Some red squirrels roaming the British Isles are infected with leprosy and it's been circulating among them for centuries, New Scientist reports.
Researchers led by the University of Edinburgh's Anna Meredith report in Science this week that they examined 70 Eurasian red squirrel Sciurus vulgaris cadavers from Great Britain and 40 from Ireland, both with and without signs of disease. They also included four Scottish gray squirrel cadavers in their study. Twenty-one percent of the squirrels without signs of disease and all 13 with clinical signs of disease harbored Mycobacterium leprae or M. lepromatosis DNA, the researchers say. In addition, serological tests of nine diseased and 14 healthy red squirrels found that 13 of the 23 blood samples harbored leprosy antibodies.
Meredith and her colleagues also sequenced M. lepromatosis samples from seven squirrels as well as a sample from a leprosy patient in Mexico and compared the to the reference M. lepromatosis genome, which is from a different Mexican leprosy patient. Using this, they constructed a phylogenetic tree. While the two samples from Mexico only differed by a handful of SNPs, the Mexican and British Isles samples differed by 400 SNPs. Based on the bacterium's mutation rate, the researchers estimated the strains diverged some 27,000 years ago, while the British and Irish strains diverged about 200 years ago.
In addition, they reported that the M. leprae sample from Brownsea Island in England was most closely related to medieval M. leprae strains, suggesting that leprosy has been circulating among squirrels for quite some time.
"That for me was a real gobsmacker," co-author Stewart Cole from Ecole Polytechnique Fédérale de Lausanne tells NPR's Goats and Soda blog. "The very same strain that would cause disease in humans back in the Middle Ages was still present in the squirrels. That's awesome."