For many people, leprosy brings to mind Biblical stories of diseased people cast out from society. It’s a condition that today is largely found in developing countries, whereas in other, mostly Western nations it’s a pestilence of the past that was eradicated decades ago. But recent research has shown the disease not only persists in Britain but, perhaps more alarmingly, is also being carried by one of our best loved and most endangered native mammals, the red squirrel.
The study by researchers at the University of Edinburgh and EPFL in Switzerland found red squirrels from England, Scotland and Ireland were infected with leprosy. In particular, a group from Brownsea Island on the south coast of England had a strain of the disease virtually identical to one that infected humans in the middle ages.
So could it be that leprosy was never entirely eradicated from Britain but instead has lingered on in wildlife reservoir hosts in isolated areas? Possibly, but the whole picture is more complex, not least because of the history of the red squirrel in the British Isles. Understanding what’s going on could help us in our efforts to protect and regrow the red squirrel population.
Leprosy is an infectious disease caused by bacteria that can persist in the body for years without causing symptoms but can eventually lead to skin lesions, eyesight problems and nerve decay. This can cause sufferers to lose the ability to feel pain and so repeatedly damage parts of their body (leading to the myth that leprosy causes limbs to drop off).
The disease seems to cause similar symptoms in red squirrels, with individuals commonly exhibiting alopecia, swollen eyes, ears and digits. How serious a problem the leprosy is for British red squirrels has still to be fully investigated, although high numbers of animals sampled in this study tested positive for the disease. Given the plight of the species, which have gone from a population of millions to just 120,000 in a few centuries, it cannot be good.
The new research, published in the journal Science, compared genetic sequence data from diseased squirrels with those taken from contemporary human cases from Mexico and the skeletons of leprosy victims from medieval Europe. The results show that the leprosy in the squirrels was caused not only by the classic bacteria Mycobacterium leprae (long thought to be the sole causative agent of leprosy), but also by the more recently discovered Mycobacterium lepromatosis.
The M leprae strain found on Brownsea Island seems to be almost identical to that of medieval victims from England and Europe. This suggests the disease has persisted in British wildlife long after its eradication from the human population. Using genetic analysis, the researchers also showed the British and Irish strains of M lepromatosis had a common ancestor just 200 years ago. By comparison, they separated from the strain found in Mexico 27,000 years ago. This suggests the disease may have actually been imported to Ireland when conservationists first tried to reintroduce red squirrels to the country from Britain in the 19th century.