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Probably not in the way the question implies. Ancient DNA proves that at least one medieval red squirrel in Winchester carried Mycobacterium leprae, and that its bacterial strain was closely related to a strain found in medieval people from the same city. That makes squirrels a plausible part of medieval leprosy ecology. It does not prove that squirrels infected people, show which host infected the other, or demonstrate that squirrels were responsible for a substantial share of medieval cases.
What the Winchester discovery actually found
The central evidence comes from a peer-reviewed study led by Christian Urban, published in Current Biology in 2024. The researchers examined archaeological material from two sites in Winchester, a medieval city associated with a leprosarium and with the fur trade. Their sample contained 25 human specimens and 12 squirrel specimens. From those remains they reconstructed four medieval M. leprae genomes, including one from a red squirrel, at an average genome coverage of 2.2-fold. The coverage figure describes how much sequence data was recovered; it is not an infection rate or an estimate of how common leprosy was.
All four reconstructed strains belonged to branch 3 of the M. leprae family tree. The squirrel genome was closely related to a newly reconstructed medieval human genome. More specifically, it was more closely related to some medieval human strains from Winchester than to modern English red-squirrel strains. The authors interpret that pattern as evidence that M. leprae circulated in non-human hosts in the Middle Ages in a way that had not previously been detected. The published study is available through the University of Cambridge repository.
Why related DNA is not proof of squirrel-to-human transmission
Closely related bacterial genomes show that the hosts shared a disease history, but they do not record the direction of transmission. A squirrel could have infected a person, a person could have infected a squirrel, or both could have acquired related bacteria from another host or from an environmental chain that has not been identified. Ancient DNA also cannot, from this single set of remains, establish when a transmission event occurred.
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The Winchester result therefore supports a narrower conclusion: medieval red squirrels were capable of carrying the same bacterial species that infected medieval humans, and the squirrel strain was part of a closely connected local lineage. It does not establish a proven route involving fur, bites, pets, carcasses, or routine handling. The city’s fur-trade connections and its leprosarium explain why Winchester is an informative setting, not how infection moved between species.
Nor does the study show how often squirrels were infected. Twelve squirrel archaeological samples and one reconstructed squirrel genome cannot be converted into a prevalence percentage for medieval England. The evidence also cannot tell us what proportion of human leprosy cases, if any, involved squirrels.
How the medieval evidence compares with modern squirrel findings
Modern studies demonstrate that British red squirrels can carry leprosy-causing bacteria, but they involve different dates, places, and sometimes a different species of bacterium. Those distinctions matter when interpreting the medieval result.
| Evidence | Bacterium identified | Where and when | What it establishes | What it does not establish |
|---|---|---|---|---|
| Urban and colleagues, 2024 | M. leprae | One medieval red squirrel from Winchester; comparison with medieval human remains from the city | A medieval squirrel carried M. leprae; its strain was closely related to medieval human strains | Transmission direction, timing, frequency, or population-level importance |
| Avanzi and colleagues, 2016 | M. lepromatosis in squirrels from England, Ireland, and Scotland; M. leprae in squirrels from Brownsea Island, England | Modern British red squirrels | Both bacterial species can occur in living British red squirrels; infection was detected in visibly diseased and apparently healthy animals | That modern infections caused medieval human disease or reveal its transmission route |
| UK surveillance paper, 2024 | M. leprae and M. lepromatosis | Wild red squirrels in Scotland and northern England | Both organisms remain detectable in contemporary squirrel surveillance | A demonstrated route from squirrels to people; the animals’ ecological role remains unresolved |
The 2016 findings are reported in the PubMed record for Avanzi and colleagues. The study found M. lepromatosis in squirrels from three countries—England, Ireland, and Scotland—and detected M. leprae in squirrels from Brownsea Island in England. It also found infection in animals that looked ill and in animals that appeared healthy, showing why visible disease in a squirrel cannot be used as a complete surveillance test.
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A 2024 molecular and serological surveillance study likewise reported both M. leprae and M. lepromatosis in UK red squirrels, while explicitly stating that their role in disease ecology still needs to be worked out. Its findings are available in PubMed Central.
The species distinction: M. leprae versus M. lepromatosis
“Leprosy bacteria” is not a single interchangeable label in this evidence. The medieval human remains discussed in the Winchester study yielded M. leprae. Modern squirrel studies often find M. lepromatosis, although they have also detected M. leprae in particular animals and locations.
A critical review of UK squirrel evidence notes that modern squirrel infections more commonly involve M. lepromatosis, whereas M. leprae is the species identified in the medieval human remains considered here. That difference prevents a simple chain of reasoning from “modern squirrels carry a leprosy bacterium” to “modern squirrels explain medieval human infection.” The review, The role of red squirrels in leprosy dynamics in the United Kingdom, is available at PubMed Central.
The correct comparison must keep four variables together: bacterial species, date, geography, and genetic relationship. A result for M. lepromatosis cannot automatically establish the history of M. leprae; a modern English squirrel cannot automatically stand in for a medieval Winchester squirrel; and detection alone does not reveal the direction of spread.
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What “spread” would require scientists to demonstrate
To answer the title literally, scientists would need more than one infected animal and a related genome. Several additional findings would strengthen a transmission claim.
- Direction. A dated series of host genomes would need to show that a squirrel lineage preceded a closely matching human lineage, or the reverse. Genetic similarity by itself cannot provide that ordering.
- Repeated contact across places and times. Comparable squirrel and human strains from multiple medieval settlements would make a sustained ecological connection more credible than a single paired observation.
- A plausible exposure pathway. Historical or archaeological evidence could help identify regular contact, but Winchester’s fur trade is context, not proof that fur handling transmitted infection.
- Population scale. Broad sampling would be needed to estimate how common infection was in squirrels and whether it could account for a meaningful fraction of human cases.
- Species matching. The bacterium in the suspected animal-to-human chain would need to be identified precisely. Findings for M. leprae and M. lepromatosis cannot be merged as if they were one organism.
The current evidence does not satisfy those tests. It establishes host status and a close local relationship, not a complete transmission narrative.
Why Winchester matters without proving a local outbreak source
Winchester provides unusually useful archaeological context. It was a medieval city with a leprosarium, so human remains associated with the disease are available for comparison. It also had links to the fur trade, which makes the presence of squirrels and contact with animal materials historically plausible. Those facts help explain why researchers could detect a squirrel strain alongside human strains.
They should not be overread. A leprosarium does not show that its patients acquired infection from squirrels, and a fur trade does not establish that pelts carried viable bacteria into people. The study did not identify a specific event, occupation, animal product, or behavior as the transmission mechanism.
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What the genome result says about medieval disease ecology
The strongest historical implication is ecological rather than causal. Medieval England contained more than one host in which M. leprae could persist. The squirrel genome reveals a non-human host that had not been represented in the known medieval record, and its relationship to Winchester human strains suggests that the local bacterial population was not confined to people.
That finding broadens the range of questions historians and epidemiologists can ask. Animal remains from other settlements may contain additional bacterial genomes. Future work could test whether squirrel-associated strains were geographically restricted, whether they changed through time, and whether the apparent relationship at Winchester recurs elsewhere. None of those possibilities is established by the current sample, but they are the logical next questions raised by it.
The result also illustrates why ancient DNA must be interpreted with sampling limits in view. Four genomes reconstructed from 37 archaeological samples are valuable discoveries, yet they represent selected remains that survived, were excavated, and yielded recoverable DNA. They are not a census of medieval people or squirrels.
Does this change the risk from red squirrels today?
It does not establish a present-day human transmission route. Modern surveillance has found leprosy bacteria in wild red squirrels, but the ecological significance of those infections remains unresolved. The medieval finding is historical evidence of host involvement, not a warning that ordinary contact with squirrels is a demonstrated source of human disease.
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It is also important not to infer risk from appearance. The 2016 study detected infection in both visibly diseased and seemingly healthy squirrels. That observation supports surveillance and careful scientific testing; it does not identify a routine exposure pathway for the public.
The most accurate answer to the title
Red squirrels could have participated in medieval England’s leprosy ecology, and a Winchester squirrel demonstrably carried M. leprae closely related to bacteria found in medieval humans. But the available DNA does not prove that squirrels spread leprosy to people, establish which species infected the other, or show that squirrels drove the prevalence of the disease.
That qualified conclusion matches the authors’ own interpretation: “In particular, the medieval squirrel strain is more closely related to some medieval human strains from Winchester than to modern red squirrel strains from England, indicating a yet-undetected circulation of M. leprae in non-human hosts in the Middle Ages.” The statement supports previously unrecognized circulation in an animal host; it does not claim a proven squirrel-to-human outbreak.
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