Why did the incidence of leprosy suddenly drop in the late Middle Ages? To answer this question, biologists and archaeologists have excavated the strains of medieval pathogens that caused the disease from ancient human graves that have gone through centuries The genome has been reconstructed, thus clarifying this ambiguous historical period and proposing some new methods to understand the epidemic. The results of their research were published in Science on June 13.
Leprosy is a common disease in medieval Europe. The leprosy haze is still firmly rooted in our collective memory: a man wrapped in coarse cloth, ringing a bell on the street to inform the world of his arrival. This scene is not without foundation. It is estimated that almost 1 in 30 people in certain areas were infected with the disease.
However, at the turn of the 16th century, the disease suddenly retreated from most of the land. This incident was very sudden and puzzling. Could it be that the pathogens that cause leprosy have evolved into a less harmful form? To find out, biologists and archaeologists have jointly formed an international research team. They excavated the human remains buried in the medieval tomb to collect and reproduce the pathogenic bacteria of leprosy, thus deciphering the nearly complete genome from five strains of M. leprosy. Reconstructing these bacterial genomes is not easy, because the materials obtained are from ancient human bones and contain less than 0.1% of bacterial DNA. The researchers developed an extremely sensitive method to separate the two DNAs and reconstruct the target genome with an unprecedented level of precision. The co-author of the study, Pushpendra Singh, a scientist at the Swiss Federal Institute of Technology in Lausanne (EPFL), said: "Without using any contemporary strains as a basis, we reconstructed this genome."
The results of the study are indisputable: the genomes of these medieval strains are almost identical to the contemporary strains, and the mode of transmission has not changed. Only about 800 mutations occurred in 16 ancient and contemporary Leprosy genomes. "If the cause of the decline in leprosy cases is not due to the pathogen, then it must be the host, which is ourselves; so this is where we need to focus," said Stewart Cole, director of the FEEL Global Health Institute and co-director of the study. Many signs indicate that humans have developed resistance to this disease. Researchers believe that the decline of leprosy in Europe in the 16th century was not because its pathogens lost their pathogenicity, but because other infectious diseases such as plague or tuberculosis began to appear or because improved social conditions restricted their spread.
The researchers also discovered an interesting thing: a medieval strain of M. leprosy in Sweden and the United Kingdom is almost identical to the strain currently found in the Middle East. "We don't have enough data to determine the direction of the spread of the epidemic. The pathogen may be brought to Palestine when the Crusaders invaded eastward. But the process may also operate in the opposite direction." In addition to historical significance Research is also of great scientific significance. It improves our understanding of the mechanism of action of epidemics and leprosy pathogens. As part of this research, sequencing allows us to track many other pathogens. In addition, the incredible tolerance of the genetic material of M. leprosy provides further possibilities for tracing back the history and revealing the origin of this disease that still affects more than 200,000 people worldwide every year.
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