Did leprosy come from squirrels?

Leprosy, formally known as Hansen’s disease, is a chronic condition affecting the skin, nerves, and mucous membranes. It can be treated with a combination of antibiotics over six to twelve months, but if left untreated, it can lead to severe complications such as crippled hands and feet, paralysis, and blindness.

While leprosy has not been reported in the United Kingdom for the past 70 years, it remains prevalent in regions such as Africa, Asia, and South America. Despite a significant decrease from 5.2 million cases in the 1980s to over 200,000 cases reported annually, it remains a significant health concern, particularly in just 14 countries, with India accounting for over half of reported cases.

The bacterium Mycobacterium leprae, which is the main cause of leprosy, has been found in soil and water, raising questions about other possible avenues of transmission.

A recent study published in the journal Current Biology in May 2024 suggests a surprising link between leprosy and red squirrels in medieval England.

Researchers from the University of Leicester and the University of Basel, Switzerland, examined bones from both humans and red squirrels found in archaeological digs in Winchester, England. They discovered Mycobacterium leprae, the bacterium that causes leprosy, in the bones of both humans and squirrels. Interestingly, the strains found in humans and squirrels were more closely related than those found in squirrels from both medieval and modern times.

The study, co-authored by Professor Verena Schuenemann of the University of Basel and Dr. Sarah Inskip of the University of Leicester, suggests that leprosy may have been transmitted between humans and red squirrels during the medieval period. Records from 1417 indicate that squirrel skinning was common in Winchester, potentially facilitating the transmission of the disease from squirrels to humans.

Dr. Inskip noted that this finding challenges the notion that leprosy was exclusively a human disease and highlights the complexity of disease transmission between humans and animals. While the exact mechanism of transmission remains unclear, the study suggests that close contact with infected animals may have played a role in the spread of leprosy.

While the possibility of humans contracting leprosy from squirrels is very slim, it raises questions about the role of animals in disease transmission. Dr. Stephen Walker of the London School of Hygiene and Tropical Medicine emphasized the need for further research to better understand the role of animals in the history and transmission of leprosy.

The study also draws attention to the broader issue of disease transmission between humans and animals. Throughout history, diseases such as tuberculosis, the Black Death, and more recently, COVID-19, have been transmitted from animals to humans. While the transmission of leprosy from squirrels to humans in medieval England is unprecedented, it underscores the importance of studying the complex interactions between humans and animals to prevent future disease outbreaks.

In addition to red squirrels, other animals such as armadillos and chimpanzees have been found to carry leprosy. Understanding the role of animals in disease transmission is crucial for both human and animal health and could help prevent future outbreaks of infectious diseases.

Public Health England has stated that the chances of humans catching leprosy from squirrels are very slim, and no such cases have ever been reported. While leprosy is commonly believed to spread from person to person through physical contact, it is not highly contagious, with 95% of people exposed to Mycobacterium leprae not developing the disease. The exact method of transmission remains unknown, but it is believed to require lengthy close contact with an infected individual over several months and may be spread through coughing or sneezing.

There may be a risk of transmission in countries where humans come into close contact with animals more frequently. Dr. Inskip suggested that the study may warrant further investigation into local animals, as some of them may carry the bacteria responsible for leprosy.

Prof. Verena Schünemann of the University of Basel, the senior author of the study, commented, “The history of leprosy is far more complex than previously thought.” She added that the role of animals in transmitting leprosy had never been considered before, and as such, the full history of the disease remains unclear. Dr. Stephen Walker, Associate Professor at the London School of Hygiene and Tropical Medicine, stated that while there is no doubt animals may play a role in disease transmission, the extent of their involvement requires further research.

It is known that animals can transmit diseases; for example, tuberculosis is believed to have been initially contracted from cattle around 10,000 years ago during the Neolithic Age, when humans began farming. The Black Death, which originated with marmots in Asia around 1350 CE, is another example. Similarly, the Spanish flu in 1918 is thought to have come from birds, and COVID-19 has been linked to animals sold for meat at the Huanan market in Wuhan, China, although recent studies suggest that raccoon dogs may have been the source. The transmission of leprosy from squirrels to humans in the 10th and 11th centuries, however, has never been previously considered.

The red squirrel is an endangered species found throughout Eurasia, with just 160,000 individuals remaining in the United Kingdom. The species is particularly threatened by habitat loss, competition from gray squirrels, and the lethal effects of squirrel poxvirus.

In conclusion, the study sheds new light on the history and transmission of leprosy and underscores the need for further research into the complex interactions between humans and animals. Understanding the role of animals in disease transmission is crucial for preventing future outbreaks and protecting both human and animal health.