Study suggests leprosy was carried by red squirrels in mediaeval England

A growing body of evidence suggests leprosy may have had zoonotic origins and spread to human beings from some species of animals. In The British Isles in mediæval times, the disease may have spread to humans from Eurasian red squirrels, a new study published in the journal Current Biology suggested, the British Broadcasting Corporation and The Hindu newspaper reported. The researchers studied human and red squirrel bones from archaeological sites in Winchester, southern England, and found they had closely related strains of the bacterium that causes the once dreaded and widespread disease.

Leprosy is a chronic infectious disease caused by Mycobacterium leprae. It has a gestation period ranging from two years to as long as 20 years and attacks the skin, nerves, and mucous membranes. In severe cases, it can lead to disfigurement of the face, loss of fingers and toes, and blindness. The long gestation period often results in delayed diagnosis of the disease, but it is now curable with multi-drug therapy.

No one is thought to have caught leprosy in the U.K. for 70 years at least, the BBC reported, but the disease remains endemic in many parts of the world, including Asia, Africa and South America, with more than 200,000 cases reported every year. India accounts for over 50% of these cases, according to a report published in the Indian Journal of Leprosy last year, though the disease is no longer as widespread in the country as it used to be until the mid- to late-20th century.

Also called Hansen’s disease, leprosy is among the oldest known diseases in human history. “For thousands of years, humans were thought to be the only natural host of M. leprae until the discovery of M. leprae in several wild animals in recent decades, such as armadillos and, more recently, red squirrels and chimpanzees,” the authors of the study wrote, according to a report in The Hindu.

While scientists have traced the evolutionary history of the bacterium that causes leprosy, how it may have spread to people from animals in the past is not known beyond some hints that red squirrels in England may have served as hosts. Armadillos have been found to carry the bacterium and are suspected of passing it to humans. Some modern red squirrels in the U.K. also have the bacteria, but there has never been a reported case of transmission to humans and experts say the risk is very low.

A research report published in the journal Frontiers in 2019 said, “Nothing is known about the presence of leprosy bacilli in other wild squirrel species despite two others (Siberian chipmunk [Tamias sibiricus], and thirteen-lined ground squirrel [Ictidomys tridecemlineatus]) having been reported to be susceptible to experimental infection with M. leprae. Rats, a food-source in some countries where human leprosy occurs, have been suggested as potential reservoirs for leprosy bacilli, but no evidence supporting this hypothesis is currently available.”

This is the first time a mediæval animal has been identified as a host for the disease. “The finding of leprosy in modern squirrels was surprising and then it’s incredible that we found it in the mediæval period,” study co-author Dr Sarah Inskip of the University of Leicester told the BBC. “It really goes against the narrative that it was a human disease specifically.”

It’s not clear whether the mediæval squirrels gave humans leprosy or got it from humans. But the shared strain suggests it was circulating between people and animals in the Middle Ages in a way that had not been detected before, the researchers said.

Back then, squirrel fur was used as a fine lining for clothes and some people also kept squirrels as pets. The rodents were particularly popular with women. The researchers studied 25 human and 12 squirrel samples. The human remains came from a Winchester leprosarium, or hospital for leprosy patients, and the squirrel remains from a nearby pit used by furriers.

Previous studies found that modern red squirrels from Scotland and Brownsea Island off the coast of southern England carry leprosy. Public Health England, a body that has since been replaced by was replaced by the U.K. Health Security Agency and the Office for Health Improvement and Disparities, said the probability of humans catching it from squirrels was very low and there had never been a reported transmission.

How the disease spreads among people is also not clearly known, but prolonged, close contact with someone untreated, over many months, is needed to catch the disease.

“The history of leprosy is far more complex than previously thought,” Prof Verena Schünemann of the University of Basel in Switzerland, senior author of the study, told the BBC. “There has been no consideration of the role that animals might have played in the transmission and spread of the disease in the past, and as such, our understanding of leprosy’s history is incomplete until these hosts are considered.”

Dr Stephen Walker, associate professor at London School of Hygiene and Tropical Medicine, told the public broadcaster, “There is no doubt that in certain circumstances animals do appear to play a role, but the size of that role in global terms of leprosy hasn’t been delineated and, I would agree, does need more work. I think it highlights that we still have a lot of work to do to understand the transmission of this ancient disease better, in our efforts to try and reduce the impact globally.”

The researchers in the latest study 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 all of them belonged to a single branch on the Mycobacterium leprae family tree. They also showed a close relationship between the squirrel strain and a newly constructed one isolated from the remains of a mediæval person, The Hindu reported. The study found that the mediæval squirrel strain was more closely related to human strains from mediæval Winchester than to modern squirrel strains from England, indicating that the infection was circulating between people and animals in the Middle Ages in a way that hadn’t been detected before.

“These findings on the natural reservoir of M. leprae indicated that M. leprae circulates in more wild animals than we suspected, and zoonotic infection may contribute to the epidemic of leprosy. Therefore, it is inevitable that leprosy epidemics can persist for a long time in the future, and we should remain vigilant against the spread of M. leprae between humans and wildlife,” the researchers wrote in the study.