Microbiologists at Ecole Polytechnique Fédérale de Lausanne (EPFL) and the University of Edinburgh (UoE) have discovered that red squirrels (Sciurus vulgaris) in Britain and Ireland carry the two bacterial species (Mycobacterium leprae and Mycobacterium lepromatosis) that cause leprosy in humans.

Image credits Wisconsin Department of Natural Resources / Flickr.

Leprosy was a big deal in medieval times. It’s a very noticeable condition which spreads easily through poor populations because of squalor and horrible hygiene. The disease transmits through coughing or contact with infected nasal fluids. It attacks the skin, peripheral nerves, the upper respiratory tract and eyes. Main symptoms include lesions (granulomas) formed on these tissues, and are usually accompanied by poor eyesight, weakness, and loss of pain-sensation in limbs. Because of the belief that leprosy spreads through simple touch, “colonies” sprang up throughout the world for the afflicted to live their lives sequestered from other people. It left a deep mark on cultures throughout history, and had severe a social impact on communities and groups in which it affected.

By the end of the Middle Ages however — for reasons still unclear — the number of leprosy cases fell dramatically in Europe. In the 1900s, the disease was virtually wiped out in Europe through a combination of increased hygiene and available medication. Colonies began to shut down as there were no patients to house, and for western culture it gradually became a problem of the past. On a global scale, the story is different — though largely controlled by modern antibiotics, there are still over 200,000 new cases of leprosy reported each year worldwide.

But it’s not only humans who get leprosy. Several cases have been reported in armadillos, for example, several of which have caused animal-to-human infections. Working from this possibility, the team sequenced the DNA of 110 red squirrels from England, Scotland, and Ireland. Some of the animals showed signs of leprosy, while others appeared healthy — the team found that most of them were infected with leprosy-causing bacteria.

“It was completely unexpected to see that centuries after its elimination from humans in the UK M. leprae causes disease in red squirrels,” says Stewart Cole.

“This has never been observed before.”

Squirrels from Scotland, Ireland, and the Isle of Wight carried M. lepromatosis, known to cause human leprosy in Mexico. On the other hand particular strain of M. leprae was found in animals from Brownsea Island and the skeleton of a leprosy victim buried in Winchester — just 70 km away from the island — 730 years ago. Genetic analysis of the two strains showed that they diverged from a common ancestor sometime around 27,000 years ago.

The findings show that a pathogen can stay hidden in the environment for hundreds of years after it’s been removed from humans.

“The discovery of leprosy in red squirrels is worrying from a conservation perspective but shouldn’t raise concerns for people in the UK,” says Anna Meredith.

“We need to understand how and why the disease is acquired and transmitted among red squirrels so that we can better manage the disease in this iconic species.”

But the team says there’s no cause for alarm in the general public.

“There is no reason for panic,” says Andrej Benjak, one of the paper’s lead authors.

“Autochthonous leprosy has not been detected in the UK in decades, though we cannot exclude the possibility of rare, unreported or misdiagnosed cases that originated within the UK.”

Andrej believes the study should prompt the WHO to increase leprosy monitoring efforts as part of the global Leprosy Surveillance Programme.

“The next logical step after this study is to check the red squirrel population outside the British Isles, and that includes Switzerland,” he says.

“Even if there is leprosy in red squirrels in continental Europe, the risk of transmission to people is generally low because of their limited contact with humans, and hunting red squirrels is forbidden in most European countries.”

The full paper “Red squirrels in the British Isles are infected with leprosy bacilli” has been published in the journal Science.