Science Notes: New research reveals link between red squirrels and leprosy in the medieval period | The Past

A recent study published in Current Biology has brought to light a link between red squirrels and leprosy in medieval Britain (http://www.sciencedirect.com/science/article/pii/S0960982224004469). In ‘Science Notes’ this month, we’re taking a look at how these bushy-tailed rodents may have passed the disease on to human hosts.

Leprosy, known today as Hansen’s disease, was widespread in Europe during the medieval period. It could cause ulcerations and lesions, and led to chronic infection and nerve damage, as well as loss of limbs and even blindness in extreme cases. The disease is mainly caused by Mycobacterium leprae. While its prime host is humans, the bacterium has been known to infect armadillos and chimpanzees, and has also been found in British red squirrels (Sciurus vulgaris) on Brownsea Island.

Red squirrels were highly popular animals in the medieval period, and they are often seen scampering playfully around the margins of illuminated manuscripts. Some examples show squirrels being carried by their doting owners, while others illustrate the hutches and leashes that were used in the domestication of such animals.

However, the rodents were not just fashionable as pets, but also for clothing. The English Exchequer customs records detail the importation of more than 377,200 squirrel furs in 1334 from places such as Scandinavia, Eastern Europe, Ireland, and Scotland. The skins would be bought by furriers, and transformed into the trims and linings we see on clothing in medieval art. Particularly popular was vair, a checked pattern created using alternating bellies and backs of squirrel furs, which even became used in heraldic imagery. The popularity of squirrels and the known presence of M. leprae in modern squirrels in Britain highlighted a possible link between squirrels and leprosy in the medieval period.

Research was conducted by academics from the University of Winchester School of History, Archaeology, and Philosophy, alongside academics from the Institute of Evolutionary Medicine at the University of Zurich, the Department of Environmental Sciences at the University of Basel, the Science Museum, the School of Oriental and African Studies, the Department of Archaeology at Cambridge University, the School of Archaeology and Ancient History at the University of Leicester, and the Department of Microbiology, Immunology, and Pathology at Colorado State University.

Investigations took place in Winchester, which was chosen as the ideal case study based on the number of known furriers and the proximity of the nearby leprosy hospital of St Mary Magdalen (see CA 267), which was founded c.1080 and dissolved in the 16th century when the disease was on the decline. Excavations by the University of Winchester began in 2007 under the long-standing Magdalen Hill Archaeological Research Project, and have been used by the School of History, Archaeology, and Philosophy to train their archaeology students in fieldwork. The human samples used in the study, numbering 25 taken from 11 individuals, focused on pathologically probable, non-characteristic, and non-pathological specimens taken from burials excavated at St Mary’s.

The archaeozoological samples of 12 squirrels were taken from hands and feet found in the largest furrier pit at a site in Staple Gardens, which was dated to the 12th century using stratigraphy and confirmed by radiocarbon dating of a small sample of cat bone. Shotgun data achieved by mapping reads against mitochondrial references from small animal bones on the site showed that the squirrel samples taken were most likely S. vulgaris specimens.

To conduct the study, all the human and putative Eurasian red squirrel samples had host DNA preservation and genetic evidence of M. leprae assessed using shotgun sequencing data. Several human samples that had the highest reads mapping to the M. leprae reference and its specific RLEP region were then used for targeted whole-genome enrichment, and one red squirrel sample with two reads mapping to the RLEP region was also selected. The potentially positive samples were then enriched for M. leprae fragments using myBaits v4 in-solution hybridisation.

The final genetic analysis allowed for the reconstruction of four M. leprae genomes from three humans and one Eurasian red squirrel, and although the M. leprae genome in the squirrel sample had low coverage, the findings confirmed that this example of the genome had diverged ancestrally from two of the human cases found in the leprosarium of St Mary’s. The study also showed that the strain of leprosy found in the medieval S. vulgaris samples is closer to the strains found in medieval humans than it is to those found in modern-day squirrels. Since the squirrel and human samples were taken from the same city and have been dated as contemporary to one another, it can therefore be inferred that cross-species infection of leprosy likely occurred between squirrels and humans in medieval Winchester.

This is the first time that an animal host has been identified as a carrier of M. leprae in the medieval period, and indeed the first evidence of leprosy in British squirrels outside Brownsea Island. However, there is still research to be done in this field, since the way in which the medieval squirrels were themselves infected is as yet unclear. The possibility that they were first infected by humans cannot be ruled out, so researchers say it could be that leprosy in fact jumped back and forth between human and squirrel hosts in many instances of past cross-species infection.

While it is highly unlikely that leprosy will be passed from squirrels to humans today, studies like this do allow us to reconstruct these events from the past in order that they can be prevented in the future.

Text: Rebecca Preedy / Image: Alison Day, Flickr