately we can never know for sure. Leprosy takes so many years to manifest disability or outwards signs that infected individuals could travel from the Middle East or Central Asia to Sweden at least once without sign of infection. These two individuals were unlikely traders: a 20-30 year old female and a 11-12 year old child. It is possible that they either traveled with traders or were slaves, but it is more likely that they contracted leprosy in Sweden. (There is no indication that they were related or died even in the same century.) Economou et al (2013) indicate that grave goods and practices from this cemetery suggest a multi-ethnic population.
Studies like this from Economou et al (2013) are just the first step in our understanding of the rise and fall of leprosy in Europe and Asia. Like plague, it is hard to imagine an environment where leprosy flourished in Europe or Asia, but we know it did. These ancient DNA studies offer some of the best evidence of a pathogenic landscape that seems so foreign to us.
Note and Reference:
*The difference between a SNP and a point mutation is the functional effect on the gene. A mutation causes an observable or measurable change in the function of a gene product. Most mutations are harmful, but not all. A polymorphism is a neutral change, as far as can be determined at the time of identification. Polymorphisms are usually found in a significant percentage of individuals or strains. It is not uncommon for human polymorphisms to be found in 10% or more of the populations. SNP typing can be thought of as a type of micro-genetic fingerprinting, based on the smallest possible change between people (or organisms).
Economou, C., Kjellström, A., Lidén, K., & Panagopoulos, I. (2013). Ancient-DNA reveals an Asian type of Mycobacterium leprae in medieval Scandinavia Journal of Archaeological Science, 40 (1), 465-470 DOI: 10.1016/j.jas.2012.07.005