Leprosy in humans was eradicated from Britain by the 19th century, but new research published in the journal Science reveals that a medieval strain lives on in a secluded population of squirrels. As the The Telegraph reports, 25 red squirrel specimens sampled from Brownsea Island, off England's southern coast, all tested positive for leprosy-causing bacteria.

Mycobacterium leprae is the oldest strain of bacterium linked to the disease. It was the same pathogen responsible for spreading leprosy throughout Europe during the medieval period, with the last case of it in Britain recorded in 1798. Leprosy has entered the country through outside sources since then, but the indigenous strain was believed to have been eradicated. The recent study from the University of Edinburgh and the Swiss Federal Institute of Technology shows that Brownsea Island red squirrels have been harboring it for at least a few decades.

Though every squirrel sampled from the island carried the bacteria that causes the disease, only some of them showed symptoms. Leprosy manifested itself in the animals through swelling and hair loss around the ears, nose, and paws. The disease isn’t unheard of in the creatures: Both animal versions and human strains of leprosy that have infected people in places other than Britain have been found in the squirrels in the past. But these results mark the first time a human strain native to the country has been detected in red squirrels.

In humans, leprosy—known as Hansen's disease today—causes painful growths and lesions on the skin. While cases still crop up in certain parts of the world, leprosy is exceedingly rare. It’s estimated that 95 percent of the world’s population has a natural immunity to the disease, and when it is contracted, it can be treated with antibiotics.

The news is more alarming for red squirrels than it is for humans: Thanks to threats like the squirrel pox virus and invasive grey squirrels, their numbers have already fallen from an one-time high of 3.5 million to 120,000 in the UK. The researchers behind the study will now work with conservationists on the island to figure out how to better protect the squirrels in the future. In the meantime, they recommend humans avoid contact with the animals and wash their hands regularly to avoid infection.

[h/t Telegraph]