Medieval Leprosy Strains Strike Squirrels

This is a red squirrel with leprosy on its ear. [Dorset Wildlife Trust]

Evoking fears in the general public about infectious diseases is not a difficult task these days. Although, leprosy holds a unique place in human history, particularly in Europe during the Middle Ages. The afflicted became physical and social outcasts, often thought of as being unclean, untrustworthy, and morally corrupt—the severe physical disfigurement caused by the infection did not help to allay people’s fears either. Thankfully, with the development of antibiotics in the early 20^th century, treating leprosy became much easier. Yet, with antibiotic resistance for many bacterial species on the rise, the threat from leprosy is far from over.  

Now, a team of researchers from the University of Edinburgh and the Swiss Federal Institute of Technology in Lausanne (EPFL) has discovered that red squirrels in Britain and Ireland carry the two bacterial species that cause leprosy in humans. The findings from their study were published online today in Science in an article entitled “Red Squirrels in the British Isles Are Infected with Leprosy Bacilli.”

Once rampant in medieval Europe, leprosy dramatically declined by the end of the Middle Ages for reasons that are still unclear. About a century ago, leprosy in Europe virtually disappeared, at least among humans. Leprosy is an infectious disease that mainly affects the skin, peripheral nerves, upper respiratory tract, and eyes. It is caused by the bacteria Mycobacterium leprae and the recently discovered Mycobacterium lepromatosis. Largely controlled today thanks to antibiotics, there are still over 200,000 new cases of leprosy reported each year worldwide.

Interestingly, leprosy also affects animals, such as armadillos, which have reportedly caused a few cases of animal-to-human—or zoonotic—infections. Drawing from this evidence, the investigators carried out DNA tests on 110 red squirrels from England, Scotland, and Ireland. Some of these animals showed clinical symptoms of leprosy, while others did not—nonetheless, most were found to be infected with leprosy bacteria.  

Most unexpectedly, the research team found that red squirrels from Brownsea Island, off the south coast of England, were infected with a strain of M. leprae that is closely related to one found in a skeleton of a leprosy victim that was buried in Winchester 730 years ago, roughly 40 miles from Brownsea Island.

The scientists also found that red squirrels from Scotland and Ireland and the Isle of Wight were infected with leprosy bacterium; however, in these animals, it turned out to be the other leprosy strain, M. lepromatosis. This species causes leprosy in humans in Mexico, and further analysis showed that the two strains from Mexico and Europe diverged from a common ancestor around 27,000 years ago.

“It was completely unexpected to see that centuries after its elimination from humans in the U.K., M. leprae causes disease in red squirrels,” remarked co-senior study investigator Stewart Cole, Ph.D., chair of microbial pathogenesis and laboratory head at EPFL. “This has never been observed before.”

This new study shows how a pathogen can remain undetected in the environment, even hundreds of years after it has been cleared from the human population. “The discovery of leprosy in red squirrels is worrying from a conservation perspective but shouldn't raise concerns for people in the U.K.,” noted co-senior study investigator Anna Meredith, Ph.D., personal chair of zoological and conservation medicine at the University of Edinburgh. “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.”

As with any infectious disease, the threat to public health is always a concern and keeping the public informed is imperative so as not to fall into hysteria.  

“There is no reason for panic,” stated co-author Andrej Benjak, Ph.D., a senior scientist in Dr. Cole’s laboratory. “Autochthonous (indigenous) leprosy has not been detected in the U.K. for decades, though we cannot exclude the possibility of rare, unreported, or misdiagnosed cases that originated in the U.K.” He suggests increasing efforts to monitor the disease, as part of the WHO's Global Leprosy Surveillance Programme, where there is still room for improvement.    

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