There are two microbes that produce leprosy in humans, one is Mycobacterium leprae, with a global dispersion, the other is Mycobacterium lepromatosis, the latter was first found in humans in America, suggesting a New World origin.
The M. lepromatosis is also found among the red squirrels found in Britain, which seems odd, but has an interesting explanation.
Red Squirrels and Leprosy
Charlotte Avanzi et al., (2016) (Red squirrels in the British Isles are infected with leprosy bacilli. Science 354, 744-747 DOI:10.1126/science.aah3783) investigated the matter.
Red squirrels (Sciurus vulgaris) is a Eurasian species, and spans the vast land mass from Ireland to Eastern Siberia, as shown in the map below.
They also lived in the United Kingdom, where they became extinct in the 1700s. They were reintroduced from surviving popualtions in Ireland. Now, in England, Scotland, and Wales they are protected as an endangered species because humans have been putting pressure on their habitat and also, the gray squirrel, imported from North America, (Sciurus carolinensis) has invaded their territory adapting to it and competing with them. To make matters worse, they are infected with a poxvirus and leprosy of the M. leprae variant and, surprisingly, the "American" M. lepromatosis.
The authors analyzed the DNA of the bacteria and calculated the date when these strains originated. The M. leprae variant seems to have passed from humans to squirrels in the Middle Ages, when it was prevalent among people, who also interacted with them, as pets and also bred them for food.
The other (M. lepromatosis) strain diverged from the strain currently found in human beings in America (Mexico), a long time ago. Their phylogenetic tree shows that 26,859 years ago the human and squirrel variants shared their last common ancestor. The paper states that:
"we estimated that the British Isles and Mexican strains diverged from their most recent common ancestor around 27,000 years ago, whereas the Irish and British strains diverged as recently as 200 years ago (Fig. 3A). The latter estimate is consistent with the date of the first campaign to reintroduce the red squirrel into Ireland from England between 1820 and 1856, following its extinction in the 17th century. This suggests that these animals may already have been infected with M. lepromatosis when they were reintroduced."
But where did these Irish red squirrels catch their infection?
They didn't get it from the mainland, a paper published in 2019 analyzed red squirrels captured in Belgium and the Netherlands, and did not find traces of leprosy in them. The source of the bacteria does not seem to have originated in NW Europe. They confirmed that the squirrels in the British Isles are the only known rodents carrying leprosy bacilli.
Did they arrive in infected gray squirrels brought from America?
M. lepromatosis in Ancient Native Americans
Ramirez et al, (2025) found genetic traces in the remains of two adult men who lived 4,000 years ago in Chile, close to what is now the city of Coquimbo (Ramirez, D.A., Sitter, T.L., Översti, S. et al. 4,000-year-old Mycobacterium lepromatosis genomes from Chile reveal long establishment of Hansen’s disease in the Americas. Nat Ecol Evol 9, 1685–1693 (2025). https://doi.org/10.1038/s41559-025-02771-y June 2025).
They built a phylogenetic tree and also used the British red squirrel genetic data. Their tree shows the red squirrels on a separate branch from the human one, which split from it 26,794 years ago. Within the human branch, the ancient Chilean remains sit on a fork, and the modern human cases from North America sit on another, see Fig. 3 d in the paper, shown below.
The paper found a similar split date between the human and red squirrel variants: "we estimate the median time for the most recent common ancestor (tMRCA) of M. lepromatosis to be ~26,800 years ago (95% HPDI range of 4,206 to 115,340 yr bp). Genomes obtained from human hosts yield a divergence estimate of ~12,600 years (95% HPDI: 5,304 to 49,659 yr bp), while the tMRCA for the red squirrel clade is ~440 years (95% HPDI: 73 to 2,063 yr bp)."
This is interesting, however the confidence intervals are very large: the haplotype found in ancient Chileans and the one found in contemporary leprosy patients in Mexico split between ~5,300 and ~50,000 years ago! (mean of ~12,600). The latter number makes me wonder if it was adopted because it falls neatly within the acceptable dates for human presence in America. Nevertheless, if we take the 50,000 year date for a split between the bacteria living in South American paleoindinans and those that remained in Mexico (causing the modern infections), we would have a very old date for people migrating across America.
The same could be said about the split between squirrel and human variants: ~4,200 to ~115,000 years bp. The older figure would imply Neanderthals or even Denisovans passing it on to squirrels or getting it from them!
Regarding the British squirrels, the dates are ~73 to ~2.000 years. The older date could suggest an ancient transatlantic contact c. 1AD, Romans, or navigators fishing cod in Terranova brought some gray squirrel infected with this American leprosy?
Gray squirrels, imported into Britain in the 1870s, are originally an East Coast creature (see map below). Did they harbor the infection? If so, why aren't any gray squirrels reported as carriers in these studies?
Ramirez et al suggests that "finding of two M. lepromatosis infections in South America, before the periods of known contact with either Oceanian or European populations, implies either movement of the pathogen within human groups during an early peopling event or its previously established endemicity in the continent in a separate reservoir species eventually acquired by humans. The latter would imply that its current distribution arises from a postcolonial dissemination, and would make it one of the few global diseases known to have emerged in the Americas." In other words: the first Americans c.12,600 y BP brought it with them, or the got infected from a local "reservoir species" and, the North American (Mexican) variants were due to its spread during the Spanish Colonial period.
Another paper published last year by Maria Lopopolo et al., (2025) (Pre-European contact leprosy in the Americas and its current persistence. Science 389, eadu7144 .DOI:10.1126/science.adu7144) reported that the M. lepromatosis cases have been "mostly reported in the Americas, with a few sporadic cases in Asia..." (two cases in Singapore, and two in Myanmar). I guess that the Asian cases are post-Discovery dispersal due to transpacific trade from America.
Lopopolo found more ancient samples in America, one from Canada and two from Argentina, their dates are 1,310 to 860 BP.
The phylo tree built by this team (see below) finds five clades: two of them contain the ancient variants (North America, South America), another has the red squirrel variant, and there are two more that hold the modern cases in North America (not only Mexico, but also, Costa Rica, and the U.S.). Interestingly one of these contemporary branches "represented by two present-day US strains, forms a basal clade that diverged from all others around 10 millennia ago." This basal clade 10 ky old, is, again, in my opinion someting forced into the data to coincide with the accepted dates for the peopling of America ~15 ky. How could they justify a date of 50 ky? They'd have to explain how human beings in Siberia or East Asia carried this leprosy strain into America but, at the same time, leave no trace of it in Asia.
Lopopolo also suggests that the other modern clade is recent, dating from the postcontact (after 1492) period. They also found that the split between the two varieties of leprosy Mycobacterium leprae and Mycobacterium lepromatosis took place later than previously estimated, 2 million to 700 ka vs. ~14 million years ago.
Since armadillos and humans are the carriers of the Old World leprosy, and armadillos only live in the Americas, the split between both types of leprosy requires hominins 2,000,0000 to 700,000 years ago carrying both variants. It also requires an explanation on why there are no traces of the "American" leprosy in Eurasia or Africa.
This, in my opinion, allows for an archaic hominid to have been the vector carrying the old world strain in Eurasia, Africa and Oceania, and some other group within that population, carrying the New World strain. This period spans manhy ancient hominins H. erectus, H. antecessor, H. heidelbergensis, Neanderhtals, and Denisovans! Could one of these groups have carried the strain into America. Where it jumped species to the squirrels?
There is one final, and very recent paper by S. Souguel, T. Oueslati, G. Grine, M. Drancourt published in December ( The role of red squirrels in leprosy dynamics in the United Kingdom: a critical review, One Health, Vol 21, 101114 https://doi.org/10.1016/j.onehlt.2025.101114).
The authors propose that "M. lepromatosis was imported into the United Kingdom following the progressive expansion of the British Empire, starting from the 16th century expansion into Canada through to the 19th century expansion" of the Empire that were "most probably imported from British colonies for their fur."
The authors indicate but do not provide a bibliographic reference that "lepromatosis has been further reported in red squirrels in a geographical belt limited by the 55th parallel north (in Scotland) and the 25th parallel south (in Paraguay), indicating the geographical source of its introduction." There are other species of squirrels in the Amazon, in Ecuador, Peru, Brazil, and Paraguay. But they are not red squirrels. There are also native squirrels in Argentina in the Yungas jungles in the Northwest, also in Bolivia. But, as I mentioned, the authors don't provide a source for this information (M. lepromatosis in squirrels outside of the UK). I searched online for references, and didn't find any.
The paper concludes with a teaser, "The predominance of M. lepromatosis—until recently undocumented in medieval human remains but frequently found in contemporary red squirrels— challenged the notion of direct historical zoonotic transmission. However, forthcoming research suggesting the presence of M. lepromatosis in human remains from the medieval era may significantly alter this narrative and restore plausibility to a broader zoonotic hypothesis."
Are the authors preparing a paper on European M. lepromatosis from the Middle Ages?
We will wait for it!
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia Copyright 2009-2026 by Austin Whittall ©
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