In August 2024, Jordan became the first country to officially eliminate leprosy.1 Countries participating in the World Health Organization (WHO)2 have committed not simply to bringing percentages of leprosy infections down, but to “getting to zero” in terms of absolute numbers of disease transmissions, all the while upholding the dignity of those who suffer from what can be a severely debilitating condition. The complete elimination of leprosy globally is the goal.
Is that even possible? The WHO’s 2024 global report on leprosy gathers data from 184 countries. 56 countries reported 0 cases of leprosy in 2023, the long-term consequence of declining incidence of the disease over the past century, thanks in large part to the antibiotic therapies available since the 1990s. Nevertheless, the same report counts 172,570 people worldwide currently afflicted with this now-curable disease, with 182,815 newly detected cases during the 2023 year. The 128 countries still reporting leprosy comprise all five inhabited continents. Leprosy is still a global disease.
But when did leprosy become “global”? And how might answering that question help us better understand the challenges involved in eliminating it? Neither as eternal as often assumed nor the result of globalizing forces in the modern era, leprosy as a human disease seems to be only about 5-6000 years old. Humans had globalized—moved into the five inhabited continents—long before that. But leprosy, it seems, did not initially travel with them. The genetic footprint for leprosy’s history suggests that it may have originated as a human disease somewhere near the Pacific Rim of Asia, and spread slowly from there. In the last 2000 years, however, it has diverged wildly, moving across continents and into new ones. Its almost complete globalization dates from the Middle Ages.
Which is why our North Africa-to-Italy transplant, Constantine the African (d. before 1098/99), makes an excellent focal point for thinking about leprosy’s past as well as its future. Constantine’s linking of Tunisia and Italy—across the “middle of the Middle Sea” (the Mediterranean)—parallels his chronological situation in the middle of leprosy’s medieval globalization. The accelerating long-distance trade that made his distantly-sourced pharmacopeia available also caused humans themselves to be carried across continents and oceans. (Sometimes willingly; sometimes not.) Leprosy traveled with them. Constantine could never have known it, but within about 200 years of when he was writing, leprosy had likely made its way more than halfway across the Pacific, all the way to Hawai’i. Humans carried it, but what likely helped it persist in its new environs were local animal populations. And we have seen in recent years, human-animal interfaces can be at the heart of what fuels pandemics.
Constantine as Mediator: Leprosy in the Medieval Mediterranean
In last year’s post for World Leprosy Day, we saw that Constantine had written a specialized text on this disease without stigmatizing it or its sufferers. But the crowning work of his career was translating the better part of a massive medical encyclopedia from Arabic, the Kitāb al-Malakī (Book of the King) composed by the Persian physician ʿAlī ibn al-ʿAbbas al-Majūsī by 977/78. In the chapter he devotes to the causes and symptoms of leprosy, Constantine deliberately omits al-Majūsī’s characterization of leprosy as a “cancer affecting the whole body.” Although he retains al-Majūsī’s suggestion that leprosy is contagious and that healthy people would be wise to avoid close contact with those infected, his discussion is free of any moralizing. In fact (as medical historian Anna Gili notes), it was not fear of contagion that prompted the expanding foundation of leprosaria throughout Europe in the 11th and 12th centuries, but changes in social concerns about how those most in need of aid should be cared for.
Evidence of Acceptance: Letting the Bones Tell New Stories
The possibility that care rather than scorn was the initial motivator for the European push to found leprosaria can now be seen from physical evidence as well. For the past decade, paleogeneticists and bioarchaeologists have been reconstructing the physical history of bodies afflicted by the bacteria that cause leprosy. In a new study on the cemetery of Sant Llàtzer in Barcelona, the first excavated cemetery in Spain directly linked to a leprosarium, a team of bioarchaeologists and historians have investigated both the thoughts and actions of a community’s responses to leprosy.
Used between the 12th and the 18th centuries, the leprosarium of Sant Llàtzer lay at the intersection of two roads leading to the city gates, just outside the city walls amid orchards and vineyards. Later, when the city expanded, the leprosarium was enclosed within the walls. Thus, there was nothing exclusionary in the placement of the house. Bioarchaeological analysis of remains from the in-house cemetery demonstrate that a number of those buried there did indeed bear all the hallmarks of extensive leprosy infections. Radiocarbon-dating of one such individual, whose hands show the characteristic “pencilling” or bone erosion common in leprosy, indicates that she lived at some point between 1038 and 1179; thus, she may have been a near-contemporary of Constantine the African.
In what’s called a “life-course approach”—which looks at physical evidence to reconstruct how a person lived, rather than just how they died—the bioarchaeologist Alette Blom has found that physical hardship in childhood (poor nutrition, stresses from other infectious diseases) primed the leprosy-infected for severe effects of leprosy later in life. That situation, in turn, made it more likely that individuals would need the long-term care in daily living that leprosaria provided.
Such approaches tell the story of individuals. How can we broaden our focus to tell about pandemics? First of all, accumulating data from genetics is already revealing the astounding breadth and complexity of the bacterium’s travels. As already noted in 2022, the strain of M. leprae that now finds major distribution in West Africa may have entered the continent through ancient Egypt. And we find an almost mirror-image of strain distribution in modern Brazil, following patterns of transatlantic slavery after 1500 CE.
As so many clues to the path of leprosy’s spread get filled in, it becomes increasingly urgent to ask how the disease has persisted. In 2014, an odd discovery was made. Squirrels in present-day Scotland were found to be infected with leprosy. Could squirrels have been an element in the medieval transmission of leprosy, too? And if so, in which direction: were they bringing leprosy to humans, or “catching” it from humans with whom they were interacting?
That lucky find of leprosy in squirrels (more has since been documented) combined with some deep sleuthing has shown how important the new One Health conception of disease history is. The concept of One Health looks at (among other things) disease transmission across host species. The most recent work on this question detected molecular fragments of M. leprae in squirrel remains coming from medieval Winchester. Comprehensive scouring of historical documents and manuscript images then reconstructed how people in medieval Europe interacted with squirrels. The answer was: a lot! They used them extensively for fur trimming on their clothes. And they even kept them as pets.
All this documented interaction means that, at the moment, there is no way to tell which direction leprosy was being transmitted: was it from humans to squirrels, or squirrels to humans? But it does tell us that the strains of M. leprae still found in wild squirrel populations in the UK are living traces of interactions 100s of years ago.
Leprosy as a Slow Pandemic
The COVID-19 pandemic underscored our sense that pandemics are rapid, sudden, inexplicable transmissions of disease through vast swathes of human populations. Yet we can think of leprosy’s global distribution as a pandemic, too, even though it happened over the course of centuries rather than weeks or months. It happened without the airplanes that made COVID’s global spread possible, and without the steamships that carried the 1918 Flu around the world. It happened without the railroads and major canals that made the early 19th-century cholera pandemics possible. Rather, leprosy’s pandemic shows us the paths of humanity’s most adventurous expansions in the days of sail. The global map of leprosy that George Thin published in 1891 is nothing other than a snapshot of this pandemic’s long-term effects.
One of the most tragic stories of persons suffering from leprosy is the enforced segregation in the Hawai’ian settlement of Kalaupapa in the later 19th and 20th centuries. But exactly when did leprosy arrive in the Pacific? A study completed in 2024 found that, as had already been hinted in 2020, the strains of leprosy in Hawai’i and other Pacific Islands belonged to the most ancient lineages of Mycobacterium leprae from the Pacific Rim and may have been brought to the islands by their first human inhabitants. Since Hawai’i, the easternmost island group of Polynesia, has been inhabited since around 1250, that means that leprosy had nearly circumnavigated the globe within 150 years of Constantine’s lifetime.
The biggest remaining mystery about leprosy’s medieval history is whether it had reached the Americas before contact with Europeans and Africans starting in 1492. This part of the story involves the recently discovered species of leprosy, Mycobacterium lepromatosis. This is in fact the species of leprosy that was found in squirrels in the UK in 2014. That strain seems far distant, however, from the strains now afflicting humans on the other side of the world in the Americas, where M. lepromatosis was first discovered in 2008.3 M. lepromatosis has also been confirmed in Southeast Asia, including India.
Yet unlike the “classic” leprosy, M. leprae, for which 100s of complete genome sequences are now available, allowing the creation of a rich family tree of the organism’s evolution, only three complete genome sequences are currently available for M. lepromatosis. Only one of them is of high quality. At the moment, the American strains seem to be quite closely related to each other, suggesting their dispersal in the region only in the past couple hundred years. But where did they come from? As a recent clinical report on M. lepromatosis in the American Midwest noted, “The geographic reach of this disease may be much broader than previously thought.”
In 2021, a ten-year plan, the Global Leprosy Strategy called “Towards Zero Leprosy,” began. The likely role of squirrels in leprosy’s medieval transmission between humans and animals parallels the documented role of armadillos in the Americas, which have been connected to both M. leprae and M. lepromatosis. If cross-species transmission is (and maybe, has always been) the norm for this disease, how will the world get to “zero”? Interrogating the medieval history of leprosy will not answer that question directly. But it will show us why it is a necessary question to ask. Seeing leprosy as a “pandemic in slow motion” shows us the value of framing pandemic research in a One Health framework.
1. Jordan’s achievement was not to be the first country to have zero reported leprosy cases, but to be the first to have systematically eliminated the disease by following a prescribed public health framework and received verification. See https://www.who.int/news/item/19-09-2024-jordan-becomes-first-country-to-receive-who-verification-for-eliminating-leprosy.
2. As of 20 January 2025, that group no longer includes the United States of America, which has withdrawn from the World Health Organization but which reported 369 cases of leprosy for 2023; source: WHO 2024.
3. As an increasing body of literature is compiled on Mycobacterium lepromatosis, it seems that this species of leprosy manifests somewhat differently than its cousin, M. leprae, in terms of symptoms and speed of morbid effects. However, the only way to distinguish the two definitively is by molecular analysis, which has only been available in recent years. Thus even though M. lepromatosis was only discovered in 2008, it is in no way a “new” disease. See Collin et al. 2023 for current information on its geographic extent.
Select bibliography:
Collin, Simon M., Amanda Lima, Stéfano Heringer, Vinícius Sanders, Hugo Aborghetti Pessotti, Patrícia Deps. “Systematic Review of Hansen Disease Attributed to Mycobacterium lepromatosis,” Emerging Infectious Diseases 29, no. 7 (July 2023), 1376-1385.
Crane, Adele E. “Phylogenomics and Zoonotic Spillover of Mycobacterium leprae in the Pacific Islands and Brazil,” PhD diss., Arizona State University, 2024. https://keep.lib.asu.edu/items/195249.
Gili, Anna. “Leprosy (al-ǧuḏām) and Smallpox (al-ǧudarī) in the Kitāb al-Malakī and its Two Latin Translations,” Medical and Philosophical Perspectives on Illness and Disease in the Middle Ages, a special issue of Quaderni di Noctua 7 (2024), ed. Alessandro Palazzo and Francesca Bonini, pp. 70-107.
Green, Monica H. [untitled: tweetstream on new leprosy phylogenetics study (Benjak et al 2018), in honor of World Leprosy Day 2018], archival copy posted on Academia.edu: https://www.academia.edu/118718902/
Green, Monica H. [untitled: Twitter thread on leprosy genetics, 2018-2022], collected tweets posted on Academia.edu, https://www.academia.edu/127132649/.
Green, Monica H. “World Leprosy Day 2023: Hansen’s Disease, Han’s Disease, and the Global History of Leprosy,” a Twitter essay on the history of leprosy, to commemorate World Leprosy Day, 29 January 2023, https://www.academia.edu/95928220/.
Green, Monica H. “The Stigma of Neglect: Why We Know Less Than We Should About Medieval Leprosy,” Constantinus Africanus blog, 28 Jan 2024, https://constantinusafricanus.com/2024/01/28/the-stigma-of-neglect-why-we-know-less-than-we-should-about-medieval-leprosy/.
Green, Monica H., and Jonathan Hsy. “Disability, Disease, and a Global Middle Ages,” in: Teaching the Global Middle Ages, ed. Geraldine Heng, MLA Options for Teaching (New York: PMLA, 2022), pp. 302-14.
Montes, Núria, Clara Jáuregui, Rosa Dinarès, Vanesa Triay, Andrea Fernández-Vilela, Jordi Ruiz, M. Eulàlia Subirà, and Maria Fontanals-Coll. “Tracing Leprosy: The Paleopathological Study of the Individuals Excavated from the Sant Llàtzer Leprosarium in Barcelona, Spain (12th-18th c.),” International Journal of Paleopathology 48 (March 2025), 23-33, https://www.sciencedirect.com/science/article/pii/S1879981724003267.
World Health Organization (WHO). “Global leprosy (Hansen disease) update, 2023: Elimination of leprosy disease is possible – Time to act!,” Weekly Epidemiological Record (WER) 99, no. 37 (13 September 2024), 497–521.
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