Those Brits, they just love their cute red squirrels. They think it’s a bloody shame that the more aggressive gray squirrels have pretty much taken over. The grays were brought to England from the US as playthings for 19th century noblemen because they encapsulated the American pioneer spirit. But now it looks like these imports might have been an inadvertent act of disease prevention.

Red squirrels are now protected in the UK as the grays have completely infiltrated their habitat. But there's also a disease element to this struggle, a poignant rodent reversal of Europeans’ inadvertently killing off Native Americans with the smallpox they carried. Gray squirrels are immune to the squirrel pox virus that kills the native red squirrels, but they can still carry it.

Over the centuries, of course, England has faced more dire threats than invasive gray squirrels. One of these was leprosy. It had generally been thought that leprosy could be transmitted only between humans—that it had no other host to hide in. And it was also thought to be caused by only one infectious agent: Mycobacterium leprae.

But in 2005, people in the US contracted the disease upon exposure to infected nine-banded armadillos. And in 2008, lepers in Mexico and the Caribbean were found to be infected with a related organism, Mycobacterium lepromatosis. But the big surprise was an outbreak of leprosy in red squirrels all over Great Britain in 2014. It warranted a fair degree of investigation.

Researchers found that infected red squirrels on Brownsea Island, off the southern coast of England, harbored the traditional M. leprae that has been circulating in England since Medieval times. But infected red squirrels from England, Ireland, and Scotland had the newer M. lepromatosis. Genetic sequencing of the strains indicates that the Mexican and British M. lepromatosis diverged from one another 27,000 years ago. It's not quite clear where it's been hiding since.

(Naturally, no gray squirrels were infected.)

Leprosy was eradicated from Britain during the 15th and 16th centuries, so the scientists were surprised to see it in squirrels. And, for that matter, in armadillos. They could not find anything in the leprosy bacillus to explain the unusual host range, so they looked at the hosts. It turns out that all three of us—humans, armadillos, and red squirrels—can have mutations in a receptor present on immune cells that can modify our vulnerability to leprosy.

This newly identified zoonotic mode of leprosy transmission (meaning from animals to humans) may explain the stubborn perseverance of the disease around the globe, in spite of the availability of effective treatment regimens.

Science, 2016. DOI: 10.1126/science.aaah3783 (About DOIs).