Leprosy transmission between squirrels and humans

But we can't say for sure which species infected the other...
10 May 2024

Interview with 

Sarah Inskip, University of Leicester


But first, archaeologists have found that, during the Middle Ages, leprosy was transmitting between red squirrels and humans. It was revealed following a study of squirrel bones and human remains in the English city of Winchester. I’ve been speaking with Sarah Inskip, a bioarchaeologist at the University of Leicester and a co-author on the study…

Sarah - About eight years ago, people found that modern British red squirrels were infected with a strain of leprosy that had infected humans in medieval England. We were really interested to try and find out how these modern British red squirrels effectively had a strain of a disease that hasn't really been seen in England for about two to three hundred years. In order to answer this question, we needed to go back.

Chris - Where did you go back to and how far back?

Sarah - We went back about a thousand years, and in particular we looked at medieval Winchester. One of the reasons as to why we chose Winchester is because it had a large leprosarium. This is a hospital that was set up to care for individuals that had the disease. At the same time, we also knew that within the town there were fur trader streets where there were the archaeological remains of lots of red squirrels. We picked this place because we had circulating leprosy in humans, and then we had the opportunity to look at, well, is it also circulating in the red squirrels at this time? Is there possibility for transmission between the two?

Chris - Were squirrels very much part of the fur trade back in those days then?

Sarah - Yes. It's actually really strange. We don't think of squirrels as being important for animals, but they were really important for over a thousand years in England in terms of lining garments. People would line them up, the white to the grey, to make these very pretty fur lined trims. It was far more important than I even thought was possible.

Chris - So your theory would be that the squirrels are brought in and the leprosy is in the squirrels that get used for their fur, or just the fact that they are around humans and humans have got leprosy, it got into the squirrels from us. Do you know which way round it was likely to have been?

Sarah - This is one of the really important questions and actually we don't know which way round it would have occurred because people kept them as pets as well. At some point, someone would've been working with live squirrels to get the skins processed and then people would've been working with the skins to make it ready for a garment. Each one of those would allow transmission in both directions as well. We still want to try and find out.

Chris - So what did you have to work with from ancient Winchester and how did you study it?

Sarah - From Winchester, we had access to the individuals that had leprosy within the leprosarium, and it's quite big. It has individuals that span a number of years and we were able to extract their teeth and look in the pulp chamber where you have the blood that circulates in and out of teeth. We were able to look for the DNA of the leprosy causing pathogen within that chamber from the humans, but from the squirrels we were limited to a furrier pit. A furrier pit is a fur trader's pit where they're processing the fur, they remove bits that they did need and that was the hands and feet that often get stuck to the fur. The human side was fine, we had plenty of material, but we only had hands and feet of squirrels to do this. And for us it was phenomenal that we were able to recover the bacteria that causes leprosy infection from these tiny, tiny bones. We were really, really lucky.

Chris - You were looking for the DNA of the mycobacterium leprae, the bacteria that caused the disease.

Sarah - Yes, exactly. Leprosy is caused by a bacterium and that is mycobacterium leprae. We were specifically looking for the DNA of that organism to be able to show that that person or that animal was infected with that bacteria.

Chris - How does that prove - the fact you've got the genetic code - that one organism was giving it to another, i.e. the squirrel was giving it to the human or vice versa?

Sarah - We can look at the DNA and you can think a little bit about family trees, for example. We can look at specific segments or parts of the code to see how similar they are between the bacteria that's in the squirrel and the bacteria that's in the human. We can compare that to all of the other published strains of leprosy bacteria that exist. What we found is that the codes were so similar between the medieval squirrels and the medieval people of Winchester, that we can say that there is the same sort of strain circulating between the squirrels and the humans. What was really interesting is that, when we looked at the bacteria that's infecting the modern squirrels, it was distinct enough that there is a difference between them. That actually suggests that it was a different transmission event, that modern squirrels have the leprosy from. So, for us, it was really cool because we were able to show that not only has this transference happened between human to squirrels, squirrel to human, it's happened more than once because there are enough differences in the code between the modern and the ancient squirrel.

Chris - I suppose as well as there being an important historical message in this, there's also a modern message because we see very similar things with bird flu and with coronaviruses, don't we? Where there's a possibility of a two-way street, a trafficking of infection between animals and us.

Sarah - There is a precedent for this. Leprosy was not present in the Americas until the Europeans went over there and it got into armadillos where it is still today. But we also know from work on the genetic material from those infected armadillos and the people that do pick it up that they have picked it back up from armadillos. There's been enough change in that code in armadillos and then appearing in humans, but we know in that case that it's certainly gone from human to armadillo and back to human. In our research, we show this potentially with squirrels. The key message here is that, perhaps in places where leprosy is proving difficult to eradicate, we need to maybe go and look at some of the animals that are in the local areas to see if they are acting as hosts for the bacteria so it is able to keep coming back into the human population, or the human population reciprocally can keep infecting them as well. You have a circular dynamic that goes on.

Chris - There's something like a quarter of a million cases of leprosy around the world every year, isn't there? Everyone thinks it's gone away, but it is still prevalent in some places. And you think that perhaps it's worth a look in nature because there may well be a reservoir, something like a squirrel might be harbouring it and injecting it back into the humans locally?

Sarah - Yes, absolutely. For a long time, leprosy was thought to be a human only disease, with the exception of armadillo. They have a slight quirk in their biology that seems to make them susceptible. The fact that we've found them in effectively wild squirrels potentially does suggest that, okay, it does infect more animals than just us and they may act as a reservoir for this disease. It could be other rodents, it could be other mammals. So it's certainly worth exploring these areas where there are eradication efforts, but perhaps it's proving to be persistent.


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