Pandemics have been common throughout human history. Black Death, influenza, smallpox, HIV... each century has had its own particular fight between humans and diseases. In the Middle Ages, leprosy hit England hard. And its great transmitter was none other than an adorable forest animal that at first glance seemed harmless.
As explained by researchers from the University of Basel in an article published in the journal Current Biology, it was the English red squirrels that served as important hosts for strains of Mycobacterium leprae that later ended up being transferred to people of the time.
Experts have analyzed ancient DNA obtained from archaeological sites in the medieval English city of Winchester. “With our genetic analysis we were able to identify red squirrels as the first animal that was a host of leprosy,” says Verena Schuenemann, lead author of the study.
“The medieval red squirrel strain we recovered is more closely related to human strains from the same city than to strains isolated from infected modern red squirrels. Overall, our results point to an independent circulation of M. leprae strains between humans and red squirrels during the Medieval Period,” he adds.
Leprosy is one of the oldest recorded diseases in human history and is still prevalent to this day in Asia, Africa and South America, with more than 200,000 cases each year. While scientists have traced the evolutionary history of the mycobacteria that causes it, they didn't know how it could have spread to people from animals.
"Our findings highlight the importance of involving archaeological material, in particular animal remains, in the study of the long-term zoonotic potential of this disease, since only a direct comparison of ancient human and animal strains allows reconstructions of possible transmission events to over time,” says co-author Sarah Inskip, from the University of Leicester.
The researchers studied 25 human and 12 squirrel samples to look for M. leprae at two archaeological sites in Winchester. The city was well known for its leprosarium (a hospital for people with leprosy) and its connections to the fur trade.
During the Middle Ages, squirrel skin was used in many places to decorate and line clothing. Many people also kept squirrels in captivity in the wild so they could raise them as pets.
Schuenemann and Inskip's team sequenced and reconstructed four genomes representing medieval strains of Mycobacterium leprae, including one from a red squirrel. An analysis to understand their relationships found that they all belonged to a single branch of the M. leprae family tree.
The analyzes also showed a close relationship between the squirrel strain and a newly constructed one isolated from the remains of a medieval person. The ancient squirrel strain is more closely related to human strains from medieval Winchester than to modern squirrel strains from England, indicating that the infection was circulating among people and animals in the Middle Ages in a way that had not been detected before. .
“The history of leprosy is much more complex than previously thought,” notes Schuenemann. “The role that animals may have played in the transmission and spread of the disease in the past has not been considered and, as such, our understanding of the history of leprosy is incomplete until these hosts are considered,” he adds.
This finding is relevant today, as there are still many cases of diseases in which animal hosts are not considered "although they may be important in terms of understanding the contemporary persistence of the disease despite eradication attempts."
”In the wake of COVID-19, animal hosts are becoming a focus of attention to understand the emergence and persistence of the disease,” said Inskip. In the 2020 pandemic, the Pangolin was accused for weeks of being the transmitter. of the coronavirus, as had already been done for decades with rats, which were (wrongly) blamed for spreading the Black Death.
