Pest: Rats were not the vectors

Experts are investigating how the plague could spread so much in the Middle Ages

In the 14th century, the plague reached Europe and led to the fact that people died in whole areas. The plague is arguably one of the worst pandemics in human history, claiming millions of lives. The cause of the disease is now known, but it was still unclear how the plague could spread from person to person so quickly. Researchers have now found that human lice and fleas were primarily responsible for the rapid spread and not rats and their parasites as previously thought.

The University of Oslo researchers found that the plague in the Middle Ages was probably transmitted from person to person via lice and fleas. So far, there have been assumptions that the disease was spread by rats and their parasites. The experts published the results of their study in the English-language journal "Proceedings of the National Academy of Sciences".

The plague and its effects in the Middle Ages

The plague is a terrible disease that killed almost a third of the population in Europe between 1346 and 1353 alone. In the case of outbreaks that repeatedly occurred for around 500 years, up to 60 percent of the population succumbed to the bacterium Yersinia pestis. The disease is also known as black death, thanks to its most famous outbreak, which is due to blackened and swollen lymph nodes. These occurred after bacteria had entered the skin.

Human fleas and lice were the transmitters

The biggest mystery in the disease is the physical mechanism that accelerated the rapid spread of the disease, experts say. In a long-held theory, rats, mice, or other rodents acted as hosts of the bacteria. For a long time, fleas, which first infected the animals and then humans, were the main carriers. The researchers explain that the rapid pace of transmission and spread is more likely to be explained by human fleas and lice. Such parasites were very widespread in the Middle Ages and almost everywhere.

Rats did not seem to play a major role in the spread

According to the researchers, the deadly bacterium spread from a rodent to humans and parasites at some point before the plague broke out. So it could spread ever further. The rats then no longer played a major role. The plague is treatable with antibiotics if it is detected early, but the symptoms can be fatal if not treated in time. However, in the Middle Ages, such drugs were not available to those affected. The newly developed mathematical model used in the study showed that rodents were wrongly blamed for spreading the plague, explains author Katharine Dean from the University of Oslo.

Scientists are creating a model for the spread of the plague

For their model, the scientists created a list of features of the plague based on current field observations, experimental data and estimates. For example, these included facts that the likelihood of recovering from the disease was 40 percent and that a louse with plague bacteria remained infectious for a period of about three days. During this time, one person could accommodate an average of six fleas. So-called mortality records from several centuries provided the most critical detail. Back then, observers were able to document the increase and decrease in deaths from the plague each week because the disease was so virulent and the signs of infection were so obvious, explains Boris Schmid from the University of Oslo.

There were three different scenarios for the model

By evaluating three different scenarios, the scientists were able to determine how the plague spread. In a common lice and fleas the plague. In the second scenario, rodents and their parasites spread the plague. In the third case, coughing people spread a version of the disease, the so-called lung plague.

What did the model result in?

The rodent model did not match historical death rates. The result showed a delayed, very high increase in deaths, which, however, does not reflect the mortality data. The model with the lung plague also did not match the data of the deaths at that time, say the experts. Human body lice or fleas were the main transmission pathways in medieval pandemics, the authors assume. Researchers had previously expressed some doubts about the theory that rat fleas (Xenopsylla cheopis) triggered the spread of the plague.

Plague outbreaks still occur today

Outbreaks of pests still take place today. The disease has recently been suspected or confirmed as the cause of 171 deaths in Madagascar, according to the World Health Organization (WHO). (as)

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