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Medieval Leprosy Is Spreading Among UK Red Squirrels

Leprosy has been gone from the UK for decades, but it may have been just in hiding.
Image: Piotr Krzeslak/Shutterstock

I have vaguest childhood memories of leprosy as a thing that kids talked and joked about a whole lot amongst themselves. By the late 1980s, the disease, a slow-moving bacterial infection notorious for its capacity for producing physical deformities, had been fully curable for several years and was either eradicated from most first-world countries or was very close.

In the US, where leprosy was treated by confinement for much of the 20th century, the social stigma around it was nonetheless strong enough that the illness managed to persist in so many brains and as a great way for kids to be cruel to other kids. Highly infectious and deforming, it was perfect for that.

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Leprosy has been gone from the United Kingdom for even longer than the US. By 2014, it had been 60 years since anyone in England or Wales contracted the illness (excepting a number of "imported" cases from other countries). In Europe, the disease declined precipitously during the Middle Ages, and then, about 100 years ago, it all but disappeared. The reasons for this disappearance are still unknown.

It may not have disappeared at all, it seems.

In recent years, the UK has seen a steady decline in populations of red squirrels, a punk rock variety of tree squirrel native to Eurasia. In researching the causes of this disappearance, microbiologists at EPFL and the University of Edinburgh tested over a hundred dead squirrels across England, Ireland, and Scotland for leprosy-causing bacteria strains following initial reports of diseased squirrels. The bacteria was found to be widespread, even among squirrels showing no symptoms of the infection, indicating the red squirrels are acting as reservoirs for once-vanquished microbes.

The group's results were published Thursday in Science in a study led by EPFL biochemist Charlotte Avanzi.

Red squirrel with obvious leprosy. Image: Avanzi et al

Avanzi and colleagues actually discovered two distinct leprosy strains, M lepromatosis and M leprae, among the squirrel cadavers. The latter happens to be the oldest pathogen associated with the disease, tracing back to the Middle Ages. One squirrel found on Brownsea Island off the southern coast of England was found to carry an M leprae strain matching a strain found in a skeleton excavated from a grave dating back 730 years and located in roughly the same area. Leprosy finds a way.

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Leprosy finds a way.

The researchers had 70 red squirrel bodies from England, with and without leprosy symptoms; 40 from Ireland, where no squirrel leprosy has been reported; and then another couple of bonus grey squirrels from Scotland. In all, they wound up with 101 animals without leprosy symptoms and 13 with them. As the study summarizes:

"Six Scottish squirrels (two without clinical signs), two from Ireland (no clinical signs), and one from the Isle of Wight, England contained M lepromatosis in several tissue samples from different anatomical sites, whereas all 25 red squirrels (17 without clinical signs) tested from Brownsea Island were infected with M leprae."

So, is this bad for people? Maybe. Globally, the incidence of leprosy has hit a "stubborn plateau," Avanzi and note. Its ability to just disappear into another species like this suggests one mechanism that it may be using to persist despite extensive global efforts (the WHO offers curative multi-drug therapy at no cost). It doesn't, however, suggest that people are going to start getting sick in the UK.

It may be the case that squirrels were making people sick prior to leprosy's rather abrupt Medieval decline. In earlier times, the squirrels were prized for their meat and fur, and this is how it may have wound up being spread among European humans in the first place. Nowadays, there just isn't as much direct squirrel-to-human contact—and most people in the world (about 95 percent) appear to have natural immunity to the disease.

The study has a more general implication: Disease eradication may not always be what it seems. For a long time it was thought that leprosy was largely a human-borne illness (with some rare exceptions). But now we know this is not the case, and that for centuries it remained undetected in a species right under our noses. "Beating" diseases may then be more provisional than we would like to think.

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