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Historical pandemics Tracking down the transmission of the plague

How dynamic was the spread of the most notorious infectious disease of all time? Using historical data, researchers have examined how quickly the plague spread in London in the 14th century and 300 years later with further outbreaks. In the medieval wave, the number of infected people doubled on average every 43 days, whereas in the plague wave of the 17th century it was already within eleven days. The causes of the accelerated rate of spread in the later pandemics remain unclear – there are several possible options, say the researchers.

The current Covid-19 pandemic is sure to make many people think about the former plague afflictions of mankind. But despite the difficult situation, one thing can be said very clearly: the extent of the horror that the plague once caused will not be reached by the coronavirus. Because the deadly potential of the plague was much higher. Between 1346 and 1353, it is estimated that the Black Death killed more than a third of Europe’s population. In the centuries that followed, the Grim Reaper kept going around and killing millions.

Recent advances in paleogenomics have confirmed that the historic plague pandemics were caused by the bacterium Yersinia pestis. Researchers succeeded in isolating the pathogen from the remains of London victims of the Black Death from 1348. The reconstruction of the genome of the historical strain showed that the versions of Y. pestis that still exist in some regions today are still very similar to it. These pathogens occasionally cause local outbreaks. Today, however, the spread of the plague can be kept in check with antibiotics. So far, however, it was unclear what dynamics the disease developed in the past epidemics in Europe.

Spread rates in the mirror of wills

The researchers led by David Earn from McMaster University in Hamilton have now investigated this question using the example of the city of London. Her focus was on the worst plague outbreaks in the city’s history. The first raged in 1348 and then waves followed in the 17th century, culminating in the so-called Great Plague of London in 1665. Earn and his team of statisticians, biologists, and evolutionary geneticists estimated mortality rates by analyzing historical, demographic, and epidemiological data from three sources: personal wills, church records, and the London death certificates.

As they report, the wills became the central source of information, because at the time of the medieval plague outbreak there were no death certificates. So the researchers first evaluated the data for the 17th century, when both wills and mortality were recorded. “Back then, people usually wrote wills when they were dying,” explains Earn. The same rate of spread of the plague was reflected in the information from the wills and death certificates, the researchers report. Therefore, they then assumed that in the case of the medieval plague, the wills can also serve as the sole source of information for determining the rate of spread.

Later four times faster

As McMaster University reports with reference to the results, the statistical calculations show: At the height of the spread, the number of infected people doubled about every 43 days in the 14th century. During the plague wave in the 17th century, however, the Grim Reaper dealt much more quickly: the number of infected people doubled about every eleven days, according to the evaluations. “We come to the conclusion that the rate of spread of the great plague of 1665 was four times higher than that of the epidemic of 1348,” the scientists sum up.

“It’s surprising how quickly the plague epidemics developed,” says Earn. According to the scientists, it is rather unlikely that the bacterium had changed its infectious characteristics over the course of 300 years. “Based on genetic evidence, we have good reason to believe that the bacterial strains responsible for the plague have changed very little during this period, so this is an interesting result,” says co-author Hendrik Poinar.

What was the acceleration?

As the scientists further report, the estimated speed of spread of both epidemics is consistent with the assumption that the plague bacterium was not primarily spread through contact from person to person – via a droplet infection. Because then it would have spread even faster. The growth rates of both early and late epidemics, on the other hand, are more in line with primary transmission through the bite of infected fleas, the researchers write.

So far, however, they cannot exactly say why the second wave of plague spread significantly faster. According to them, it is possible that there were more rats in London in the 17th century due to the increase in population compared to the Middle Ages. The fleas of the animals could then have ensured the faster transmission to humans. Another reason could have been a change in the climate in the 300 years. Possibly colder and more humid weather conditions favored the rate of spread of the disease in the 17th century.

The researchers now hope that their newly created database can continue to contribute to researching epidemiological patterns of the past and even our time. In this context, Earn concludes with a thought on the people who wrote their wills at the time of need: “Nobody who lived in London in the 14th or 17th century could have imagined that these records would have been used hundreds of years later could be used to study the spread of the plague, ”said the scientist.

Source: McMaster University, technical article, PNAS, doi: 10.1073/pnas.2004904117

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