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The DNA of leprosy-causing bacteria has been found in the remains of people and a red squirrel unearthed at medieval sites in the UK. This makes red squirrels the earliest known non-human hosts of the infection and suggests it may have spread between the rodents and people at the time.

In 2016, scientists found that red squirrels (Sciurus vulgaris) around the UK carry strains of Mycobacterium leprae, the bacterium that causes the chronic disease leprosy. Some of the strains were similar to ones that infected people in England more than 700 years ago.

“So, we had an inkling that maybe medieval red squirrels have had it too,” says at the University of Leicester in the UK.

To investigate further, Inskip and her colleagues examined the remains of 25 people uncovered at the site of a medieval hospital for people with leprosy in Winchester and 12 red squirrels found at a nearby site that was home to at least one fur shop between the 11^th and 13^th centuries.

Most of the human bones exhibited the characteristic lesions associated with leprosy, while the squirrel bones showed signs of inflammation, another possible sign of the disease.

By analysing the DNA in the bones, the team found genetic sequences from M. leprae in three people and one red squirrel.

“There really was leprosy circulating among medieval squirrels,” says Inskip, making the species the earliest reported non-human carrier of leprosy.

The DNA showed that the strain of M. leprae found in the medieval red squirrel was more closely related to those in the three medieval people than to those in modern red squirrels. This indicates that the infection probably spread back and forth between squirrels and people in England in the Middle Ages.

“There were a lot of opportunities for transmission in medieval Winchester,” says at the University of Basel, Switzerland, who also worked on the study. In addition to the hospital and well-known fur trade in the city, historical reports from the period suggest that people in the area often kept squirrels as pets, she says.

The findings also suggest that the leprosy strains found in modern squirrels may not necessarily have descended from the strain found in this specimen. “It may be that there has been more than one transmission event between humans and squirrels over history,” says Inskip.

Although some small populations of red squirrels have leprosy today, it is important to stress that the transmission risk to people is basically zero, says Schünemann.

“Leprosy has definitely been around for a long time and M. leprae likely has a far more robust ecological history than our previous modern-day observations might have suggested,” says formerly at the US Public Health Service. “It is important that we understand this better.”