Did Squirrels Spread Leprosy in Medieval Europe?
A recent study reveals squirrels may have transmitted the disease
Leprosy is one of the oldest recorded human diseases; some of the earliest accepted records can be traced back to India ca. 600 BC, perhaps even 1400 BC, and China around 500 BC. Yet despite centuries having passed and massive jumps in medical treatment and understanding, the disease remains prevalent in parts of Asia, Africa, and South America.
While much of the disease’s evolutionary history has been revealed in ancient DNA (aDNA), the role of animal hosts and interspecies transmission remains largely unexplored. During and after the COVID-19 pandemic, zoonotic (diseases that can spread from animals to humans, pl. zoonoses) diseases took centre stage. Yet, the scientific tools used to understand these zoonotic diseases primarily only focused on modern transmission, excluding research into historic zoonoses.
In a study published by Urban and colleagues in Current Biology, the researchers have presented the first ever One Health (a method that recognises the interconnectedness of humans, animals, and environmental health) approach to the understanding of the disease’s zoonotic past and how this understanding could help to manage and eradicate it in the present. This approach drew me to the study; like most people, I think of Biology and Archaeology as separate, and yet, it was only through collaboration between disciplines that this study could be produced. As a zooarchaeologist, I am no stranger to interdisciplinary study, using what I know about animal bones to interpret past climates, hunting behaviour, seasonality, etc. However, I usually think of my field as being useful for other histrically focused fields, such as for geologists who may want to know about the past climate or anthropologists who want to know more bout past hunting behaviours.
To use zooarchaeology for modern medical studies is something truly groundbreaking.
Zoonotic Diseases and Leprosy
Zoonotic diseases have existed for as long as humans and animals have lived together, and in recent years, they have gained more prominence in modern health agendas than ever. However, little attention has been focused on historic zoonoses due to the lack of pathological and genetic research on zooarchaeological (archaeological animal) remains.
However, without complete pathogen genomes from archaeological animals, understanding the long-term dynamics and nuances of animal-to-human transmission is hindered, thus making it harder to identify potential spillover and reemergence risks of current zoonotic diseases.
Leprosy is mainly caused by Mycobacterium leprae (M. leprae) bacteria but also, to a lesser extent, by M. lepromatosis. The disease can lead to nerve damage, chronic infection, blindness, dry skin, anosmia (smell blindness), and alopecia (hair loss). While the disease is almost unheard of in many places in the world, it persists in parts of Asia, South America, and Africa, with over 200,000 cases annually.
While there is an ongoing debate about its exact origins in either Eastern Africa or India, the earliest widely accepted cases date back to India, some 1400 to 600 BC. From there, it spread eastward to China and over the ocean to Japan. Armies and traders, such as Alexander the Great’s returning armies from India (327–325BC), brought the disease to the Middle East and Greece. The Romans then spread it throughout the Mediterranean Basin and Western Europe. By the Middle Ages (1000–1400AD), the disease ran rampant throughout Europe.
Humans are the primary host of the disease. Still, some animals are known to transmit it, including wild armadillos in the Americas, chimpanzees in West Africa, and red squirrels from the British Brownsea Island.
These modern squirrels harbour a medieval leprosy strain, which indicates a historic transmission event. The bacteria does not survive well outside the living host’s body and thus can almost exclusively spread via human-to-human or animal-to-human contact.
The Medieval Era and the Squirrel
Considering its modern and historic interspecies transmission, leprosy was the ideal first candidate for a One Health pilot study. By studying the archaeological context of ancient cases of leprosy, the disease spread and evolution could be better understood.
Throughout the medieval period, squirrels were kept as pets or used in the fur trade. Squirrel fur in England was usually acquired from Scandinavia, Eastern Europe, Russia, and the Baltic, with some squirrels also trapped in Scotland, Italy, and Ireland. These little rodents were trapped throughout the year, and their furs were used to line and trim the garments of the High and Late Medieval folk. For example, vair was a way in which the squirrel fur was laid out to decorate clothing in a checkered pattern.
In many medieval images, people dressed in squirrel fur can be found. Beyond that, there are even accounts detailing the use of squirrel fur for clothing, such as the use of squirrel fur by the nuns of Montivilliers (1265 and 1266) for their pilches (type of outer garment). Such massive quantities of squirrel fur were imported, dwarfing all other animal skins. One English customer account from 1384 recorded a jaw-dropping 377,200 imported squirrel skins (all other animal skins accounted for less than 15,000).
Beyond being used for their fur, squirrels were also widely used as pets. The practice is recorded in the various accounts of episcopal visitations (official visits by the bishop) and the recovery and depiction of squirrel collars and leashes. English nuns were also known to keep pet squirrels, as were the nuns of St. Sauveur d’Evreux, who kept small dogs, birds, and squirrels. And at St. Léger des Préaux, there were two small dogs and three pet squirrels. The pet squirrels were captured as kits and raised close to humans, often depicted sitting on their laps and shoulders.
To reflect the opportunities for transmission of leprosy between squirrels and humans, a medieval city with both leprosy and squirrels was needed. Winchester fulfilled both needs. During medieval times, it was a key city on trading routes. From historic records, it was known to be an active participant in the preparing and selling of various lined garments, including squirrel fur. One such furrier (a workshop specialising in furs) was in Staple Gardens, which operated between the 11th and 13th centuries. During excavations, it was revealed that the site contained various foot bones belonging to cats, foxes, stoats, squirrels, and unidentified small mammals.
Finding so many foot bones made sense, considering it was typical that the hands and feet of the animals often remained attached to the fur and were only later removed by the furrier when processed into garments.
The city was also the site of St Mary Magdalen, a leprosarium (hospital for leprosy patients) active between the 11th and 15th centuries. Excavations of its graves revealed many individuals with leprosy-associated lesions.
The Study Outcomes
Using archaeological remains, aDNA, and historical sources, Urban and colleagues tried to detect M. leprae in the remains of 25 human and 12 squirrel samples. They did this by using a method called shotgun sequencing. This technique involves randomly breaking up the DNA sequence into various smaller fragments and analyzing all these random fragments to look for traces of M. lepra. The shotgun method is usually used to prevent bias from skewing the results. For the human samples, they found at least 2000 DNA fragments that matched the reference leprosy bacteria. For the squirrel samples, they also found over 2000 matches to leprosy bacteria.
To ensure their results were sound, Urban and colleagues enriched the ancient DNA in the laboratory. This amplified the detectability of the small sample size and reduced the background detectability of non-relevant DNA.
Three whole genome sequences were recovered from the human samples; sadly for the squirrels, only one low-coverage or not fully completed genome sequence could be reconstructed from the aDNA. These genomes were compared to reference genomes to ascertain the accuracy of the data.
Their results provided multiple lines of evidence that M. leprae strains of medieval humans and squirrels belonged to the same genotype, meaning there was definitive interspecies leprosy transmission. The further genetic analysis allowed the researchers to reconstruct four new M. leprae genomes previously unknown, three from humans and one from a Eurasian red squirrel.
It also successfully showed that squirrels outside of Brownsea Island carried leprosy. However, it remains unclear if the squirrels transmitted leprosy to humans first or if they became infected after contact with humans.
Until this study, the historic zoonotic transmission had only been suggested via indirect evidence. However, the aDNA results provide clear, direct proof of this transmission.
The results will be instrumental in understanding the diversity of leprosy strains that circulate in animal hosts and the frequency and direction of transmission to humans, other animals, and the environment. These findings could inform future medical debate and the role of animal hosts in its continued persistence in the modern day.
The study conducted by Urban and colleagues showcases the value of interdisciplinary study. Using archaeological, historical and biological approaches, the researchers were able to definitively showcase that leprosy transmission between medieval humans and squirrels occurred.
As a zooarchaeologist, I have been taught to look for signs of disease in animal bones. Yet, I would probably only use that information to draw conclusions about the animal.
How did the disease affect the animal, and how serious was it when the creature died?
Do any other bones in the assemblage have this particular disease?
To find those markers of disease and implement them in a biomedical study is not something I would have considered. Yet, the One Health method has much potential for future medical studies.
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References
- Bennett, B.H., Parker, D.L. and Robson, M., 2008. Leprosy: steps along the journey of eradication. Public Health Reports, 123(2), pp.198–205.
- Urban, C., Blom, A.A., Avanzi, C., Walker-Meikle, K., Warren, A.K., White-Iribhogbe, K., Turle, R., Marter, P., Dawson-Hobbis, H., Roffey, S. and Inskip, S.A., 2024. Ancient Mycobacterium leprae genome reveals medieval English red squirrels as animal leprosy host. Current Biology.
